1 atomic spectroscopy

Measurement principle [VIM 2.4] of spectroscopy for the study of electromagnetic radiation absorbed and emitted by atoms.

1. Atomic spectra may be emission or absorption spectra.

2 molecular spectroscopy

Measurement principle [VIM 2.4] of spectroscopy for the study of rotational, vibrational, and electronic transitions of molecules.

1. Molecular spectra may be emission or absorption spectra.

3 nuclear magnetic resonance spectroscopy (NMR)

Measurement principle [VIM 2.4] of spectroscopy to measure the precession of magnetic moments placed in a magnetic induction based on absorption of electromagnetic radiation of a specific frequency by an atomic nucleus.

1. Nuclei having a suitable magnetic moment include ¹H, ¹³C, ¹⁵N, ¹⁹F, ³¹P.

2. The technique is used as a method of determining structure of organic molecules, or as a mechanism for quantification.

Source: [4].

4 spectrometry

Measurement of quantities related to electromagnetic radiation or charged particles as a means of obtaining information about a system and its components.

Examples: electron emission spectrometry, mass spectrometry.

Source: [5] p 1738. See also: spectroscopy.

5 spectroscopy

Study of chemical systems by the electromagnetic radiation with which they interact or that they produce.

Examples: atomic absorption spectroscopy, nuclear magnetic resonance spectroscopy.

Source: [5] p 1738. See also: spectrometry.

6 absorbance, A

deprecated: extinction

obsolete: attenuance

Logarithm of the division of incident radiant power (P₀) by transmitted radiant power (P_tr).

A = log_a(P₀/P_tr) = –log_a(T) = –log_a(1 – α_i), where T is the transmittance and α_i is the absorptance.

1. The base a of the logarithm should be specified. See decadic absorbance (log₁₀, lg), and Napierian absorbance (ln).

2. Although the term absorbance used by chemists often means the decadic absorbance as measured (see experimental absorbance), the data must be corrected for reflection, scattering, and luminescence if absorbance is to have absolute numerical significance.

3. Confusingly, the term absorbance is also widely used for the negative logarithm of the ratio of the final to the incident intensities of processes other than transmission, such as attenuated total reflection and diffuse reflection.

Source: [3] p 36. See also: internal absorbance.

7 absorptance, α

absorption factor

Absorbed radiant power (P_abs) divided by incident radiant power (P₀). α = P_abs/P₀

Source: [3] p 36.

8 absorption coefficient

See: linear decadic absorption coefficient, linear Napierian absorption coefficient, molar decadic absorption coefficient, molar Napierian absorption coefficient.

9 absorption index, k

imaginary refractive index

Imaginary part of the complex refractive index describing absorption of electromagnetic radiation.

Source: [3] p 37.

10 absorption spectrum

General term for the spectrum of electromagnetic radiation absorbed by a sample.

1. The ordinate quantity may be absorption cross-section, absorption coefficient, or absorbance.

2. If the ordinate quantity is the absorbance, but not otherwise, the spectrum may be called an absorbance spectrum.

Source: [6].

11 accidental degeneracy

Condition that occurs when two or more normal transitions have the same energy as a matter of coincidence, as opposed to because of symmetry conditions.

Source: [6].

12 aliasing

See: folding (in spectroscopy).

13 angle of incidence, θ

Angle between a beam of electromagnetic radiation and a line normal to a surface.

14 anisotropic

nonisotropic

Having properties that depend on direction.

Source: [7] p 2198. See also: isotropic.

15 apodization

Technical term for changing the shape of a mathematical function, electrical signal, or optical transmission to remove or smooth a discontinuity at the edges, literally meaning ‘removing the foot’.

1. Originally, ‘apodization’ described the procedure by which Fourier-transform infrared spectra are corrected for the side lobes that appear in the wings of spectral bands when the interferogram is not zero at its limits, by multiplying the interferogram prior to Fourier transformation by a weighting function that is zero at its limits (see Fourier transform spectroscopy). Its use has broadened in recent years to include weighting functions that are not zero at their limits.

2. Apodization is also used for manipulating FIDs (free induction decay) to enhance specific components of the signal, for example to improve signal-to-noise or resolution, and is achieved by application of specific shapes to the FID, e.g., in nuclear magnetic resonance spectroscopy: exponential, Gaussian or sine-bell; in infrared spectroscopy: Boxcar, Happ-Genzel, Norton-Beer, cosine.

Source: [6].

16 apodization function

window function

Mathematical function that effects apodization.

17 array detector

Photoelectric detector in which a large number of pixels are distributed, usually in regularly spaced lines and usually over a rectangular area.

Source: [6].

18 asymmetric Fourier-transform spectroscopy

See: dispersive Fourier-transform spectroscopy.

19 attenuated total reflection (ATR)

frustrated total internal reflection

Internal reflection from an absorbing material at angles of incidence at or above the critical angle.

See also: attenuated total reflection spectroscopy, multiple attenuated total reflection.

20 attenuation index

See: complex refractive index.

21 band-pass filter

bandpass filter

1. Optical filter that passes a spectral band of wavelengths within a certain range and centred at a certain wavelength and that rejects radiation of a wavelength outside of the range.

2. Electrical filter that passes a band of electrical signals with a certain frequency range, centred at a certain frequency, that rejects signals at frequencies outside of the range.

Source: [6]. See also: high-pass filter, low-pass filter.

22 bandwidth

See: spectral bandwidth.

23 beam splitter

Device to split a beam of electromagnetic radiation into two parts.

1. In an ideal 2-beam Fourier-transform spectrometer, the beam splitter would transmit half of the radiation and reflect half of it to create the two beams.

Source: [6].

24 Beer-Lambert law

Beer-Lambert-Bouguer law

Absorbance of a beam of collimated monochromatic radiation in a homogeneous isotropic medium is proportional to the absorption path-length, l, and to the concentration, c, or (in the gas phase) to the pressure of the absorbing species.

1. This law holds only under the limitations of the Lambert law [8] p 361, and for absorbing species exhibiting no concentration or pressure-dependent aggregation.

2. The law can be expressed as A(λ)=lg(Pλ0/Pλ)=ε(λ)cl, where the proportionality constant ε(λ) is the molar decadic absorption coefficient and Pλ0,Pλ are, respectively, the incident and transmitted spectral radiant power.

3. Spectral radiant power must be used because the Beer–Lambert law holds only if the spectral bandwidth of the ultraviolet, visible, or infrared radiation is narrow compared to line widths in the spectrum.

Source: [8] p 307.

25 Cauchy function

Function y = a/(b²+x²) where a and b are constants. If x is replaced by ν˜−ν˜0 this function yields a Lorentzian band centred at ν˜0.

Source: [6].

26 charge coupled device (CCD)

Device that stores information in the form of charge packets in an array of closely spaced capacitors.

1. The packets can be transferred from one capacitor to the next sequentially by electronic manipulation, so that the information contained in the array can be sequentially read electronically.

2. In spectroscopy, the charge packets are created by interaction with radiation and the devices are extremely sensitive detectors for near IR and visible radiation.

Source: [6].

27 circular birefringence

Difference between the refractive indices of an optically active medium for left, n^L, and right, n^R, circularly polarized radiation (n^L– n^R).

1. “Circular birefringence” is also used to describe the phenomenon of having different refractive indices for left and right circularly polarized radiation.

2. Like the refractive indices (n^L, n^R), circular birefringence changes with wavenumber.

Source: [6].

28 complex refractive index, nˆ

refractive index

Speed of light in a given medium divided by speed of light in vacuum.

1. nˆ=n+iκ. The real part, n, is usually called the ‘refractive index’, and is the entire refractive index when no radiation is absorbed. The imaginary part, κ, describes absorption (see absorption index).

2. The older literature, and some physics literature today, uses nˆ=n(1+iκ) where κ is called the attenuation index. For simplicity, this usage is discouraged.

Source: [3] p 37.

29 decadic absorbance, A₁₀, A

Absorbance calculated with logarithm base 10.

1. Confusion with Napierian absorbance must be avoided.

Source: [3] p 36.

30 degeneracy, g

Number of states that have the same energy level.

Source: [6]. See also: accidental degeneracy.

31 dephasing

phase relaxation

Loss of coherence between the upper and lower states of a transition.

1. The term ‘dephasing’ arises from the density matrix formalism that is used in a phenomenological approach to simplify the description of the relaxation dynamics of a molecule coupled to its surroundings.

2. Dephasing leads to either homogeneous broadening or inhomogeneous broadening depending on the system. The continually changing intermolecular interactions cause a shift of the frequency of the intramolecular vibration. If these interactions change much more slowly than the amplitude of this shift, heterogeneous broadening results and causes a Gaussian band. If these interactions change much faster than the amplitude of this shift, homogeneous broadening occurs and the observed spectral band is a notionally narrowed Lorentzian band. In the general case, the broadening due to dephasing is neither homogeneous nor heterogeneous and the line shape is neither Gaussian nor Lorentzian.

Source: [6].

32 derivative spectroscopy

Measurement method [VIM 2.5] of spectroscopy in which the absorbance, or other spectral ordinate, is differentiated n times with respect to wavenumber or frequency to give the n^th derivative spectrum, i.e., the spectrum of the n^th derivative of the original spectrum.

1. The technique is used to transform changes of slope in the original spectrum into more prominent features in the derivative spectra and is extensively used in the near-infrared to flatten spectral baselines to improve subsequent chemometric study.

Source: [6].

33 detector gate width

detector integration time

Time for signal integration after start of detection.

1. Optimization of gate width allows detection of species with different decay rates.

Reference [51].

34 difference spectroscopy

Measurement method [VIM 2.5] of spectroscopy in which spectral subtraction is used to help the study and identification of individual species in a mixture.

Source: [6].

35 diffuse reflectance, R

remittance

remission fraction

Diffuse reflected (remitted) radiant power (P_rem) divided by incident radiant power (P₀). R = P_rem/P₀.

1. The sum of remittance, transmittance, and absorptance equals one.

2. Some authors distinguish between remittance and diffuse reflectance by excluding specular reflection in the latter.

Source: [6].

36 diffuse reflection

remission

Reflection in which electromagnetic radiation incident on a scattering surface at a certain angle is reflected (remitted) over all angles.

1. Diffuse reflection is a complicated process and involves transmission, reflection, and scattering.

Source: [6]. See also: remittance.

37 diffuse transmission

Process in which radiation is transmitted by a scattering sample and leaves the sample in directions other than that required by Snell’s law of refraction. The process is complicated and involves transmission, reflection, and scattering.

Source: [6]. See also: transmission.

38 dipole coupling

transition-dipole transition dipole coupling

Coupling of different motions through dipole-dipole forces.

1. In vibrational spectroscopy of crystals, intermolecular vibrational coupling through interaction of resonant transition dipoles throughout the whole crystal.

Source: [6].

39 dispersive Fourier-transform spectroscopy

asymmetric Fourier-transform spectroscopy

Fourier-transform spectroscopy using a Michelson or other two-beam interferometer with the sample placed in one arm of the interferometer.

1. This method provides information about the phase change as well as the amplitude change caused by the sample.

Source: [6].

40 dispersive spectrometer

Spectrometer in which electromagnetic radiation is separated spatially into its component wavenumbers by a dispersive element, such as a prism or a diffraction grating.

Source: [6].

41 double-beam spectrometer

Spectrometer in which beams of electromagnetic radiation from the source reach the detector via two paths, one through the sample and the other through a reference.

1. The radiant power in each beam is measured at each wavenumber.

2. The output of a double-beam spectrometer is termed a double-beam spectrum. The ordinate of a double-beam spectrum is related to the radiant power that reaches the detector in the sample beam divided by that in the reference beam. The spectrum has a flat baseline where the sample does not absorb.

3. The reference is usually a material having the same composition as the sample but lacking the analyte.

Source: [6].

42 electric dipole moment, p, μ

dipole moment

Vector quantity of a dipole that in an electric field (E) has a potential energy E_p = – p · E

1. The direction of the dipole moment is from the negative to the positive charge.

2. Electric dipole moment has also the magnitude of equal separated charges of opposite sign times the distance between them. It is usually expanded as: μ = μ_o + Σ_k (∂μ/∂Q_k) Q_k + Σ_k (∂²μ/∂Q_k²)Q_k² + Σ_i<j (∂μ/∂Q_i) (∂μ/∂Q_j)Q_iQ_j + higher order terms, where the Q_k, etc. are the normal coordinates and μ_o is the equilibrium dipole moment of the molecule.

3. SI unit: C m. Common unit: Debye, D ≈ 3.335 64 × 10⁻³⁰ C m. See Green Book (3^rd edition) p26 note 9 for alternative ways of expressing dipole moment [3].

Source: [3] p 26.

43 electromagnetic radiation (EM)

Flow of energy through space propagated as synchronized sinusoidal waves of the electric field, E, and the magnetic field H.

1. Electromagnetic radiation is characterized by frequency ν, wavelength λ, and speed c, where νλ = c.

2. The wavelength and velocity change when radiation enters a medium. In its interaction with atoms and molecules, radiation behaves like particles, called photons, with zero mass, energy hν, and momentum h/λ, where h is the Planck constant h = 6.626 070 15 × 10⁻³⁴ J s. [9]

Source: [6].

44 electromagnetic transition

Transition accompanied by the emission or absorption of a photon.

1. The frequency of the photon (v) is related to the energy difference of the transition by hv = ΔE = |E₂ – E₁|, where h is the Planck constant.

2. In general, an electromagnetic transition obeys certain rules called “electromagnetic selection rules”.

Source: [10].

45 electron emission spectrometry

Measurement principle [VIM 2.4] of spectrometry for the study of electrons emitted by atoms.

46 electronic transition

Transition in an atom, ion, or molecule from an electronic energy level E₁ to another energy level E₂.

Source: [10].

47 emittance, ε

emissivity

Radiant excitance (M) emitted by the sample divided by radiant excitance emitted by a black body (M_bb) at the same temperature. ε = M/M_bb

1. The term “excitance” can be replaced by “power” or “flux” if the sample and the black body have the same area and by intensity if, in addition, the emission is measured as a collimated radiation beam.

Source: [3] p 35.

48 energy level of a free atom, ion or molecule

energy level

energy state

Set of one or more stationary quantum states of a free atom, ion, or molecule having a particular internal energy.

1. SI unit: J. Common unit: electron volt, symbol eV, where 1 eV = 1.602 176 634 × 10⁻¹⁹ J. Energy levels are also expressed as kJ mol⁻¹.

Source: [10].

49 excimer laser

Pulsed gas laser that typically emits radiation in the ultraviolet range.

1. The lasing medium is made up of a gas mixture containing a halogen and a noble gas such as Ar and F₂ or Xe and Cl₂.

Source [51].

50 excitation energy

Minimum energy required to bring a system to a specified higher energy level.

1. Usually excitation energy refers to the energy of transition from the ground state to a higher energy level (excited state).

Source: [2] p 223.

51 excited state of a free atom, ion, or molecule

excited state

Stationary quantum state of a free atom, ion, or molecule of energy greater than that of the ground state.

Source: [11] p 205.

52 experimental absorbance, A₁₀

Decadic absorbance as measured with no corrections for other processes.

1. The term emphasizes that radiation might be lost from the beam by reflection, luminescence, scattering, and possibly vignetting (e.g., spectral shifts introduced from changing a radiation beam diameter or shape), and not solely by absorption.

2. Internal absorbance is used to term absorbance corrected for other processes.

Source: [6].

53 folding (in spectroscopy)

aliasing

The appearance of peaks at incorrect frequencies in a mathematically-processed spectrum due to insufficient density of datapoints in the time-domain spectrum or from peaks outside of the sampled frequency range.

1. Folding is a consequence of undersampling in spectroscopy.

2. In vibrational spectroscopy, when a spectrum is sampled correctly only up to a wavenumberν˜M, radiation of wavenumber ν˜>ν˜M appears to have a wavenumber between 0 and ν˜M, as follows:

This process can be simulated by folding a spectrum that shows the correct wavenumbers back onto itself at ν˜M and its multiples.

Source: [6]. See also: Nyquist criterion.

54 Fourier-transform spectroscopy

Fourier-transform spectrometry

Measurement method [VIM 2.5] whereby spectra are collected based on measurements of the temporal coherence of a radiative source, using time-domain measurements of electromagnetic radiation or other type of radiation.

1. This procedure can be applied to a variety of spectroscopies, including optical spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy, and electron spin-resonance spectroscopy.

2. There are several methods for measuring the temporal coherence of the radiation, including the CW (continuous wave) Michelson or Fourier-transform spectrometer and the pulsed Fourier-transform spectrograph (which is more sensitive and has a much shorter sampling time than conventional spectroscopic techniques).

Source: [8] p 344.

55 free induction decay (FID)

Indication [VIM 4.1] from a pulsed spectroscopy experiment that takes the form of damped sine waves arising from the excited system returning to equilibrium.

1. In nuclear magnetic resonance spectroscopy, the FID consists of sine waves oscillating at the Larmor frequency (ω) and dampened by net dephasing time relaxation (T₂*), sin(ωt)exp(−t/T2∗).

Source: [6].

56 frequency, ν, f

Number of cycles of periodic motion in unit time.

1. In spectroscopy, the periodic motion is of the electric and magnetic vectors of electromagnetic radiation.

2. Frequency is related to the energy change of an electromagnetic transition, ΔE, induced when electromagnetic radiation is absorbed through ΔE = hν, where h is the Planck constant.

3. SI unit: Hz, where 1 Hz = 1 s⁻¹.

57 Fresnel reflection

Reflection of radiation from a surface that is smooth and large with respect to the wavelength of the radiation. A reflected beam, of intensity governed by the Fresnel equations, is produced at an angle of reflection equal to the angle of incidence.

1. May be termed specular reflection for non-scattering samples.

Source: [6].

58 frustrated multiple internal reflection (FMIR)

See: multiple attenuated total reflection.

59 frustrated total internal reflection

See: attenuated total reflection.

60 full width at half maximum (FWHM), W, Γ

full width at half height (FWHH)

Interval between the two points across a section of a spectrum where the ordinate is equal to half the maximum ordinate of the section.

1. W is a measure of line or band width.

2. FWHM has units of wavelength, wavenumber, frequency, or energy.

3. For infrared absorption spectra, the FWHM must be measured from a spectrum that is plotted linear in absorbance.

4. In Raman spectra, the FWHM can be measured directly from the spectrum of scattered light intensity unless the instrument response function varies significantly across the band.

5. Inclusion of “full” distinguishes FWHM from half-width at half maximum.

Source: [6], [8].

61 Gaussian band

Spectral band with the shape HG exp[−4ln2(ν˜−ν˜0)2/W2], where H_G is the band maximum (peak height), W is the full width at half maximum, and ν˜0 is the peak wavenumber.

Source: [6].

62 ground state of a free atom, ion or molecule

ground state

Ground energy level of minimum internal energy of a free atom, ion, or molecule.

1. It is conventional to assign the relative energy value of zero to this level.

Source: [6].

63 Hadamard transform spectrometer

Multiplexing spectrometer that uses the optical components of a dispersive spectrometer with a Hadamard encoding mask.

Source: [6].

64 half width at half maximum (HWHM)

half width at half height (HWHH)

One half of the full width at half maximum.

Source: [6].

65 high-pass filter

1. Optical filter that excludes radiation of wavenumber lower (wavelength higher) than a certain value and passes radiation of wavenumber above (wavelength lower) than this value.

2. Electrical filter that blocks signal with a modulation frequency lower than a certain cut-on value and passes signals of frequency above this value.

Source: [6]. See also: band-pass filter, low-pass filter.

66 Hilbert transforms of a spectrum

Functions that interrelate the real part, f ΄, and imaginary part, f ˝, of the refractive index, nˆ=n+ik, or relative permittivity, ϵˆ=ϵ′+iϵ″, through

where P means that the principal part of the integral is taken at the singularity.

Source: [6].

67 imaginary refractive index

See: absorption index.

68 instrument line shape (ILS)

Idealized form of a feature in a spectrometer corresponding to a transition.

1. Ideal line shapes include Lorentzian, Gaussian, and Voigt functions, whose parameters are the line position, maximum height, and half-width.

2. For Fourier-transform spectra, ILS varies with the apodization function used.

3. Source: [6].

69 integrated intensity

Area under a spectral band in an absorption spectrum where the absorption is linearly proportional to the amount of absorber in unit area.

1. The area under a band in a transmittance spectrum is not linearly proportional to the amount of absorber.

2. The absorption quantity is usually a Beer-Lambert absorption coefficient, but recently ν˜αm”(ν˜), the wavenumber times the imaginary part of local molar polarizability, has been used for neat liquids to correct for dielectric effects.

Source: [6]. See also: infrared intensity and associated entries within the Green Book [3] p 37.

70 integration range of a spectrum

integration range

Frequency or wavenumber range over which the integrated intensity of an absorption band is measured by numerical integration of the area under the band.

1. For Gaussian bands, integration over one full width half maximum to each side of the band centre yields 98 % of the area under the band.

2. Lorentzian bands integration over 3.1 or 15.9 full width half maximum to each side yields 90 % and 98 % of the band area.

Source: [6].

71 intensified charge coupled device detector (ICCD)

Charge coupled device with a prior amplification system to improve sensitivity.

1. The intensifier used is based on a microchannel plate (MCP), which converts the incoming photons to electrons by means of a photocathode. Generated electrons are multiplied in a second step by the MCP and then reconverted to photons using a phosphorous screen. Finally, derived photons are detected with a CCD device.

Source: [51].

72 intensity of radiation, I, E

intensity

irradiance

deprecated: radiant flux

Radiant power (P) per unit area (A) that is received at a surface. I = dP/dA.

1. Intensity and irradiance are formally the same quantity, but the term intensity is usually used for collimated beams of radiation. (See [3] p 35 Note 5).

2. SI unit: W m⁻².

Source: [3] p 35. See also: spectral intensity, radiant intensity.

73 intensity spectrum

See: single beam spectrum.

74 interference (in analytical spectroscopy)

Effect that causes a change in the measured absorbance, or of the intensity for a given concentration, due to the presence of one or more components accompanying the analyte in the material submitted for, or the reagents used in, the analysis.

1. Interference in analytical spectroscopy should not be confused with interference of beams in an interferometer.

Source: [10].

75 interferometer

Device in which beams of electromagnetic radiation are superimposed causing interference in order to extract information.

See also: continuous scan interferometer, step-scan interferometer, Fourier-transform spectroscopy.

76 internal absorbance, A_i

Absorbance in the absence of reflection, scattering or luminescence.

1. Internal absorbance is a rarely used term in quantitative absolute intensity studies to emphasize that the necessary corrections of the transmittance for reflection and other cell effects have been made. Instrument manufacturers usually use the raw, uncorrected transmittance when calculating the absorbance. See experimental absorbance.

2. Formally, A_i = –log₁₀(1 – α_i), A_i = –log₁₀(T_i) for a sample that does not scatter or luminesce, where _i is the internal absorptance and T_i is the internal transmittance.

Source: [6].

77 internal absorptance, α_i

Absorptance fully corrected for surface effects and effects of the cell, such as reflection, scattering, luminescence, and vignetting losses.

1. If scattering and luminescence in the sample are negligible, α_i + Τ_i =1, where Τ_i is internal transmittance.

Source: [3] p 36.

78 internal transmittance, T_i, τ_i

internal transmission

Transmittance fully corrected for surface effects and effects of the cell, such as reflection, scattering, luminescence, and vignetting losses.

1. If scattering and luminescence in the sample are negligible, α_i + Τ_i =1, where α_i is internal absorptance.

Source: [6].

79 irradiance

See: intensity of radiation.

80 isotropic

Having properties that are independent of direction.

Source: [7] p 2198. See also: anisotropic.

81 Kramers–Kronig transforms of a spectrum

Functions based on the physical principle of causality that interconvert the real and imaginary parts of complex optical quantities when they are known over a sufficiently wide (strictly infinite) wavenumber range. They are frequently used to interconvert the real part, f′, and imaginary part, f″, of the refractive index, nˆ=n+ik, the dielectric constant (relative permittivity) , ϵˆr=ϵr′+iϵr″, or the logarithm of the complex reflection coefficient re^iφ through

where P means that the principal part of the integral is taken at the singularity.

1. All functions used to model vibrational spectra obey the Kramers–Kronig transforms as long as the real parts are even functions of wavenumber and the imaginary parts are odd functions of wavenumber, so that the Kramers–Kronig transforms are equivalent to the Hilbert transforms.

Source: [6].

82 laser

Source of ultraviolet, visible, or infrared radiation that produces light amplification by stimulated emission of radiation, from which the acronym is derived.

1. The radiation emitted is coherent except for superradiance emission.

2. All lasers contain an energized substance that can increase the intensity of radiation passing through it.

Source: [8] p 362.

83 light

Electromagnetic radiation visible to the human eye.

1. In general, spectroscopic usage, ‘light’ is synonymous with ‘radiation’ when the specific wavelength is not relevant.

Source: [6].

84 line width

linewidth

Extent of a spectral line, usually measured as the full width at half maximum.

1. Line width may be expressed in terms of wavelength, wavenumber, or frequency.

See also: natural line width.

85 linear decadic absorption coefficient, a, K

decadic absorption coefficient

Decadic absorbance (A₁₀) divided by path-length (l). a = A₁₀/l.

See also: linear Napierian absorption coefficient, molar decadic absorption coefficient, molar Napierian absorption coefficient.

Source: [3] p 36.

86 linear Napierian absorption coefficient, α

Napierian absorption coefficient

Napierian absorbance (A_e) divided by path-length (l). α = A_e/l.

1. α is the reciprocal of the optical absorption depth

See also: linear decadic absorption coefficient, molar decadic absorption coefficient, molar Napierian absorption coefficient.

Source: [3] p 36.

87 liquid crystal tunable filter (LCTF)

Optical device based on polarization interference caused by transmission through a series of birefringent liquid crystal layers of different thicknesses that allow a narrow wavelength region to be selected and tuned over a broad spectral range.

Source: [6].

88 lock-in amplifier

Amplifier that amplifies only those signals that have, within a specified spectral bandwidth, the same frequency as, and a specified phase relation with, the selected reference signal.

Source: [6].

89 Lorentzian band

Band with the shape HL W2/[W2+4(ν˜−ν˜0)2], where H_L is the peak height, W is the full width at half maximum, and ν˜0 is the peak wavenumber.

Source: [6].

90 low-pass filter

1. Optical filter that excludes radiation of wavenumber greater (wavelength lower) than a certain value and passes radiation of wavenumber below (wavelength greater than) this value.

2. Electrical filter that blocks signal with a modulation frequency above a certain cut-on value and passes signals of frequency below this value.

Source: [6]. See also: high-pass filter, band-pass filter.

91 magic angle

54.74°, the angle at which the Legendre polynomial P₂(cos θ) = ½(3cos²θ − 1) equals zero.

1. The name arises from solid state NMR, in which rapidly spinning a sample about an axis at 54.74° to the magnetic field ‘magically’ converts into sharp lines the very broad bands that are due to coupling of the magnetic dipoles in the solid.

2. This angle is important in the estimation of the molecular orientation in polymer fibres by vibrational spectroscopy. In particular, if for a particular vibrational band, the dichroic ratio measured parallel and perpendicular to the fibre axis is unity for many degrees of orientation of the fibre, then the oscillating dipole must lie very close to 54.74° to the axis of the polymer chain.

Source: [6].

92 molar decadic absorption coefficient, ε

molar absorption coefficient

deprecated: extinction coefficient

deprecated: molar absorptivity

Decadic absorbance (A₁₀) divided by path-length (l) and amount concentration (c). ε = A₁₀/(cl).

1. ‘Extinction coefficient’ or ‘molar absorptivity’ have been widely used for the molar absorption coefficient, unfortunately often with values given in ill-defined units. Use of these terms has been discouraged since the 1960s, when international agreement with non-chemical societies reserved the word where the absorption is linearly proportional to the amount of absorber in unit area ‘extinction’ for diffusion of radiation, i.e., the sum of the effects of absorption, scattering and luminescence.

See also: linear decadic absorption coefficient, linear Napierian absorption coefficient, molar Napierian absorption coefficient.

Source: [3] p 36.

93 molar Napierian absorption coefficient, κ

Napierian absorbance (A_e) divided by path-length (l) and amount concentration (c). κ = A_e/(cl).

See also: linear decadic absorption coefficient, linear Napierian absorption coefficient, molar decadic absorption coefficient.

Source: [3] p 36.

94 multiple attenuated total reflection (MATR)

multiple internal reflection (MIR)

frustrated multiple internal reflection (FMIR)

Process in which attenuated total reflection occurs sequentially several times with the same sample.

Source: [6].

95 Napierian absorbance, A_e, B

Absorbance calculated with logarithm base e (natural or Napierian logarithm).

1. Confusion with the decadic absorbance must be avoided.

Source: [3] p 36.

96 net absorption cross-section, σ_net

absorption cross-section, σ

Molar Napierian absorption coefficient divided by the Avogadro constant. σ_net = κ/N_A.

Source: [3] p 37.

97 noise

Random fluctuations occurring in a signal that are inherent in the combination of instrument and method.

1. Noise can be defined as peak-to-peak noise, which is the difference between the largest and smallest values of these fluctuations in a given region, or as root-mean-square noise, which is the standard deviation of the noise in that region. For digital data with approximately 100 independent data points, the peak-to-peak noise is about five times greater than the root-mean-square.

Source: [13] p 1663.

Different kinds of noise are defined in Table 97.1 (Source [6]).

98 nominal spectral resolution

nominal resolution

resolution

Spectral resolution setting on an instrument that a user sets to record a spectrum.

1. The actual resolution achieved is related to this, to the width of the bands studied and to the alignment of the instrument.

Source: [6].

99 nonisotropic

See: anisotropic.

100 Nyquist criterion

Result of information theory that a sine wave can be completely reconstructed if its ordinate is known at two or more points in a cycle.

Source: [6].

101 Nyquist frequency, f _Nyquist

Highest frequency or wavenumber that can be characterized by sampling at a given rate in order to be able to fully reconstruct the signal without artefacts.

1. The Nyquist sampling theorem states that an FID or interferogram must be sampled at a rate at least twice the highest frequency in the FID or interferogram in order to reproduce the correct frequencies in an NMR or Fourier-transform infrared spectrum, respectively, without folding (folding in spectroscopy) artefacts. f_Nyquist = ½ ν.

Source: [14].

102 optical throughput of a spectrometer, G

optical throughput

étendue

Volume in phase space, a function of the area of the emitting source (S) and the solid angle into which it propagates (Ω). d2G=dS dΩ.

103 path-length, l, s

path length

Length of the optical path through the sample.

1. In a transmission cell the path-length is usually taken as the distance between the windows.

2. SI unit: m. Commonly used units: μm, mm.

Source: [6]. See also: optical path difference.

104 phase relaxation

See: dephasing.

105 radiance, L

Radiant power emitted or passing through a small transparent element of surface in a given direction from the source about the solid angle Ω, divided by the solid angle and by the orthogonally projected area of the element in a plane normal to the given beam direction θ, dS_⊥ = dS cos θ, L = d²P/dΩdS_⊥.

1. SI unit: W sr⁻¹ m⁻².

Source: [8], [3] p 34.

106 radiant energy density, ρ, w

Radiant energy (Q) per unit volume (V). ρ = dQ/dV.

1. SI unit: J m⁻³.

Source: [6], [3] p 34.

107 radiant excitance, M

emitted intensity

deprecated: emitted radiant flux

Radiant power emitted per unit area of the source. M = dP/dA_source

1. SI unit: W m⁻².

Source: [3] p 34.

108 radiant intensity, I_e

Radiant power per solid angle in the direction of the point from which the source is being viewed. I_e = dP/dΩ.

1. Radiant intensity should not be confused with intensity of radiation (irradiance).

2. SI unit: W sr⁻¹.

Source: [3] p 34, 35.

109 radiant power, P, Φ

radiant flux

Radiant energy (Q) in unit time (t). P = dQ/dt.

1. IUPAC notes the previous use of ‘flux’ for ‘flux density’ in the third edition of the Green Book ([3] p 81), but now recommends that ‘flux’ is simply a rate of flow (of energy, mass, heat, radiation).

2. SI unit: W.

Source: [3] p 34.

110 radiofrequency (RF)

Frequency or band of frequencies in the range 10⁴ to 10¹² Hz.

111 received radiant flux density, I, E, ϕ₀

incident flux density

irradiance

Radiant power incident on unit area.

1. SI unit: W m⁻².

2. In atomic absorption spectroscopy incident flux density at a given wavelength is given the symbol ϕ₀(λ).

Source: [6]. See also: intensity of radiation.

112 reflectance, ρ, R

Spectral intensity reflected by the sample divided by spectral intensity incident on the sample.

1. For non-scattering and non-luminescent samples the sum of absorptance (α), transmittance (τ), and reflectance (ρ) equals one.

Source: [6].

113 reflection

Process by which electromagnetic radiation is reflected by a sample. For non-scattering samples the term means Fresnel reflection; for scattering samples the term includes regular reflection and volume reflection or diffuse reflection.

Source: [6].

114 refractive index

See: complex refractive index.

115 regular reflection

See: specular reflection.

116 relative permittivity, ε_r

3 × 3 tensor that gives, to first order, the electric displacement, D, in a material in terms of the applied electric field, E, through D = ε_rε_οE, where ε_ο is the permittivity of vacuum.

1. For isotropic materials the tensor is diagonal with the 3 diagonal elements equal, so a single quantity ε_r suffices.

2. The equation D= ε_rε_οE is valid at all frequencies, and ε_r is a complex quantity, ϵˆr=ϵr′+iϵr″, where ϵr″ describes absorption. ϵr′ is termed ‘real dielectric constant’, and ϵr″ is termed ‘imaginary dielectric constant’.

3. Relative permittivity is the square of the refractive index, thus ϵˆr=nˆ2 or, for a non-absorbing isotropic material, ε = n².

Source: [3] p 16, [6].

117 remission

See: diffuse reflection.

118 remission fraction

See: diffuse reflectance.

119 remittance

See: diffuse reflectance.

120 resolving power, R

Wavelength (or wavenumber or frequency) divided by the spectral resolution at that wavelength (or wavenumber or frequency); R = λ/δλ (=ν˜/δν˜ = ν/δν).

1. Resolving power characterizes the performance of a spectrometer, or the degree to which a spectral line (or laser beam) is monochromatic.

Source: [3] p 36.

121 scattering coefficient, s, μ_s

Negative Napierian logarithm of the ratio of the intensity of radiation scattered (I_s) by a non-absorbing sample to the incident intensity (I₀), divided by the path-length, l.

1. SI unit: m⁻¹.

Source: [6].

122 signal-to-noise ratio (SNR, S/N, SNR _dB ), R_S/N

Power of signal divided by power of noise.

1. When signal and noise are measured across the same impedance, SNR is often calculated as the root-mean-squared amplitude of the signal divided by the root-mean-squared amplitude of the noise.

2. The value of signal-to-noise ratio may be expressed in decibel (SNR_dB) as ten times the logarithm to base 10 of signal-to-noise ratio.

3. Initialisms should not be used to denote a quantity in expressions and formulae. R_S/N is recommended.

Source: [6].

123 single beam spectrum

intensity spectrum

Output of a single beam spectrometer as a function of wavenumber or wavelength.

1. A single-beam spectrum is often termed an ‘intensity spectrum’ (the measured radiant power is the intensity times the area of the detector) and does not have a flat baseline.

Source: [6].

124 single-beam spectrometer

Spectrometer that measures radiant power that reaches the detector at each wavelength without reference to another spectrum in real time.

1. Most Fourier-transform spectrometers are single-beam spectrometers.

Source: [6].

125 slit width of a dispersive spectrometer

slit width

Distance between the blades of the entrance or exit slits in a monochromator.

1. Spectral slit width is the range of wavenumbers/wavelengths or frequencies that the physical slit widths allow to be passed by the monochromator and is often determined as the full width at half maximum measured by the instrument for a line that is known to be extremely sharp.

Source: [6].

126 spectral band

Region of the absorption/transmission spectrum in which the absorbance/transmission passes through a maximum/minimum.

1. The use of the term “spectral band” implies a wider region of the spectrum than “spectral line” (see spectral line of an atom).

127 spectral bandwidth

bandwidth

Upper frequency minus lower frequency in a continuous band of frequencies.

1. SI unit: hertz, Hz.

Source: [6].

128 spectral intensity, I_ṽ(ṽ), E_ṽ(ṽ)

spectral irradiance

Intensity of radiation per unit wavenumber at wavenumber ṽ. I_ṽ(ṽ) = dI/dṽ.

1. Spectral intensity and spectral irradiance are formally the same quantity, although the term spectral intensity is usually used for collimated beams of radiation.

2. SI unit: W m⁻¹. Common unit: W m⁻²/cm⁻¹.

Source: [6], [3] p 34.

129 spectral radiance, L_ṽ(ṽ), L_λ(λ)

spectral brightness

Radiance in unit wavenumber (wavelength) interval at wavenumber ṽ (wavelength λ).

1. SI unit: W m⁻³ sr⁻¹. Common unit: W m⁻² sr⁻¹ nm⁻¹.

Source: [6], [8] p 2276.

130 spectral radiant energy density, ρ_ṽ(ṽ), ρ_λ(λ)

Radiant energy density in unit wavenumber (wavelength) interval at wavenumber ṽ (wavelength λ).

1. SI unit: J m⁻².

Source: [6].

131 spectral range of a spectrometer

Wavelength range for which a spectrometer can be used.

1. Spectral range depends essentially on the radiation source, the optical components of the wavelength selector, and the detector.

Source: [10].

132 spectral resolution of an instrument, δ ν ˜ , δν, δλ

spectral resolution

resolution

Smallest difference between wavenumbers (or wavelengths or frequencies) at which different spectral properties may be distinguished, measured as the full width at half maximum.

1. An equivalent definition is the minimum separation of two infinitely sharp lines of equal intensity of radiation that allows the presence of two lines to be seen in the measured spectrum.

2. Resolution depends strongly on the instrument line shape “function” of the spectrometer and on the natural line width.

Source: [3] p 36. See also: nominal spectral resolution.

133 spectral slit width

See: slit width of a dispersive spectrometer.

134 spectral subtraction

Ordinate of one spectrum minus ordinate of another spectrum.

1. Spectral subtraction may be used to improve the ability to detect of minor components in the spectrum of a mixture.

2. Successful use of the technique requires spectra, such as absorbance and Raman spectra, in which the ordinate is linear in concentration; a precisely agreeing wavenumber scale; and a very high signal-to-noise ratio.

Source: [6].

135 spectrometer

Measuring instrument [VIM 3.1] that separates electromagnetic radiation into its component frequencies and transforms the radiant power at each frequency into an electrical signal.

Source: [6].

136 spectrum (vibrational)

Graph of a quantity derived from the radiant power at each wavelength (frequency or wavenumber) as ordinate plotted against the wavelength (frequency or wavenumber) of electromagnetic radiation as abscissa.

1. In mid-infrared and Raman spectroscopy, wavenumber is mostly used today, while it is more commonly replaced by wavelength in near infrared spectroscopy.

2. Quantities plotted include absorbance, absorption coefficient, transmittance, reflectance, luminescence.

Source: [6].

137 specular reflection

regular reflection

Reflection of electromagnetic radiation from a surface such that each incident ray is reflected at the same angle to the surface normal as the incident ray, but on the opposing side of the surface normal in the plane formed by incident and reflected rays.

1. An image reflected by the surface in this way is reproduced in mirror-like (specular) fashion.

138 stray light

Electromagnetic radiation that does not follow the usual path through a spectrometer and consequently appears in a spectrum at a frequency or wavenumber different from its initial value. The term is also used to refer to radiation that passes around the sample instead of through it, and consequently is not modified by the sample and seriously affects absolute and relative intensities.

Source: [6].

139 superradiance emission

See: laser.

140 transition

Change of energy level of a system.

1. In spectroscopy, a transition is caused by interaction of an atom, ion, or molecule with electromagnetic radiation.

Source: [6]. See also: electronic transition and electromagnetic transition.

141 transition-dipole transition dipole coupling

See: dipole coupling.

142 transmission

Process in which electromagnetic radiation passes through a material, entering it on one side and leaving it on another.

1. For non-scattering, non-luminescent samples, the transmitted beam obeys Snell’s law of refraction, and the term internal transmission (see above) is used when surface reflection effects are not included in transmission within the sample.

2. For scattering, non-luminescent materials, transmission consists of two processes, direct transmission and diffuse transmission, and is sometimes called total transmission.

Source: [6].

143 transmission factor

See: transmittance.

144 transmission fraction

See: transmittance.

145 transmission spectrum

transmittance spectrum

General term for the spectrum of transmitted electromagnetic radiation.

1. The ordinate quantity is usually the percentage of the incident radiation that is transmitted, usually called the percent transmission, with range 0 to 100 %. If the ordinate quantity is the fraction transmitted, i.e., the transmittance, the ordinate range is 0 to 1 and the spectrum may be called a transmittance spectrum.

Source: [6].

146 transmittance, T, τ

transmission factor

transmission fraction

Transmitted radiant power at wavenumber ν˜ divided by incident radiant power.

1. For non-scattering, non-luminescent samples, the sum of absorptance (α), transmittance (T), and reflectance (ρ) equals one.

2. For scattering, non-luminescent samples, the term ‘transmission fraction’ is usually used instead of transmittance.

Source: [3] p 36. See also: percent transmission.

147 two-dimensional (2-D) correlation spectroscopy

Measurement principle [VIM 2.4] of spectroscopy in which spectra are recorded at different magnitudes of an applied external perturbation of the sample and are processed to yield a 2-D correlation spectrum.

1. The external perturbation may be time dependent (chemical reactions, physical relaxation processes), or static (temperature change, concentration change).

2. Correlation spectroscopy is used in infrared spectroscopy, nuclear magnetic resonance spectroscopy (COSY, TOCSY), fluorescence spectroscopy and others.

Source: [6]. See also: [15], [16].

148 two-dimensional (2-D) correlation spectrum

Three-dimensional surface in which two of the dimensions show frequency axes (ν₁, ν₂) and the third axis shows a correlation function of the spectral intensities observed at ν₁ and ν₂.

1. The shape of the surface shows whether the bands at the two wavenumbers are or are not correlated, and hence allows deductions to be made about the extent to which the different parts of the molecule are linked in their response to the applied external perturbation.

Source: [6]. See: dynamic spectrum.

149 undersampling

Sampling less frequently than is required by the Nyquist criterion.

1. Undersampling occurs when an analogue signal is digitized with less than two data points per wavelength of the shortest wavelength (highest wavenumber) radiation that reaches the detector.

Source: [6]. See: folding in vibrational spectroscopy.

150 vacuum wavelength

See: wavelength in vacuum.

151 Voigt function

Convolution of Gaussian and Lorentzian (Cauchy) functions.

Source: [6].

152 wavelength in medium, λ

wavelength

Distance travelled by a wave of electromagnetic radiation in one cycle in a non-absorbing medium of real refractive index n. λ = λ₀/n.

1. In spectroscopy, the unqualified term ‘wavelength’ frequently means the vacuum wavelength, and symbol λ frequently means λ₀.

2. SI unit: m. Common unit: nm or μm.

Source: [6].

153 wavelength in vacuum, λ₀

vacuum wavelength

Distance travelled by a wave of electromagnetic radiation in one cycle in vacuum.

1. SI unit: m. Common unit: nm or μm.

Source: [6].

154 wavenumber in medium, σ

wavenumber

Reciprocal of wavelength of electromagnetic radiation. σ = 1/λ.

1. In spectroscopy, the unqualified term ‘wavenumber’ (sometimes referred to as “reciprocal centimetre” cm⁻¹) often means vacuum wavenumber, and symbol σ often means ṽ.

2. SI unit: m⁻¹. Common unit: cm⁻¹.

Source: [6].

155 wavenumber in vacuum, ṽ

vacuum wavenumber

Reciprocal of the vacuum wavelength of electromagnetic radiation. ṽ = 1/λ₀.

1. Wavenumber is related to the energy change, ΔE, induced when the radiation is absorbed by ΔE = hc₀ṽ, where h is the Planck constant and c₀ the speed of the radiation.

2. Wavenumber should only be used when no confusion with frequency is possible.

3. The symbol ν is often used instead of ṽ; this usage should be discouraged.

4. SI unit: m⁻¹. Common unit: cm⁻¹.

Source: [6].

156 window function

See: apodization function.

157 working range of a spectrometer

working range

Range of absorbance or intensity which a spectrometer can measure with accuracy [VIM 2.13] and precision[VIM 2.15].

1. Working range varies in different parts of the spectral range.

Source: [10].

158 zero filling

Addition of zeros to a free induction decay or interferogram to decrease the frequency or wavenumber spacing between points in the Fourier-transformed spectrum.

Source: [6].

159 zero path difference

In a Michelson interferometer or other two-beam interferometer, the position of the moving mirror at which the optical paths are equal.

Source: [6].

160 ¹³C-HMQC-NOESY

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with 1 carbon and 2 hydrogen dimensions.

1. In the initial HMQC step, magnetization is transferred from ¹H to ¹³C and back again. This is then followed by a NOESY step in which the magnetization is transferred to any other hydrogen nucleus close by before detection.

2. The technique requires ¹³C labelling.

3. The spectrum is used to obtain restraints for structure calculations.

Source: [17]. See also: [18].

161 ¹³C-NOESY-HSQC

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with one carbon and two hydrogen dimensions.

1. Magnetization is exchanged between all hydrogens using the nuclear Overhauser effect before being transferred to neighbouring ¹³C nuclei and then back to ¹H for detection.

2. The spectrum is used to obtain restraints for structure calculations.

Source: [19]. See also: [6], [18].

162 ¹⁵N-NOESY-HSQC

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with 1 nitrogen and 2 hydrogen dimensions.

1. Magnetization is exchanged between all hydrogens using the nuclear Overhauser effect before being transferred to neighbouring ¹⁵N nuclei and then back to ¹H for detection. This spectrum can be used to obtain restraints for structure calculations.

Source: [19]. See also: [18].

163 ¹⁵N-TOCSY-HSQC

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with 1 nitrogen and 2 hydrogen dimensions.

Note An isotropic mixing step transfers magnetization between ¹H spins. Then the magnetization is transferred to neighbouring ¹⁵N nuclei and back to ¹H for detection. This can help identify amino acid types.

Source: [19]. See also: [18], total correlation spectroscopy.

164 180° pulse

Pulsed radiofrequency signal in NMR that rotates the bulk magnetization vector (M₀) of a nuclear spin to its opposing direction, effectively inverting the signal, (e.g., from the +z to −z axis).

Source: [20].

165 90° pulse

Pulsed radiofrequency signal that rotates the bulk magnetization vector (M) of a nuclear spin to its orthogonal plane.

1. In a simple 1D NMR experiment, this is from the z axis to the xy plane.

Source: [20].

166 acquisition time (AT), t_a

Duration of the digitization of a free induction decay.

1. t_a = ½ N/f_SW with number of data points N and NMR spectral width f_SW

Source: [14]. See also: dwell time.

167 adiabatic pulse

Radiofrequency pulse that inverts spins by using a frequency sweep during the pulse.

1. Adiabatic pulse allows very high spectral bandwidth and accurate flip angles to be achieved with high tolerance to spatial variations in RF field intensity and is thus of value in excitation of nuclei with frequency ranges.

2. the sweep must be slow enough to satisfy the adiabatic condition dθ/dt≪γBeff where B_eff is the effective radiofrequency magnetic flux density and θ is the angle between B_eff and abscissa.

Source: [21].

168 Bloch equations

Set of coupled differential equations which can be used to describe the behaviour of a magnetization vector under any conditions.

1. When properly integrated, Bloch equations yield X’, Y’, and Z’ components of magnetization within the rotating frame of reference as a function of time

Source: [22].

169 Bloch Siegert shift

Frequency difference seen for a nuclear magnetic resonance signal when a radiofrequency field is applied during the acquisition time.

1. The shift arises from the effective magnetic field generated by the applied RF field. The resonances are always moved away from the frequency of the irradiating field and are inversely proportional to the difference in frequency between the irradiation and the resonance.

Source: [21].

170 Boltzmann distribution of nuclear spins

Distribution of nuclear spins among their possible energy levels at thermal equilibrium.

1. For a positive gyromagnetic ratio, produces population excess (polarization) in the direction of B₀. The population difference between the spin state can be expressed as

where T is the thermodynamic temperature, k is the Boltzmann constant, N_i is the number of spins in excited state i, N is the total number of spins and ε_i and ε_j are the energies of states i and j respectively.

171 CBCA(CO)NH/HN(CO)CACB

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with nitrogen, carbon and hydrogen dimensions.

1. Magnetization is transferred from ¹Hα and ¹Hβ to ¹³Cα and ¹³Cβ, respectively, and then from ¹³Cβ to ¹³Cα. From here it is transferred first to ¹³CO, then to ¹⁵NH and then to ¹HN for detection. Along with CBCANNH and HSQC this forms the standard set of experiments needed for backbone assignment.

Source: [23]. See also: [18].

172 CBCANH/HNCACB

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with nitrogen, carbon and hydrogen dimensions.

1. Magnetization is transferred from ¹Hα and ¹Hβ to ¹³Cα and ¹³Cβ, respectively, and then from ¹³Cβ to ¹³Cα. From here it is transferred first to ¹⁵NH and then to ¹HN for detection. For each NH group there are two Cα and Cβ peaks visible. Along with the CBCA(CO)NNH and HSQC this forms the standard set of experiments needed for backbone assignment.

Source: [17]. See also: [18].

173 CC(CO)NH

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with nitrogen, carbon and hydrogen dimensions.

1. Magnetization is transferred from the side-chain hydrogen nuclei to their attached ¹³C nuclei. Then isotropic ¹³C mixing is used to transfer magnetization between the carbon nuclei. From here, magnetization is transferred to the carbonyl carbon, on to the amide nitrogen and finally the amide hydrogen for detection. The chemical shift is evolved simultaneously on all side-chain carbon nuclei, as well as on the amide nitrogen and hydrogen nuclei, resulting in a three-dimensional spectrum.

2. This method is a useful spectrum for obtaining carbon side-chain assignment.

Source: [24]. See also: [18].

174 chemical shift anisotropy in NMR (CSA)

Chemical shift difference between isotropic and anisotropic states in nuclear magnetic resonance spectroscopy.

1. Nuclei which are part of a specific functional group resonate at different frequencies depending on shielding by the local electronic environment and thus can give information on their relative orientation.

2. Chemical shift anisotropy also contributes to nuclear spin relaxation.

Source: [25].

175 chemical shift in NMR, δ

The fractional variation of the resonance frequency of a nucleus in nuclear magnetic resonance spectroscopy relative to the resonance frequency of the nucleus in a given reference in consequence of its magnetic environment.

1. The chemical shift for nucleus X in sample s, δX,s is δX,s=(νX,s−νX,r)νX,r where νX,s is the resonance frequency of X in the sample and νX,r is the resonance frequency in the reference.

2. Chemical shift is usually reported in ‘parts per million’ or ppm, where the difference of frequencies in the numerator have unit Hz, and the frequency of the reference in the denominator has unit MHz.

3. The alternative use of Hz/MHz is recommended as best practice in the Green Book to remove ambiguity regarding the symbol ppm.

Source: [4] p 1807.

176 combined rotation and multiple pulse spectroscopy (CRAMPS)

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy for obtaining high-resolution solid-state nuclear magnetic resonance spectra on nuclei with high gyromagnetic ratio such as ¹H and ¹⁹F.

1. CRAMPS is based on coherent-averaging theory, which attempts to describe the time evolution of a system in terms of an average Hamiltonian derived from the density matrix formalism of quantum mechanics.

Source: [26].

177 constant time evolution

Segment of a pulse sequence in nuclear magnetic resonance spectroscopy that allows evolution of chemical shift during a fixed time period rather than through the traditional symmetrical incremented delays.

Source: [21].

178 correlation spectroscopy in NMR (COSY)

Homonuclear two-dimensional nuclear magnetic resonance spectroscopy correlation experiment showing cross peaks representing spin-spin coupling between pairs of nuclei.

1. COSY is the simplest and original 2D experiment, but still one of the most common along with its many variations including those with multiple quantum filters, solvent suppression and optimization for long range couplings.

2. COSY is often used to show the spin-spin coupling between hydrogen nuclei 2 or 3 bonds apart (H,H-COSY).

Source: [27]. See also: correlation spectroscopy.

179 correlation spectroscopy through long-range coupling (COLOC)

Heteronuclear (typically ¹³C) detected long range 2D correlation experiment in nuclear magnetic resonance spectroscopy that has mostly been superseded by heteronuclear multiple bond correlation (HMBC) experiments and its variants.

1. COLOC is often used to show spin-spin coupling between hydrogen and carbon nuclei 2 and 3 bonds apart.

Source: [27]. See also: [18].

180 correlation time, τ, τ_c

Parameter related to the mean time during which a molecule maintains its spatial geometry.

1. For an internuclear vector, correlation time is approximately equal to the average time for it to rotate through an angle of one radian.

Source: [22].

181 coupling constant

See: spin-spin coupling constant.

182 cross polarization with magic angle spinning NMR (CP/MAS)

Combined use of cross polarization with magic-angle spinning for solid-state nuclear magnetic resonance spectroscopy.

Source: [27].

183 cross polarization

Technique in solid-state nuclear magnetic resonance spectroscopy that enables polarization from abundant spins such as ¹H or ¹⁹F to be transferred to dilute spins such as ¹³C or ¹⁵N to enhance signal-to-noise ratio.

1. Cross polarization requires that nuclei are dipolar coupled to one another.

184 cross-relaxation appropriate for minimolecules emulated by locked spins (CAMELSPIN)

See: rotating-frame NOE spectroscopy.

185 decoupling

See: heteronuclear decoupling, homonuclear decoupling.

186 decoupling in the presence of scalar interactions (DIPSI)

Pulse sequence in nuclear magnetic resonance spectroscopy often used for isotropic mixing in total correlation spectroscopy (TOCSY) experiments due to its efficiency over large spectral bandwidths.

Source: [21].

187 diffusion ordered spectroscopy (DOSY)

Pseudo-two-dimensional nuclear magnetic resonance spectroscopy experiment that resolves NMR signals of a one-dimensional nuclear magnetic resonance spectrum in a second dimension through the diffusion times of their corresponding molecules.

1. DOSY is used to differentiate signals from different molecules based on their effective sizes.

2. DOSY relies on spatially encoding the analyte by use of pulsed-field gradients.

Source: [21].

188 dipolar coupling in nuclear magnetic resonance spectroscopy

magnetic dipole–dipole interaction in NMR

Direct though-space interactions of nuclear magnetic moment vectors proportional to the inverse cube of internuclear distance.

1. In isotropic solution, dipolar couplings average to zero as a result of diffusion but are present in solid-state nuclear magnetic resonance spectroscopy and their effect on nuclear spin relaxation are measurable through nuclear Overhauser effects (NOEs).

Source: [22].

189 distortionless enhancement by polarization transfer (DEPT)

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy used for determining the presence of primary, secondary and tertiary heteronuclei.

1. DEPT is a 1D technique which is used typically for ¹³C.

2. The DEPT experiment for ¹³C differentiates between CH, CH₂ and CH₃ groups by variation of the selection angle parameter (the flip angle of the final ¹H pulse): 135° angle gives all CH and CH₃ in a phase opposite to CH₂; 90° angle gives only CH groups, the others being suppressed; 45° angle gives all carbons with attached protons with the same phase.

Source: [27].

190 double pulsed-field gradient spin-echo excitation (DPFGSE)

Pulse sequence in nuclear magnetic resonance spectroscopy in which pulsed-field gradients are applied to a doubled spin echo sequence to cleanly and precisely excite particular resonances.

Source: [28].

191 double resonance

Nuclear magnetic resonance spectroscopy experiment where excitation is applied independently to two distinct frequency ranges.

1. Double resonance is most commonly applied for spin decoupling.

Source: [20].

192 double-labelled protein

Shorthand for a protein which has been uniformly ¹⁵N- and ¹³C-labelled.

193 double-quantum filtered correlation spectroscopy (DQF-COSY)

Variant of the COSY experiment with a double quantum filter to remove, or at least reduce, impact of large singlet peaks on dynamic range and artefacts.

Source: [27].

194 dwell time (DW), τ_d

Time interval between sampled data points.

1. τ_d = 1/fSW, where fSW is the NMR spectral width

Source: [21]. See also: acquisition time.

195 echo time (TE), T_E

In nuclear magnetic resonance spectroscopy, time interval between the middle of the first radiofrequency pulse and the peak of the spin echo.

Source: [21].

196 electronic reference to access in vivo concentrations (ERETIC)

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy that generates an electronic signal in a nuclear magnetic resonance spectrometer which is detected simultaneously to the sample free induction decay during the acquisition.

1. This approach is used to simplify the quantification of signals in the resulting NMR spectra by avoiding the need for the use of internal or external reference materials.

Source: [29].

197 Ernst angle, θ_E

When signal averaging in a single-pulse nuclear magnetic resonance spectroscopy experiment, flip angle giving the best signal-to-noise ratio for a given combination of spin-lattice relaxation time and repetition rate.

1. Named after Richard Ernst. It is employed to allow the maximum signal to noise to be generated in a fixed amount of time. cos(θE)=e−(td+ta)/T1. where t_d is the interpulse delay, t_a is the acquisition time, and T₁ is longitudinal relaxation time.

Source: [21].

198 exchange spectroscopy (EXSY)

Nuclear magnetic resonance spectroscopy experiment, which is the same as a NOESY, but for the purposes of observing nuclei exchanging chemical environments during the course of the experiment through conformational or chemical exchange.

Source: [27].

199 excitation sculpting

Robust method of generating selective excitation through combination of shaped pulses and pulsed-field gradients in nuclear magnetic resonance spectroscopy.

Source: [21].

200 fast field cycling NMR relaxometry (FFC-NMR)

Relaxometry to measure the nuclear spin-lattice relaxation rate constant as a function of the applied magnetic field strength in nuclear magnetic resonance spectroscopy.

1. FFC-NMR is used to study molecular dynamics of molecules across a range of applications including molecular motion in solids and ligand-metal binding.

Source: [30].

201 flip angle, α

tip angle

Rotation that the net magnetization experiences during the application of a radiofrequency pulse in nuclear magnetic resonance spectroscopy.

1. For a strong and rectangular RF-pulse of constant amplitude (B₁) and duration (t_p), the resultant flip angle is approximately proportional to the frequency (f₁) of the B₁ field: α=γB1tp, where γ is the gyromagnetic ratio.

202 gated decoupling

Application of spin decoupling only during selected time periods in a pulse experiment in nuclear magnetic resonance spectroscopy.

1. Gated decoupling is used to eliminate either ¹H – ¹³C spin-spin coupling or the nuclear Overhauser effect from a 1D ¹³C spectrum.

2. If the decoupler is turned off during acquisition this is usually referred to as ‘gated decoupling’ while decoupling turned on during acquisition is usually referred to as ‘inverse gated decoupling’.

Source: [20].

203 globally optimized alternating-phase rectangular pulses (GARP)

Spin decoupling pulse sequence in nuclear magnetic resonance spectroscopy used to provide decoupling over a large frequency range.

1. GARP is typically used to remove ¹³C sidebands in ¹H nuclear magnetic resonance spectra.

Source: [27].

204 gradient pulse

See: pulsed-field gradient.

205 gradient-selected experiment

Nuclear magnetic resonance spectroscopy experiment where pulsed-field gradients are employed to filter out unwanted resonances from a spectrum rather than the classic phase-cycling approach.

Source: [27].

206 gyromagnetic ratio, γ

see magnetogyric ratio

207 H(CCO)NH

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with one nitrogen, and two hydrogen dimensions.

1. Magnetization is transferred from the side-chain hydrogen nuclei to their attached ¹³C nuclei. Then isotropic ¹³C mixing is used to transfer magnetization between the carbon nuclei. From here, magnetization is transferred to the carbonyl carbon, on to the amide nitrogen and finally the amide hydrogen for detection.

2. Requires ¹⁵N and ¹³C labelling

3. Useful spectrum for obtaining hydrogen side-chain assignments.

Source: [31]. See also: [18].

208 HBHA(CO)NH

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with one nitrogen and two hydrogen dimensions.

1. Magnetization is transferred from ¹Hα and ¹Hβ to ¹³Cα and ¹³Cβ, respectively, and then from ¹³Cβ to ¹³Cα. From here it is transferred first to ¹³CO, then to ¹⁵NH and then to ¹HN for detection.

2. Requires ¹⁵N and ¹³C labelling

3. Useful spectrum for obtaining Hα and Hβ assignments.

Source: [32]. See also: [18].

209 HCCH-COSY

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with one carbon and two hydrogen dimensions.

1. Magnetization is transferred from the side-chain hydrogen nuclei to their attached ¹³C nuclei. Magnetization is then exchanged between neighbouring ¹³C nuclei via the J-coupling and finally transferred back to the side-chain hydrogen atoms for detection.

2. The nuclear magnetic resonance spectrum can be useful in aiding side-chain assignment.

Source: [33]. See also: [18].

210 HCCH-TOCSY

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with one carbon and two hydrogen dimensions.

1. Magnetization is transferred from the side-chain hydrogen nuclei to their attached ¹³C nuclei. This is followed by isotropic ¹³C mixing and finally the transfer back to the side-chain hydrogen atoms for detection.

2. The nuclear magnetic resonance spectrum is used for side-chain assignment.

Source: [34]. See also: [18], total correlation spectroscopy.

211 Hahn echo

See: spin echo.

212 heteronuclear decoupling

decoupling

Spin decoupling in nuclear magnetic resonance spectroscopy between two different nuclei (e.g., ¹³C and ¹H) by appropriate excitation of the decoupled nucleus.

1. Heteronuclear decoupling is normally performed to simplify spectra or to enhance signal-to-noise ratio by combining individual components of the NMR signal.

2. Notation: ¹³C{¹H} denotes observation of ¹³C nucleus with simultaneous decoupling of ¹H nuclei.

Source: [20]. See also: homonuclear decoupling.

213 heteronuclear multiple bond correlation NMR (HMBC)

Two-dimensional nuclear magnetic resonance spectroscopy that gives correlations between carbons and protons that are separated by two, three, and, sometimes in conjugated systems, four bonds.

1. Direct one-bond correlations are suppressed.

2. HMBC gives connectivity information much like a proton-proton COSY.

3. The intensity of cross peaks depends on the spin-spin coupling constant, which for three-bond couplings follows the Karplus relationship. For dihedral angles near 90°, the coupling is near zero. Thus, the absence of a cross peak does not confirm that carbon-proton pairs are many bonds apart.

4. Because of the wide range (0 to14 Hz) of possible carbon-proton couplings, two experiments are often performed. One optimized for 5 Hz couplings and the second optimized for 10 Hz. This gives the optimum signal-to-noise ratio. Alternatively, a comprise value of 7 to 8 Hz can be used. There are also “accordion” versions that attempt to sample the full range of couplings.

Source: [35].

214 heteronuclear multiple-bond correlation over two bonds (H2BC)

Variant of the heteronuclear single quantum correlation (HSQC) and heteronuclear multiple bond correlation (HMBC) experiment in nuclear magnetic resonance spectroscopy that almost exclusively correlates ¹H nuclei with heteronuclei separated by two covalent bonds.

Source: [19]. See also: [18].

215 heteronuclear multiple-quantum correlation NMR (HMQC)

Simplest implementation of an inverse detected ¹H heteronuclear correlation nuclear magnetic resonance spectroscopy experiment.

Source: [27].

216 heteronuclear multiple-quantum correlation with additional TOCSY transfer (HMQC-TOCSY)

Variant of heteronuclear multiple bond correlation (HMBC) spectroscopy that shows correlations from a heteronucleus (e.g., ¹³C) to not only its directly bound ¹H but also to all other ¹H nuclei within the spin system.

Source: [27]. See also: total correlation spectroscopy.

217 heteronuclear Overhauser effect spectroscopy (HOESY)

Heteronuclear variant of nuclear Overhauser effect spectroscopy (NOSEY) showing through space interaction between ¹H and a heteronucleus, typically ¹³C.

Source: [27].

218 heteronuclear shift correlation NMR (HETCOR)

Directly-observed one-bond heteronuclear correlation experiment in nuclear magnetic resonance spectroscopy.

1. HETCOR has largely been replaced by the indirectly observed heteronuclear single quantum correlation experiment due to its greater sensitivity. It does however have the advantage of greater resolution in the heteronuclear dimension.

Source: [27].

219 heteronuclear single quantum correlation (HSQC)

heteronuclear single quantum coherence

¹H (inverse) detected two-dimensional nuclear magnetic resonance spectroscopy heteronuclear experiment that correlates ¹H nuclei with their directly coupled nucleus through a single bond.

Source: [27].

220 heteronuclear single quantum correlation with additional TOCSY transfer (HSQC-TOCSY)

Variant of the heteronuclear single quantum correlation (HSQC) experiment that shows correlations from a heteronucleus (e.g., ¹³C) to not only its directly bound ¹H but also to all other ¹H nuclei within the spin system.

Source: [27]. See also: total correlation spectroscopy.

221 heteronuclear single quantum multiple-bond correlation (HSQMBC)

Variant of the heteronuclear single quantum correlation (HSQC) experiment that provides pure absorption, antiphase line shapes for precise, direct measurement of ⁿJ(C,H) spin-spin coupling constants.

Source: [19]. See also: [18].

222 HN(CA)CO

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with nitrogen, carbon and hydrogen dimensions.

1. the magnetization is transferred from ¹H to ¹⁵N and then via the N-Cα J-coupling to the ¹³Cα. From there it is transferred to the ¹³CO via the ¹³Cα-¹³CO J-coupling. For detection the magnetization is transferred back the same way: from ¹³CO to ¹³Cα, ¹⁵N and finally ¹H.

2. each NH group will show correlations to both their own and adjacent carbonyls group.

Source: [36]. See also: [18].

223 HN(CO)CA

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with nitrogen, carbon and hydrogen dimensions.

1. The magnetization is passed from ¹H to ¹⁵N and then to ¹³CO. From here it is transferred to ¹³Cα and the chemical shift is evolved. The magnetization is then transferred back via¹³CO to ¹⁵N and ¹H for detection. This is similar to the HNCA, but is selective for the Cα of the preceding residue.

Source: [37]. See also: [18].

224 HNCA

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with nitrogen, carbon and hydrogen dimensions.

1. The magnetization is passed from ¹H to ¹⁵N and then via the N-Cα J-coupling to the ¹³Cα and then back again to ¹⁵N and ¹H hydrogen for detection. Each NH group will show correlations to both their own and the previous residue’s ¹³Cα.

Source: [38]. See also: [18].

225 HNCO

Three-dimensional nuclear magnetic resonance spectroscopy for the assignment of protein structure with nitrogen, carbon and hydrogen dimensions.

1. Magnetization is passed from ¹H to ¹⁵N and then selectively to the carbonyl ¹³C via the ¹⁵NH–¹³CO J-coupling. Magnetization is then passed back via¹⁵N to ¹H for detection.

Source: [38]. See also: [18].

226 homonuclear decoupling

decoupling

Spin decoupling in nuclear magnetic resonance spectroscopy between two signals of the same nuclear isotope.

1. Homonuclear decoupling is normally performed through selective excitation of specific NMR signals within a spectrum or through pure shift experiments such as PSYCHE.

Source: [20]. See also: heteronuclear decoupling.

227 homonuclear Hartmann-Hahn spectroscopy (HOHAHA)

See: total correlation spectroscopy.

228 imaginary NMR spectrum

imaginary spectrum

In nuclear magnetic resonance spectroscopy, one of two equally sized blocks of frequency data 90° out of phase to each other produced by Fourier transformation of the time domain signal.

1. Usually the imaginary frequency data is not displayed and is used for phase correction of the real spectrum.

2. In simple 1D spectra this equates to the NMR signals being in dispersion mode.

Source: [21].

229 incredible natural-abundance double-quantum transfer experiment (INADEQUATE)

Nuclear magnetic resonance spectroscopy experiment designed to show homonuclear spin-spin coupling correlations between low natural abundance nuclei such as ¹³C.

1. 2D variant shows each coupled pair of nuclei gives a pair of peaks on the INADEQUATE spectrum which both have the same vertical coordinate, which is the sum of the chemical shifts of the nuclei; the horizontal coordinate of each peak is the chemical shift for each of the nuclei separately.

Source: [27].

230 insensitive nuclei enhanced by polarization transfer (INEPT)

Pulse sequence in nuclear magnetic resonance spectroscopy that involves the transfer of nuclear spin polarization from spins with large Boltzmann population differences to nuclear spins of interest with low Boltzmann population differences.

1. INEPT uses spin-spin coupling for the polarization transfer in contrast to nuclear Overhauser effect (NOE) which arises from dipolar cross-relaxation.

2. INEPT is a building block for many heteronuclear NMR experiments.

Source: [27].

231 inverse gated decoupling

See: gated decoupling.

232 inversion pulse

Pulse in nuclear magnetic resonance spectroscopy that completely inverts the magnetization of a spin, e.g., +z to –z.

1. The inversion pulse can be a simple 180°or composite pulse.

233 inversion recovery sequence

inversion recovery

Spin echo sequence preceded by a 180° inversion pulse used to determine spin-lattice relaxation times in nuclear magnetic resonance spectroscopy.

1. The sequence is typically denoted as 180° – τ – 90°.

Source: [20]. See also: inversion time.

234 inversion time (TI), T_I

Time between the inversion pulse and the sampling pulse(s) in an inversion recovery sequence.

1. T_{I ,null} is the value of T_I when longitudinal magnetization (M_z) is close to zero at the end of the TI interval.

235 J-modulated spin-echo (J-MOD)

Nuclear magnetic resonance pulse sequence that refocuses spin vectors that have fanned out due to field inhomogeneity or chemical shift differences.

1. J-MOD is typically denoted as 90° – τ – 180° – τ (echo).

Source: [20]. See also: spin echo.

236 J-resolved spectroscopy (J-RES)

Two-dimensional nuclear magnetic resonance spectroscopy resolving chemical shift information in one dimension and spin-spin coupling in the second.

1. J-RES exists as both homonuclear and heteronuclear variants.

2. The homonuclear variant can generate homonuclear decoupled projection when processed with a tilt function.

Source: [27].

237 Knight shift, K

Frequency shift in the nuclear magnetic resonance spectrum of a paramagnetic species that refers to the relative shift K in NMR frequency for atoms in a metal (e.g., sodium) compared with the same atoms in a nonmetallic environment (e.g., as a salt).

1. The observed shift reflects the local magnetic field produced at the metal nucleus by the magnetization of the conduction electrons.

Source: [39].

238 Larmor angular frequency, ω, ω_L

Larmor frequency

Frequency at which nuclei with a nuclear spin precess (Larmor precession) around the direction of an external magnetic induction B₀. ωL=−γB0, where γ is the gyromagnetic ratio of the nucleus.

Source: [3], [20].

239 Larmor precession

Precession of the magnetic moment (M) of an object at an angular frequency, ω_L, about a static magnetic induction (B₀), named after Joseph Larmor.

1. Larmor precession can be visually compared to the precession of a tilted gyroscope in an external torque-exerting gravitational field.

Source: [20].

240 longitudinal relaxation

See: spin-lattice relaxation.

241 magic angle spinning (MAS)

Technique in solid-state nuclear magnetic resonance spectroscopy to remove or reduce the influence of anisotropic interactions by rapid sample rotation about the magic angle.

1. Much like in the case of solutions, MAS effectively averages orientation-dependent interactions and allow high resolution spectra.

2. The rate of spinning must be greater than or equal to the magnitude of the anisotropic interaction to average it to zero.

where B₀ is external magnetic induction and θ is the axis of rotation of the sample at an angle of approx. 54.74° to the external magnetic induction.

Source: [21].

242 magnetic dipole–dipole interaction in NMR

See: dipolar coupling in nuclear magnetic resonance spectroscopy.

243 magnetic flux density

See: static magnetic flux density.

244 magnetogyric ratio, γ

gyromagnetic ratio

Magnetic moment µ of a nuclide divided by its spin angular momentum J.

μ = γ J = γ ( h / 2 π ) I ( I + 1 ) , where I is the spin quantum number of the nuclide and h is the Planck constant.

1. γ can be either positive (e.g., ¹H, ¹³C) or negative (¹⁵N, ²⁹Si). For particular values of the magnetic induction B₀ and the magnetic quantum number m, γ determines the energy level of the nucleus. E=−mγ(h/2π)B0.

2. SI unit: rad s⁻¹ T⁻¹ ≡ C kg⁻¹.

Source: [40].

245 multiple-quantum magic angle spinning NMR (MQMAS)

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy used to obtain high-resolution NMR spectra of quadrupolar nuclei by removal of the anisotropy of the quadrupole interaction, involving creating a triple-quantum (or 5Q) coherence.

Source: [41].

246 multiplet

Feature in a nuclear magnetic resonance spectrum that is split but is too complex to easily interpret.

1. A multiplet is distinguished from multiplicity of NMR peaks that give well-resolved doublet, triplet, quartet, quintet etc.

247 multiplicity of NMR peaks

multiplicity

Splitting into more than one peak of an NMR signal due to interaction with other magnetic vectors, normally adjacent nuclei.

1. Multiplicity can arise from spin, spin (J) coupling and, in solids and restrained media, dipolar coupling.

2. In its simplest form multiplicity results in patterns associated with the 2nI+1 rule (singlet, doublet, triplet, quartet, pentet etc.) although complex multiplets arise through variation in spin-spin coupling constants and second order multiplets exist where peak overlap occurs.

Source: [14].

248 net dephasing time, T₂*

T2-star

Measured time constant associated with spin-spin relaxation for loss of magnetization in the xy plane.

1. T₂* includes losses due to B₀ inhomogeneity as well as spin-spin relaxation and is always less than or equal to the spin-lattice relaxation time (T₂).

Source: [20], [42] p 2493.

249 non-uniform sampling (NUS)

Technique employed within a high dimensionality nuclear magnetic resonance spectroscopy experiment that acquires only a subset of the data points traditionally acquired linearly thus enabling a reduction in experiment time or an increased resolution in the resultant spectra.

1. NUS uses reconstruction methods to allow the extraction of complete sets of chemical shift information.

250 nuclear electric quadrupole moment, eQ

quadrupole moment of a nucleus

Parameter that describes the effective shape of the ellipsoid of nuclear charge distribution.

1. Non-zero quadrupole moment indicates that the charge distribution is not spherically symmetric.

2. By convention, the value of eQ is taken to be positive if the ellipsoid is prolate and negative if it is oblate.

Source: [43].

251 nuclear magnetic resonance relaxometry (NMRR)

relaxometry

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy to measure nuclear relaxation variables.

1. NMRR generally involves analysis of time domain NMR signals to generate simplified NMR data rather than conventional full frequency resolved NMR spectra.

252 nuclear magnetic resonance spectrum

NMR spectrum

Representation of nuclear magnetic resonance spectroscopy data with at least one dimension a frequency domain.

1. An NMR spectrum typically involves the Fourier transform of a free induction decay. See Fourier-transform spectroscopy.

253 nuclear Overhauser effect (nOe, NOE), η

nuclear Overhauser enhancement

Change of intensity of one resonance when the spin transitions of another are somehow perturbed from their equilibrium populations.

1. The nuclear Overhauser effect at nucleus i for perturbation of spin S is expressed as a relative intensity change between the equilibrium intensity (I) and that in the presence of the nOe (I_o). ηi(S)=(I−Io)/Io. The effect may be given as a percentage η ×100 %.

2. nOe allows a measure of through space rather than through bond interactions between nuclei and is proportional to the inverse 6^th power of the internuclear distance.

3. η can be positive or negative depending on the motional properties of the molecule and the signs of the gyromagnetic ratios.

Source: [21], [42].

254 nuclear Overhauser effect difference spectroscopy

nOe difference spectroscopy

One dimensional nuclear magnetic resonance spectroscopy that yields, through subtraction of spectra generated with and without selective excitation of a specific resonance, the nuclear Overhauser enhancement of nuclei spatially close to a selectively excited nucleus.

Source: [27].

255 nuclear Overhauser effect spectroscopy (NOESY)

nuclear Overhauser enhancement spectroscopy

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy based on the nuclear Overhauser effect to observe nuclei that are close to each other in space.

1. NOESY is typically run as a homonuclear ¹H experiment.

2. A NOESY spectrum yields through space correlations via dipolar cross-relaxation.

3. For small molecules, NOE may be observed between protons that are up to 0.4 nm apart, while the upper limit for large molecules is about 0.5 nm.

4. NOESY also detects chemical and conformational exchange, when it is termed exchange spectroscopy (EXSY).

Source: [27].

256 nuclear quadrupole resonance spectroscopy (NQR)

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy where splitting of the nuclear spin states is determined by the electrostatic interaction of the nuclear charge density with the external electric potential of the surrounding electron cloud.

1. Unlike NMR, NQR transitions of nuclei can be detected in the absence of a magnetic field.

Source: [43].

257 nuclear spin

See: spin of a nucleus.

258 observation frequency

Frequency corresponding to the centre of the spectrum window for a given nucleus under observation and the nominal frequency used to generate the hard, full spectrum pulses.

Source: [14].

259 Pake doublet

Characteristic line shape seen in solid-state nuclear magnetic resonance spectroscopy arising from dipolar coupling between two nuclei.

260 phase correction

Linear combination of the real and imaginary parts of a 1-D nuclear magnetic resonance spectrum to produce peaks with pure absorption mode line shape.

1. The phase correction is normally described by the automatic or manual setting of a fixed zero order and a frequency dependent first order phase constant.

Source: [21].

261 phase cycling

Component of nuclear magnetic resonance spectroscopy experiment that repeats a pulse sequence changing only the phases of the pulse(s) and the phase-sensitive detector reference.

1. the resultant FID’s are then added to suppress undesirable signal components and/or to produce the desired effect of a pulse sequence (e.g., multiple quantum filter)

Source: [20].

262 powder pattern

Very broad and distinctive line shape seen in a static solid-state nuclear magnetic resonance spectroscopy experiment with a typical magnitude of 10² to 10⁵ Hz.

1. A powder pattern is a summation of the multitude of signals arising from crystals with different orientations within the magnetic induction.

263 precession

Classical description of the behaviour of nuclear magnetic moments, in which the vectors rotate about the B₀ axis at their Larmor angular frequencies.

Source: [21].

264 presaturation

Technique to suppress a particular nuclear magnetic resonance spectroscopy signal where a long low-power pulse is applied to that specific resonance prior to the main pulse sequence.

1. Presaturation is typically used as a method for solvent suppression.

Source: [27].

265 pulse sequence

Timed series of radiofrequency pulses and magnetic field gradients that define a part or whole of a nuclear magnetic resonance spectroscopy experiment.

266 pulsed-field gradient (PFG)

gradient pulse

Short, timed pulse in nuclear magnetic resonance spectroscopy that momentarily destroys the magnetic field homogeneity within the sample through a spatial-dependent field intensity.

1. The net result of the pulse is that spins are dispersed in the transverse plane (defocussed) and produce zero net magnetization.

2. A PFG is characterized by its power, shape, duration and axis.

3. PFG techniques are used in magnetic resonance imaging, spatially-selective NMR, and diffusion ordered NMR spectroscopy (DOSY).

Source: [21].

267 pure shift yielded by chirp excitation (PSYCHE)

Nuclear magnetic resonance spectroscopy experiment generating homonuclear spin-spin decoupled spectra.

1. The effect is achieved through the concatenation of partial free induction decays generated with a series of swept adiabatic pulses.

Example: ¹H spectrum with no ¹H,¹H coupling.

Source: [44].

268 quadrature detection in nuclear magnetic resonance spectroscopy

Collection of time domain nuclear magnetic resonance spectroscopy data on both the x and y axes of the rotating frame of reference to allow discrimination of positive and negative frequencies.

Source: [21].

269 quadrupole moment of a nucleus

See: nuclear electric quadrupole moment.

270 quadrupolar nuclide

Nuclide with spin quantum number I > ½ and a non-spherical distribution of charge in the nucleus giving rise to a nuclear electric quadrupole moment (eQ).

271 quadrupolar relaxation

Relaxation mechanism in nuclear magnetic resonance spectroscopy arising from intramolecular quadrupolar interactions with electric field gradients.

Source: [20]. See also: quadrupolar nuclide.

272 radiation damping

Line-broadening effect on intense signals in high field nuclear magnetic resonance spectroscopy.

1. Radiation damping occurs when the rotating transverse magnetization of the sample is intense enough to induce large enough electromotive force in the radiofrequency coil strong enough that it feeds back to the sample. This causes a rotation of magnetization back to the +z axis and results in line-broadening of the intense peak.

2. Radiation damping can also produce other effects including changes to amplitude and phase of adjacent peaks.

Source: [27].

273 radiofrequency (RF) coil

In nuclear magnetic resonance spectroscopy, an inductor-capacitor resonant circuit used to set up B₁ magnetic inductions in a sample and to detect the signal from the sample.

274 real spectrum

In nuclear magnetic resonance spectroscopy, one of two equally sized blocks of frequency data 90° out of phase to each other produced by Fourier transformation of the time domain signal.

1. Usually only the real frequency data is displayed and in simple 1D spectra equates to the NMR signals being absorption mode (in-phase).

Source: [21]. See also: imaginary spectrum, Fourier-transform spectroscopy.

275 relaxation reagent

Paramagnetic species added to a sample in a nuclear magnetic resonance spectroscopy experiment to promote more rapid spin-lattice relaxation and allow a faster pulse repetition rate.

Example: Tris(acetylacetonato)chromium(III) (chromium(III) acetyl acetonate, Cr(acac)3 ) is typically used.

Source: [14].

276 relaxometry

See: nuclear magnetic resonance relaxometry.

277 repetition time (TR), T_r

Time duration between repetitions of a nuclear magnetic resonance pulse sequence including the pulse sequence, acquisition time and relaxation delay.

Source: [21].

278 residual dipolar coupling (RDC)

Dipolar coupling, but of a reduced magnitude to that seen in solids, observed in a liquid sample between spins in a molecule where there is incomplete averaging of spatially anisotropic dipolar couplings.

1. RDC is typically achieved through partial alignment of molecules in the liquid state through spatially aligned gels or liquids.

2. RDC is used to provide 3D structural information on molecules.

279 rotating-frame NOE spectroscopy (ROESY)

cross-relaxation appropriate for minimolecules emulated by locked spins (CAMELPSIN)

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy yielding through-space correlations via spin-spin relaxation.

1. ROESY is similar to the NOESY experiment (see nuclear Overhauser effect spectroscopy) and is useful for determining which signals arise from protons that are close to each other in space even if they are not bonded.

2. ROESY has the advantage over the NOESY experiment that its response does not go through a null at specific correlation times and is thus preferred for molecules having molecular masses of a few thousand Da.

Source: [27].

280 rotational echo double resonance (REDOR)

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy allowing recoupling of heteronuclear dipolar coupling under magic angle spinning.

Source: [45].

281 saturation transfer difference spectroscopy (STD)

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy that allows detection of transient binding of small molecule ligands to macromolecular receptors.

1. Range of applicable dissociation constants is approximately 10⁻³ M to 10⁻⁸ M. The experiment relies on ligand-signal attenuation through spin diffusion saturation propagating from the macromolecule.

Source: [46].

282 selective decoupling

Spin decoupling of a specific resonance by selective irradiation.

283 selective excitation

selective irradiation

Application of a frequency selective radiofrequency pulse(s) to a single resonance or a discrete band of frequencies.

1. Selective excitation is usually achieved through the application of long, low power (soft) pulses.

Source: [21].

284 selective pulse

Radiofrequency pulse with a narrow frequency spectral bandwidth (long, low-power pulse) to excite nuclei in a limited chemical shift range (see selective excitation).

Source: [14].

285 single crystal nuclear magnetic resonance spectroscopy

Solid-state nuclear magnetic resonance spectroscopy on a single crystal in a similar manner to that used in X-ray diffraction.

1. A large crystal is mounted on a ‘tenon’, which is in turn mounted on a goniometer head. If the orientation of the unit cell is known with respect to the tenon, then it is possible to determine the orientation of the NMR interaction tensors with respect to the molecular frame.

Source: [47].

286 solid-state nuclear magnetic resonance spectroscopy (SSNMR)

Application of nuclear magnetic resonance spectroscopy to solids.

1. SSNMR is characterized by the presence of anisotropic (directionally dependent) interactions. Chemical shift anisotropy (CSA) and internuclear dipolar coupling result in severe line-broadening of the corresponding NMR signals and specific hardware and methods are required to perform SSNMR to mitigate some of these issues including magic angle spinning (MAS).

Source: [22].

287 solvent suppression

In nuclear magnetic resonance spectroscopy reduction or removal of large, unwanted resonances from solvents to improve spectrum quality and dynamic range through selective excitation and exploiting differences in relaxation times between solvent and analyte signals.

Source: [20].

288 spectral width in NMR, f_SW

spectral width

Frequency range that describes the width of the spectrum window of a specific nucleus in nuclear magnetic resonance spectroscopy.

289 spin of a nucleus

nuclear spin

spin

Intrinsic angular momentum of a nucleus (or other sub-atomic particle).

1. The magnitude of nuclear spin is (h/2π)I(I+1) where I is the spin quantum number and h is the Planck constant.

2. In nuclear magnetic resonance spectroscopy, only nuclei with spin I > 0 are observable.

3. The spin of a nucleus is dependent on the numbers and alignments of the spins of its individual protons and neutrons.

4. Nuclei with a spin of ½ generate the simplest nuclear magnetic resonance spectra and are thus the most commonly studied.

Source: [40].

290 spin decoupling

Irradiation of a nucleus in nuclear magnetic resonance spectroscopy to prevent spin-spin coupling.

See also: homonuclear decoupling, heteronuclear decoupling.

291 spin echo (SE)

Hahn echo

Refocusing of spin magnetization usually by 2 consecutive radiofrequency pulses.

1. The simplest form of the spin-echo pulse sequence consists of 90°-pulse, a 180°-pulse, and then an echo. The time between the middle of the first RF pulse and the peak of the spin echo is called the echo time (T_E).

2. SE is typically used within a pulse sequence to refocus chemical shift by pulses 90°, T_E/2, 180°, T_E/2. (See also: flip angle.)

Source: [21].

292 spin system

Group of magnetic nuclei that interact amongst themselves through spin-spin coupling but do not interact with any nuclei outside the spin system.

Source: [48].

293 spin-lattice relaxation

longitudinal relaxation

T ₁ relaxation

T1 relaxation

Mechanism in nuclear magnetic resonance spectroscopy by which the component of the magnetization vector along the direction of the static magnetic induction reaches thermodynamic equilibrium with its surroundings through loss of energy of the excited nuclear spins to the surrounding molecular lattice. The resultant exponential decay of signal is characterized by the spin-lattice relaxation time T₁ (reciprocal of the rate constant of this mechanism).

1. T1 dictates the time required between individual NMR scans to ensure the system has returned to equilibrium.

Source: [14]. See also: spin-lattice relaxation time.

294 spin-lattice relaxation time, T₁

spin-lattice time constant

Reciprocal of the rate constant of longitudinal relaxation.

See also: spin-spin relaxation time, net dephasing time.

295 spinning sidebands

Satellite peaks in a nuclear magnetic resonance spectrum symetrically spaced either side of a main peak at a frequency offset related to the rate of spinning of the sample.

1. In solid state nuclear magnetic resonance spectroscopy this occurs if the sample is spun at a rate less than the magnitude of the anisotropic interaction,

Source: [48].

296 spin-spin coupling

spin coupling

Effect of the relative orientation of the magnetic fields of adjacent nuclei on each other and the resultant splitting of their signals in nuclear magnetic resonance spectroscopy (multiplicity).

1. For two proximal nuclei A and B, magnetic field at nucleus A is the sum of the nuclear shielding and the magnetic field at nucleus B. If nucleus B has two possible orientations in field due to spin ½ then nucleus A will experience two possible magnetic fields and therefore give two distinct frequencies. The frequency difference between these two lines is called the spin-spin coupling constant J_AB.

2. Spin-spin coupling is transmitted through intervening bonding electrons, in contrast to the through space dipolar mechanism.

3. SI unit: Hz.

Source: [14].

297 spin-spin coupling constant, J

coupling constant

Frequency difference between two nuclear magnetic resonance lines arising from spin-spin coupling.

1. Parentheses may be used (for example) to indicate the species of nuclei coupled, e.g., J(¹³C, ¹H) or, additionally, the coupling path, e.g., J(POCF). Where no ambiguity arises, the elements involved can be, alternatively, given as subscripts, e.g., J_CH. The nucleus of higher mass should be given first. ⁿJ indicates coupling through n bonds.

Source: [42] p 2592.

298 spin-spin relaxation

transverse relaxation

T2 relaxation

Loss of magnetization in the xy plane (in the absence of B₀ inhomogeneities) through the interchange of energy between nuclear spins resulting in some precessing faster than the Larmor frequency and some slower.

1. The resultant exponential decay of signal is characterized by the spin-spin relaxation time T₂ (reciprocal of the rate constant of this mechanism).

Source: [14]. See also: net dephasing time.

299 spin-spin relaxation time, T₂

transverse relaxation time

Reciprocal of the rate constant of spin-spin relaxation.

Source: [42] p 2493. See also: spin-lattice relaxation time, net dephasing time.

300 static magnetic flux density, B₀

magnetic flux density

static magnetic field

Flux density of the magnetic field of a nuclear magnetic resonance spectrometer about whose direction the nuclear magnetic moment precesses.

1. The value of B₀ is expressed in Tesla or as the nominal proton precession frequency (e.g., 14 T or 600 MHz).

2. Static magnetic induction is related to the magnetic field H through the equation B = μH where μ is the permeability of the material.

Source: [20], [42] p 2492, [3] p 17.

301 symmetrization

Method of removing artefacts from a two-dimensional nuclear magnetic resonance spectrum that are symmetrical about the diagonal (e.g., HH COSY) or the f1 axis (e.g., tilted J-resolved spectra). Values equidistant from midline are compared and replaced the lower (or average) value of the two.

Source: [21].

302 t ₁ in two-dimensional nuclear magnetic resonance spectroscopy

t ₁ in 2-D NMR

Time domain arising from the regular increments of the delay period in two-dimensional nuclear magnetic resonance spectroscopy.

303 t ₂ in two-dimensional nuclear magnetic resonance spectroscopy

t ₂ in 2-D NMR

Time domain in two-dimensional nuclear magnetic resonance spectroscopy arising from direct free induction decay detection.

304 t ₁ noise in two-dimensional nuclear magnetic resonance spectroscopy

t ₁ noise in 2-D NMR

Streaks of spurious artefact signals in a two-dimensional nuclear magnetic resonance spectrum parallel to the f1 axis at the f2 of a strong resonance.

Source: [21].

305 T1 relaxation

See: spin-lattice relaxation.

306 T2 relaxation

See: spin-spin relaxation.

307 three-dimensional nuclear magnetic resonance spectroscopy

3-D NMR

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy in which data are collected in three time domains.

1. A 3-D NMR experiment may be constructed from a 2-D NMR experiment by inserting an additional indirect evolution time and a second mixing period between the first mixing period and the direct data acquisition.

See also: triple-resonance nuclear magnetic resonance spectroscopy.

308 time domain

Condition in nuclear magnetic resonance spectroscopy where the independent variable of all functions is time.

Example: The display of an FID.

Source: [14].

309 time domain nuclear magnetic resonance spectroscopy (TD-NMR)

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy run at low magnetic field strengths where data analysis is performed directly on the free induction decay, often through the extraction of relaxation time constants

See also: relaxometry.

310 tip angle

See: flip angle.

311 total correlation spectroscopy (TOCSY)

homonuclear Hartmann-Hahn spectroscopy (HOHAHA)

Two-dimensional nuclear magnetic resonance homonuclear correlation experiment, which creates correlations between all protons within a given spin system, not just between geminal or vicinal protons as in COSY.

1. TOCSY uses a spin lock to allow propagation of magnetization through scalar couplings.

Source: [27].

312 total suppression of spinning sidebands (TOSS)

Technique to suppress spinning sidebands in solid-state nuclear magnetic resonance spectroscopy with cross polarization in magic angle spinning experiments.

Source: [27].

313 transverse relaxation

See: spin-spin relaxation.

314 transverse relaxation optimized spectroscopy (TROSY)

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy, usually in protein NMR, that improves peak sharpness for molecules having a molecular mass of greater than 100 kDa.

1. TROSY relies on the cancellation of the dipolar coupling and chemical shift anisotropy (CSA) components of the transverse relaxation (see spin-spin relaxation).

Source: [25].

315 transverse relaxation time

See: spin-spin relaxation time.

316 triple-resonance nuclear magnetic resonance spectroscopy

triple-resonance NMR

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy using three NMR-active nuclei.

1. For proteins the three nuclei are usually ¹H hydrogen, ¹³C carbon, and ¹⁵N nitrogen, which requires suitable labelling of the sample.

317 two-dimensional nuclear magnetic resonance spectroscopy

2-D NMR

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy in which data are collected in two time domains: acquisition of the free induction decay (t₂) and a successively incremented delay (t₁).

1. The resulting data matrix is subjected to two successive Fourier transforms to produce a spectrum with two frequency axes, usually either chemical shift/chemical shift (correlation spectroscopy) or chemical shift/J-coupling (see J-resolved spectroscopy).

Source: [20].

318 WALTZ decoupling

See: wideband, alternating-phase, low-power technique for residual splitting.

319 wideband, alternating-phase, low-power technique for residual splitting

WALTZ decoupling

Cluster of pulses applied repeatedly for heteronuclear decoupling.

1. WALTZ is commonly used for proton decoupling during the acquisition of ¹³C spectra.

Source: [21].

320 Zeeman levels

Energy levels of a nucleus arising from the interaction of its magnetic dipole with an external magnetic field.

Source: [20].

321 zero-field nuclear magnetic resonance spectroscopy

zero-field NMR

Measurement method [VIM 2.5] of nuclear magnetic resonance spectroscopy in which nuclear magnetic resonance spectra are acquired in an environment carefully screened from magnetic fields, including from the Earth’s field allowing the direct detection of J-spectra in the absence of Zeeman interactions (see Zeeman levels).

Source: [49].

322 ablation by sputtering

Bombardment of a sample surface with ions resulting in the removal of atoms from their lattice sites, and thus an ongoing discharge process that causes continuous ablation of the sample.

Source: [50].

323 ablation efficiency in LIBS

Mass of sample removed in laser-induced breakdown spectroscopy divided by the energy delivered by the laser beam.

Source: [51].

324 ablation rate

Depth of sample layer removed in the glow discharge process per unit time.

1. Ablation rate is usually expressed in nm s⁻¹.

Source: [50].

325 ablation threshold fluence in LIBS

Minimum fluence required to achieve ablation of the sample.

Source: [51].

326 absorption path-length

Path-length of a beam of radiation in an absorbing medium.

Source: [10].

327 acid transient effects in ICP spectrometry

Increase in signal equilibration time or in wash out time caused by a modification in inorganic acid concentration.

Source: [52].

328 additional gases in flame atomic spectroscopy

Gases added to a combustion mixture.

1. An inert diluent is non-reactive, an auxiliary gas is reactive.

Source: [10].

329 aerosol

Sol in which the dispersed phase is a solid, a liquid or a mixture of both and the continuous phase is a gas (usually air).

1. Owing to their size, the particles of the dispersed phase have a comparatively small settling velocity and hence exhibit some degree of stability in the Earth’s gravitational field.

2. An aerosol can be characterized by its chemical composition, its radioactivity (if any), the particle size distribution, the electrical charge and the optical properties.

3. In inductively-coupled plasma spectrometry the sample is delivered to the plasma as an aerosol, where the continuous gas phase is argon.

Source: [53] p 1805.

330 aerosol transport phenomena in ICP spectrometry

Processes occurring inside the spray chamber or desolvation system responsible for the modification of the primary aerosol to finally yield the tertiary aerosol.

1. These phenomena include solvent evaporation, droplet coalescence or coagulation, inertial droplet losses, gravitational settling and turbulence losses.

Source: [52].

331 analyte transport rate in ICP spectrometry

Mass of analyte reaching the plasma per unit time.

Source: [52].

332 atomic absorption spectroscopy (AAS)

Measurement method [VIM 2.5] of atomic spectroscopy for measuring the amount of a chemical element based on the measurement of the absorption of characteristic electromagnetic radiation by atoms in the vapour phase.

Source: [40].

333 atomic emission spectroscopy (AES)

Measurement method [VIM 2.5] of atomic spectroscopy for measuring the amount of a chemical element based on the measurement of the intensity of characteristic electromagnetic radiation emitted by atoms or molecules.

Source: [40].

334 atomic excitation source

excitation source

Atomizer intended to convert free atoms to an excited state.

Source: [10].

335 atomic fluorescence spectroscopy (AFS)

Measurement method [VIM 2.5] of atomic spectroscopy for measuring the amount of a chemical element based on the measurement of the re-emission of characteristic electromagnetic radiation by atoms, following the absorption of radiation in the vapour phase.

1. The wavelengths of the absorbed and re-emitted radiation may be identical (atomic resonance fluorescence spectroscopy) or different.

Source: [40].

336 atomic vapour

Vapour containing free atoms of analyte.

Source: [10].

337 atomization

Conversion to an atomic vapour.

Source: [10].

338 atomization curve in electrothermal AAS

Graph of absorption against atomization temperature.

Source: [54].

339 atomizer

Device used in atomic spectroscopy to achieve atomization.

1. Examples are a flame or electrothermal atomizer.

Source: [10].

340 axial viewing mode

Inductively-coupled plasma optical emission spectrometer configuration in which the entrance slit is aligned with the main plasma axis.

1. The large amount of light captured in this method, includes information from the sample of interest plus background, which can be considerable.

2. Matrix interferences, which originate in the cooler plasma tail, can degrade precision and accuracy.

Source: [52].

341 background continuum emission in LIBS

Intense continuous background usually observed in laser-induced breakdown spectra

1. The continuum emission originates from Bremsstrahlung radiation, which predominates the first part of the plasma life time and decreases with further plasma evolution.

Source: [51].

342 background equivalent concentration, c_BE

Concentration of a given element measured at a given wavelength providing the same intensity as the background.

Source: [52].

343 breakdown threshold fluence in LIBS

Minimum fluence required to achieve measurable emission signals

1. Usually the breakdown-threshold fluence is orders of magnitude higher than the ablation-threshold fluence.

Source: [51].

344 Bremsstrahlung emission

Portion of the continuum spectrum originating from the energy lost by high velocity electrons as they interact with positively charged ions without combining with them.

Source: [52].

345 Bremsstrahlung radiation

Photons with a broad energy distribution produced by losses in kinetic energy due to the deceleration of charged particles connected with the emission of electromagnetic radiation.

Source: [51].

346 capacitive microwave plasma

Plasma generated by a capacitor in presence of a microwave field.

Source: [52].

347 characteristic line

Spectral line of an atom used for the measurement of analyte concentration by atomic spectroscopy.

1. Characteristic lines include resonance and non-resonance lines.

Source: [10].

348 chemical modifier for electrothermal atomic absorption spectroscopy

Substance added to an electrothermal atomizer to obtain a better analyte atomization, or to alter the vaporization and/or atomization of interferents, thus mitigating interferences.

Source: [54].

349 continuum source in atomic spectroscopy

Source that emits electromagnetic radiation with relatively constant intensity over a broad spectral region.

1. The use of such a source allows for correction of spectral interferences.

Source: [54], [55].

350 depth resolution

Capability to distinguish between two consecutive layers in a layered material.

1. With GD-OES nanometre or even atomic-layer depth resolution can be obtained.

2. Usually the parameters used in the sputtering process (discharge parameters, selected gas and pressure) restrict the achievable depth resolution.

Source: [50].

351 depth-profiling

Process by which the sample is sputtered ‘atomic layer by layer’ using suitable conditions.

1. Depth profile analysis can be accomplished by time resolved measurement of the generated emission signals.

2. Depth profiling using signal versus time measurement requires knowledge of the ablation rate, which must be known, or has to be calculated, from measurements of suitable materials.

Source: [50].

352 desolvation in atomic spectroscopy

desolvation

Removal of the solvent in atomic spectroscopy, giving rise to particles of the solute either in the solid or gas phase.

Source: [10].

353 detection efficiency in inductively-coupled plasma spectrometry

Probability that a given atom in the plasma-probed volume generates a detectable signal.

Source: [52].

354 direct current glow discharge optical emission spectroscopy (DC GD-OES)

Measurement method [VIM 2.5] using optical emission spectroscopy in which atomization and excitation occur in a glow electrical discharge sustained by a direct current electric field.

1. Generated argon ions impact with high energy on to the surface of the cathode, which can thus be ablated in a purely mechanical way (cathodic sputtering) or the cathode can be heated to assist evaporation (thermal volatilization).

2. DC GD-OES is only applicable for electrically conducting samples.

Source: [50].

355 direct-injection burner

Burner in a flame atomic spectrometer in which the fuel, the oxidant and the sample solution are injected into the flame. (See flame in flame atomic spectroscopy).

1. This burner generally produces a turbulent flame.

Source: [10].

356 discharge

See: electric discharge.

357 dispersion of a sample in atomic spectroscopy

Conversion of the whole or part of a liquid or solid sample into a physical form sufficiently finely divided to allow it to be atomized (see atomization) upon introduction into the atomizer.

Source: [10].

358 double-pulse laser-induced breakdown spectroscopy

Measurement method [VIM 2.5] of laser-induced breakdown spectroscopy in which reactions are induced by the interaction of the evolved plasma and the remaining part of the laser pulse, or with a following laser pulse provided from a second laser system.

Source: [51].

359 dual viewing mode

Inductively coupled plasma spectrometer that allows axial viewing mode or radial viewing mode.

Source: [52].

360 Echelle spectrometer

See: GD-OES spectrometers with simultaneous detection.

361 electric discharge

discharge

Transmission of electrical current by a plasma in an applied electric field through a normally non-conducting medium.

1. An inductively-coupled plasma is a kind of electric discharge.

362 electron number density

Number of free electrons in vapour phase per volume unit.

1. SI unit: m⁻³.

Source: [56].

363 electrothermal atomic absorption spectroscopy (ETAAS)

Measurement method [VIM 2.5] of atomic absorption spectroscopy in which the sample is atomized by an electrothermal atomizer.

Source: [54].

364 electrothermal atomizer

Atomizer that is heated by the passage of electrical current through its body to the temperature required for analyte atomization.

1. An electrothermal atomizer typically consists of tube, rod, strip and filament made of refractory material that is heated by a low voltage, high current device, providing a means of obtaining variable temperatures according to the nature of the analyte.

2. Electrothermal atomizers are often made of graphite (polycrystalline electrographite), and glassy carbon, termed ‘graphite furnace’.

Source: [54]. See also: electrothermal atomic absorption spectroscopy.

365 electrothermal vaporization

Technique for sample introduction based on the use of a solid furnace heated at a controlled temperature to vaporize the sample prior to its introduction into an inductively-coupled plasma.

Source: [54], [55].

366 E _sum

Energy that must be supplied to an atom to transfer it from the ground atomic state to a given ionic excited state.

Source: [10].

367 excitation source

See: atomic excitation source.

368 flame atomic spectrometer

Spectrometer to make measurements by flame atomic spectroscopy.

369 flame atomic spectroscopy

flame atomic spectrometry

Measurement method [VIM 2.5] of atomic spectroscopy that uses a flame to excite atoms.

370 flame background in flame atomic spectroscopy

See: spectral background of an atomizer or excitation source.

371 flame in flame atomic spectroscopy

Continuously flowing mixture of hot gases with a stationary position that derives its heat content from a strongly exothermic, irreversible chemical reaction between a fuel and an oxidant.

1. The flame in general consists of a primary-combustion zone, a secondary-combustion zone and an interconal zone.

372 flow spoiler

Component of a flame atomic spectrometer for creating turbulence in the stream of mist in the spray chamber and removing the largest droplets from this mist by deposition.

Source: [10].

373 fluence, F, H

Energy of a beam of electromagnetic radiation delivered per unit area. F=∫I dt=∫(dP/dA)dt where I is intensity and Pradiant power.

1. SI unit: J m⁻². Common unit J cm⁻².

Source: [51], [3] p 35.

374 fraction atomized

See: local fraction atomized.

375 gate delay in laser induced breakdown spectroscopy

time delay in laser induced breakdown spectroscopy

Time between the application of a laser pulse and the start of detection in laser-induced breakdown spectroscopy.

1. The gate delay varies from some hundreds of nanoseconds to several microseconds.

2. Use of a suitable gate delay allows the high intensity background continuum emission present in the early stages of plasma formation to be removed.

Source: [51].

376 GD-OES spectrometers with simultaneous detection

Spectrometers for glow discharge optical emission spectroscopy that allow simultaneous monitoring of selected spectral ranges or the whole emission spectra.

1. GD-OES spectrometers mostly use the so called Paschen–Runge mounting, where the entrance slit, the curved grating and different detectors are aligned along the Rowland circle. Alternatively, Echelle spectrometers (see GD-OES spectrometers with simultaneous detection) are used, which use an Echelle grating in combination with a prism for wavelength separation.

Source: [50].

377 generator coupling efficiency in inductively-coupled plasma spectrometry

Fraction of the energy generated being used for creating and maintaining the plasma in an inductively-coupled plasma spectrometer.

Source: [52].

378 glow discharge source

Component of an atomic emission spectrometer employing a glow electric discharge used in atomic emission spectroscopy for qualitative and quantitative analysis of solid materials.

1. Glow discharge allows volatilization as well as excitation of analytes and sample matrices.

Source: [50].

379 glow electric discharge

glow discharge

Electric discharge in gas at low pressure (100 to 1000 Pa).

1. In atomic emission spectroscopy argon is used as the inert working gas that generates the plasma.

2. In the prevailing electrical field the produced ions are accelerated towards the cathode, resulting in a continuous bombardment of the sample surface.

Source: [50].

380 glow-discharge optical emission spectroscopy (GD-OES)

Optical emission spectroscopy in which a glow electric discharge excites atoms at the surface of a sample, which is made the cathode of the discharge.

381 Grimm-type cathode

Cathode for a glow electric discharge consisting of a flat sample, which is cooled during the discharge process so that thermal volatilization is suppressed and the sample material is removed by sputtering only.

Source: [50].

382 hollow-cathode discharge source

Glow discharge source with a graphite cathode, into which the sample is inserted in the form of drillings, a pressed pellet or a dry solution residue.

1. For this source, sample volatilization takes place mainly by thermal effects.

Source: [50].

383 hollow-cathode lamp

line source

Glow discharge source of spectral lines of an atom in which the cathode is made of the desired element.

1. It is often referred to as line source, as opposed to a continuum source.

Source: [55].

384 incident power

Power supplied by a generator to the plasma.

Source: [56].

385 inductively-coupled plasma (ICP)

Plasma produced by induction by means of a high-frequency (about 2 500 Hz) electromagnetic field. The region of gas at high temperature and free from the field is taken as the region of observation.

Source: [52]. See also: inductively-coupled plasma optical emission spectroscopy, inductively-coupled plasma mass spectrometry.

386 inductively-coupled plasma spectrometry

inductively-coupled plasma spectroscopy

ICP

Measurement method [VIM 2.5] of atomic spectroscopy that uses an inductively-coupled plasma to excite and ionize atoms.

1. ICP methods are described by their method of detection of ions in the plasma and include inductively-coupled plasma optical emission spectroscopy, and inductively-coupled plasma mass spectrometry.

387 inductively-coupled plasma mass spectrometry (ICP-MS)

Measurement method [VIM 2.5] of atomic spectrometry that uses an inductively-coupled plasma to excite and ionize atoms with measurement of the number and kind of ions in the plasma using mass spectrometry.

388 inductively-coupled plasma optical emission spectroscopy (ICP-OES)

Measurement method [VIM 2.5] of atomic spectroscopy that uses an inductively-coupled plasma to excite and ionize atoms with measurement of the number and kind of ions in the plasma from their emission of electromagnetic radiation (see optical emission spectroscopy).

389 inductively-coupled plasma robustness

plasma robustness

Ability of an inductively-coupled plasma to withstand slight modification in the operating conditions and/or matrix composition with no significant changes in its fundamental characteristics (i.e., temperatures, electron number density) or the analytical figures of merit (i.e., sensitivity, detection limits or signal-to-noise ratio)

Source: [52].

390 inductively-coupled plasma thermal pinch

Decrease in the inductively-coupled plasma volume normally caused when an organic volatile solution is delivered to it. Organic vapours diffuse towards the outermost area of the plasma and its volume decreases.

Source: [52].

391 inductively-coupled plasma torch

Quartz component of an inductively coupled plasma spectrometer where the plasma is generated and stabilized.

1. In ICP the torch is an open three concentric tubes assembly allowing the introduction of the three gas streams constituting the plasma. The inner tube of the torch can be made of a different material such as alumina and is usually called the injector.

Source: [52].

392 initial plasma radiation zone in inductively-coupled plasma optical emission spectroscopy

Inductively-coupled plasma volume in inductively-coupled plasma optical emission spectroscopy where the analytically relevant light emission begins to be observed.

Source: [52].

393 injector in an inductively-coupled plasma torch

See: inductively-coupled plasma torch.

394 interference curve in atomic spectroscopy

Graph of absorbance or intensity plotted as a function of the concentration of an interfering element in an atomic spectroscopy experiment.

395 inverse Bremsstrahlung

Absorption of photon energy by free electrons.

1. Inverse Bremsstrahlung increases the kinetic energy of the electrons, which is important for heating the plasma.

Source: [51]. See also: Bremsstrahlung emission, Bremsstrahlung radiation.

396 ionic cloud in atomic spectroscopy

Gas phase containing free ions of analyte.

Source: [10].

397 ionization buffer in atomic spectroscopy

Buffer that decreases and stabilizes the ionization of free atoms of analyte by increasing the concentration of free electrons in the atomizer.

Source: [10].

398 ionization energy

Minimum energy that must be supplied to remove an electron from an atom in the ground state.

Source: [10].

399 laser ablation

Removal of material as an aerosol from a surface on irradiation by a laser beam.

Source: [51].

400 laser induced breakdown spectroscopy (LIBS)

Measurement method [VIM 2.5] of atomic spectroscopy that uses a focused laser beam to ablate, atomize and excite atoms from a sample surface with subsequent measurement of the generated electromagnetic radiation.

Source: [51].

401 laser-induced breakdown spectrum

Emission spectrum generated as a consequence of the decay of the excited states generated in a laser-induced plasma that consists of a continuous background and element-specific atomic or ionic lines and molecular bands.

1. LIBS spectra cover a wide wavelength range: commonly LIBS measurements are taken between the wavelengths of 200 nm and 1000 nm.

Source: [51].

402 laser-induced plasma

Local plasma formed by the interaction between a laser beam and a solid sample and initiated by the production of primary electrons by multiphoton ionization and thermionic photoemission mechanisms.

1. As well as electrons, laser-induced plasma contains ions, and neutral atoms as well as excited species of the ablated matter. Decay of these excited species generates the light emission analysed in laser-induced breakdown spectroscopy.

Source: [51].

403 line

See: spectral line of an atom.

404 line-broadening in atomic spectroscopy

Increase in the theoretical width of a spectral line owing to thermal motion of emitting atoms (Doppler effect), to electric field (Stark effect), to self-absorption and to pressure (Lorentz effect).

1. Line-broadening increases the measurement uncertainty [VIM 2.26] of a measurement.

Source: [10].

405 line profile

Graph of the variation of emitted radiation intensity as a function of wavelength (emission line) or the variation of the absorption factor as a function of wavelength (absorption line).

Source: [10].

406 line source

See: hollow-cathode lamp.

407 local fraction atomized

fraction atomized

Number of free atoms divided by the total number of atoms of the analyte present in the gaseous phase in the observation volume in atomic spectroscopy, the latter being the volume confined by the atomizer or immediately adjacent to it.

Source: [10].

408 low sample consumption system in inductively-coupled plasma spectrometry

Liquid-sample introduction system to an inductively-coupled plasma spectrometer adapted to work at sample flow rates below 100 μL min⁻¹.

1. The system is normally composed of a micro-nebulizer and a low inner volume spray chamber.

Source: [52].

409 L’vov platform

See: platform atomization in electrothermal atomic absorption spectroscopy.

410 measurement efficiency in inductively-coupled plasma spectrometry

Number of detector events per atom in the sample.

Source: [52].

411 microwave cavity

Resonant cavity that permits focusing a microwave field inside a discharge tube thus giving rise to a standing wave.

Source: [52].

412 microwave-induced plasma

microwave plasma

Plasma generated in a microwave cavity.

1. The plasma results as a consequence of ionization of a given gas in presence of electrons and a microwave field.

Source: [52].

413 multiphoton ionization

Absorption of multiple photons by an atom such that the accumulated energy of the absorbed photons is greater than the ionization potential of the atom.

Source: [51].

414 nebulization

Conversion of a liquid into an aerosol consisting of droplets suspended in a gas stream.

Source: [10].

415 nebulizer

Device for producing nebulization.

Source: [10].

416 non-specific emission or attenuation

Electromagnetic radiation, within the bandpass filter used, emitted by, or absorption caused by, atoms, molecules and radicals that are subject to the atomizer by all components, apart from the analyte, that are present there during measurement.

1. These effects include scattering or absorption effects by solid particles.

Source: [10].

417 normal plasma analytical zone in inductively-coupled plasma optical emission spectroscopy

Inductively-coupled plasma volume mainly responsible for emission of analytically-useful light because processes leading to excited species, such as droplet desolvation, element vaporization, atomization (or ionization) and excitation have been completed.

Source: [52].

418 observation height

Vertical distance between the optical axis of observation in a flame atomic or inductively coupled plasma optical spectrometer and the horizontal plane of the top of the burner or the ICP load coil, respectively.

Source: [10].

419 optical emission spectroscopy (OES)

Measurement principle [VIM 2.4] of atomic emission spectroscopy in which the emitted electromagnetic radiation is in the range of frequencies from the infrared, through visible, to the ultra-violet region.

1. OES is used as the detector in inductively-coupled plasma optical emission spectroscopy, and glow-discharge optical emission spectroscopy.

420 overall efficiency of atomization

In atomic spectroscopy, mass of analyte converted into free atoms in the atomizer divided by mass of analyte entering the dispersion device.

Source: [10].

421 Paschen-Runge mounting

See: GD-OES spectrometers with simultaneous detection.

422 Penning excitation/ionization in inductively-coupled plasma spectrometry

Collisional excitation and ionization mechanism of inductively-coupled plasma spectrometry involving analyte atoms and metastable argon atoms.

Source: [52].

423 percentage transmission of sample

In atomic spectroscopy, flux of mass of sample divided by the flux of mass of solvent blank, measured under the same conditions, expressed as a percentage.

Source: [10].

424 permanent modifier in electrothermal atomic absorption spectroscopy

Substance added together with the sample or standards in electrothermal atomic absorption spectroscopy that is refractory, or forms a refractory species (e.g., a carbide), and thus does not need to be renewed for several atomization cycles.

1. This is a particular type of chemical modifier for electrothermal AAS.

Source: [57].

425 plasma

Matter brought into the gaseous state, largely ionized, and emitting and absorbing electromagnetic radiation.

1. In practice, the term ‘plasma’ is restricted to cases where the temperature is greater than 7000 K.

2. A plasma may be generated by an electric discharge.

Source: [56].

426 plasma arc

Plasma formed by an arc-discharge.

1. The discharge is blown through a suitable orifice to form a plasma jet. The high-temperature, electric field-free gaseous region is designed as the region of observation.

Source: [10].

427 plasma excitation temperature

Temperature governing the number density of atomic species in their ground state and excited states following a Boltzmann distribution.

Source: [56].

428 plasma gases in inductively-coupled plasma

Gas streams used to maintain and stabilize the inductively-coupled plasma. The external stream is the main constituent of the plasma and confines the plasma avoiding melting of the torch walls; the intermediate gas stream keeps the plasma at a given height above the top of the torch and, the central stream carries the aerosol and promotes its injection in the plasma central channel.

Source: [52].

429 plasma induction zone

Inductively-coupled plasma volume where there is a maximum interaction between the electromagnetic field and the plasma gas (mostly argon). This is the plasma’s hottest area and has a toroidal configuration.

Source: [52].

430 plasma ionization temperature

Temperature associated with the energy level of plasma ions.

Source: [56].

431 plasma local thermodynamic equilibrium (LTE)

Condition for which the temperatures estimated from different particles (atoms, molecules, ions, electrons) constituting a plasma are locally coincident.

Source: [56].

432 plasma robustness

See: inductively-coupled plasma robustness.

433 plasma shielding in laser induced breakdown spectroscopy

Prevention of laser pulse energy from reaching a sample surface as a result of the presence of a laser-induced plasma.

1. The main process leading to plasma shielding is the absorption of the laser energy by electrons (inverse Bremsstrahlung) and multiphoton ionization (mainly relevant for shorter laser wavelengths).

Source: [51].

434 plasma termination in laser induced breakdown spectroscopy

Extinction of the plasma after a laser pulse as a consequence of self-absorption (quenching) and recombination of electrons and ions.

1. The time elapsed between initiation and extinction of the plasma ranges from tenths of microseconds to a few milliseconds.

Source: [51].

435 platform atomization in electrothermal atomic absorption spectroscopy

L’vov platform

Sample support made of graphite that is inserted into the atomizer tube in electrothermal atomic absorption spectroscopy. The sample, a liquid or a solid, is deposited onto this platform, from which it undergoes all the transformations leading to atomization. The use of a platform delays atomization, until a more stabilized temperature in the gas phase is reached.

Source: [54].

436 pneumatic nebulizers

Nebulizers for which the aerosol generation principle is based on the exposure of the liquid sample to a high velocity gas stream.

1. Most common designs include cross flow, concentric, parallel path, entrained and Babington and V Groove.

Source: [52].

437 preheating inductively-coupled plasma zone

preheating plasma zone

Inductively-coupled plasma volume where tertiary aerosols suffer from complete solvent evaporation, element salt vaporization and dissociation into atoms.

Source: [52].

438 premix burner

Burner in a flame atomic spectrometer in which the fuel, the oxidant and the aerosol are mixed before reaching the flame. (See flame in flame atomic spectroscopy).

1. A premix burner generally produces a laminar flame (see flame in flame atomic spectroscopy).

Source: [10].

439 primary aerosol

Aerosol generated by a nebulizer.

1. Usually, primary aerosols contain coarse droplets, they are polydisperse in terms of drop diameters and they are turbulent.

Source: [52].

440 pulse duration of a laser

pulse width

pulse length

Time duration of laser pulses.

1. Pulse duration depends on the laser system used and can vary between hundreds of femtoseconds to a few nanoseconds.

Source: [51].

441 pulse energy of a laser

Total optical energy content of a laser pulse. Integral of optical power over pulse duration.

1. Pulse energies range from microjoules to millijoules for Q-switched lasers, whereas mode-locked lasers achieve much lower pulse energies (picojoules, nanojoules or sometimes several microjoules).

Source: [51].

442 pulse frequency

See: repetition rate of a pulsed laser.

443 pulsed laser

Laser system that emits electromagnetic radiation in the form of pulses of a defined and constant duration.

1. A pulsed laser produces no continuous optical wave.

Source: [51].

444 pyrolysis curve in electrothermal atomic absorption spectroscopy

Graph of absorption against pyrolysis temperature in electrothermal atomic absorption spectroscopy.

Source: [54].

445 quartz atomizer

Atomizer that is used to atomize those elements that form volatile species (e.g., hydrides) at relatively low temperatures (e.g., 1000 °C or below).

1. These can be heated externally or positioned in a flame (see flame in flame atomic spectroscopy).

Source: [55].

446 radial viewing mode

Inductively coupled plasma optical emission spectrometer configuration in which the plasma and the entrance slit are perpendicular and from which the signal is taken perpendicularly to the main plasma axis.

1. This configuration yields lower sensitivities than the axial viewing mode.

Source: [52].

447 radiofrequency (RF) generator in an inductively-coupled plasma spectrometer

Component of an inductively-coupled plasma spectrometer responsible for the generation of alternating current in the radiofrequency portion of the electromagnetic spectrum (see Table 1).

Source: [52].

448 radiofrequency glow discharge optical emission spectroscopy (RF-GD-OES)

Optical emission spectroscopy in which a radiofrequency-powered glow discharge is operated with frequencies in the low megahertz range.

1. The use of these frequencies establishes a negative direct current (DC)-bias voltage on the sample surface. The DC-bias is the result of an alternating current waveform that is centered about negative potential; as such it more or less represents the average potential residing on the sample surface. Radio-frequency has ability to appear to flow through insulators (non-conductive materials).

Source: [50].

449 reference flux density, ϕ_r(λ)

Intensity transmitted by the reference medium in a double-beam spectrometer.

Source: [10]. See also: sample flux density.

450 reflected power

Incident power that is not absorbed by the plasma.

Source: [56].

451 repetition rate of a pulsed laser

pulse frequency

Number of laser pulses emitted by a pulsed laser system per unit time.

1. For laser systems with pulse duration in the nanosecond regime repetition rates range from 1 to 20 Hz, whereas for femtosecond laser systems increased repetition rates up to 1000 Hz are possible.

2. SI unit: Hz.

Source: [51].

452 residence time in electrothermal atomic absorption spectroscopy

Time in which the analyte is confined within the observation volume in electrothermal atomic absorption spectroscopy.

Source: [54].

453 resonant inelastic X-ray scattering (RIXS)

resonant X-ray emission

resonant X-ray Raman

Measurement method [VIM 2.5] of inelastic X-ray spectroscopy that uses a hard or soft X-ray beam to excite transitions of core-level electrons into empty energy levels. The subsequent transition of an electron into the core state is accompanied by X-ray emission with characteristic momentum and energy.

1. The incident photon energy is selected for resonance with an X-ray absorption edge of the system, such that the observed shifts in momentum and energy provide element-specific chemical sensitivity, with the capacity to distinguish the same element located at inequivalent sites.

Source: [58].

454 Rowland circle

See: GD-OES spectrometers with simultaneous detection.

455 Saha’s ionization equation

Saha–Langmuir equation

Equation that relates the ionization state of an element present in a gas in thermal equilibrium to the temperature and the pressure of the medium.

1. For a gas composed of a single atomic species, the Saha equation is written:

where ni is the number density of atoms with i electrons removed, g_i is the degeneracy of the ith state, ϵi is the ionization energy of the ith state, n_e is the number density of electrons, λ is the thermal de Broglie wavelength of an electron, k is the Boltzmann constant, and T is the thermodynamic temperature of the gas.

1. The thermal de Broglie wavelength is given by:

where h is the Planck constant, k the Boltzmann constant, T temperature, and m_e the mass of an electron.

Source: [56].

456 sample flux density, ϕ_s(λ)

Intensity transmitted by the atomizer when the latter is supplied with the sample solution or a reference solution.

Source: [10]. See also: reference flux density.

457 saturator

Buffer containing interfering element(s) in sufficient quantity to reach the limit of enhancement or dispersion (i.e., saturation) of the interference curve.

Source: [10].

458 self-absorption

Partial absorption of electromagnetic radiation emitted by excited atoms in an atomic emission source by atoms of the same kind present in the source.

1. As a result of self-absorption, the observed intensity of a spectral line may be less, and its width greater, than would be the case for a source having a very small optical path and the same concentration of emitting atoms per unit volume.

2. Self-absorption may occur in all emitting sources, whether they are homogeneous or not, thermal or non-thermal.

Source: [10].

459 self-reversal in atomic emission

Absorption of electromagnetic radiation emitted from the centre of an ionic cloud by outer layers of the emitting vapour, which are cooler than the centre.

1. The intensity measured at the centre of a line is less than the intensity measured on either side of the centre.

2. In extreme cases, the intensity at the centre of the line is so weak that only the wings remain, giving the appearance of two fuzzy lines.

Source: [10].

460 sequential detection in glow discharge optical emission spectrometry

Sequential spectrometers use a dispersive element (grating) in combination with entrance and exit slits for selection of specific wavelengths. Geometric arrangements include the Czerny-Turner and the Ebert-geometry, both offering high resolution measurements. However, they do not permit simultaneous recording of different spectral lines, even for single emission line intensity, and adjacent spectral background has also to be measured sequentially.

Source: [50].

461 solvent blank flux density, ϕ_T(λ)

Intensity transmitted by the atomizer when the latter is supplied with the solvent blank.

Source: [10].

462 solvent nucleation

Solvent condensation on solid particles or droplets contained in an aerosol.

Source: [52].

463 solvent transport rate in an inductively-coupled plasma spectrometer

Mass of solvent, both in liquid and vapour forms, reaching an inductively-coupled plasma per unit of time.

Source: [52].

464 spectral background of an atomizer or excitation source

Electromagnetic radiation, within the bandpass used, emitted by, and/or in the case of atomic absorption spectroscopy, absorbed in, the atomizer when nothing is supplied to the atomizer but the gases involved in its operation.

1. In the case of flame atomic spectroscopy, the term “flame background” is used.

Source: [10].

465 spectral line of an atom

spectral line

line

In atomic spectroscopy, a very narrow band of frequencies of electromagnetic radiation emitted or absorbed by atoms, which undergo a single electronic transition.

1. The radiation is centred on a peak whose wavelength characterizes the line and which corresponds to the emission or absorption maximum.

2. A distinction is made between lines corresponding to transition of neutral atoms (e.g., Ba I 553.548 nm and 577.762 nm) and ions (e.g., Ba II 455.403 nm).

Source: [10].

466 spray chamber

Chamber of a nebulizer in which the sprayed liquid is converted to mist. Some of the droplets of this mist may evaporate, coalesce, or deposit in the chamber and subsequently drain away as waste.

Source: [10].

467 sputter rate

Mass of material removed from a sample surface in unit time

1. Usually the sputter rate is given in unit µg s⁻¹.

Source: [50].

468 stop-flow conditions in electrothermal atomic absorption spectroscopy

Stoppage to the otherwise constant flow of argon gas to the atomizer in electrothermal atomic absorption spectroscopy, that can be programmed to occur during atomization, for the purpose of increasing the residence time of the analyte atoms and thus the associated signal.

Source: [54].

469 synchrotron X-ray spectroscopy

X-ray spectroscopy performed using a high intensity, continuous polychromatic synchrotron X-ray source.

Source: [59].

470 tertiary aerosol in an inductively-coupled plasma spectrometer

Aerosol that reaches an inductively-coupled plasma.

1. Such aerosols are finer, less polydispersed and less turbulent than primary aerosols.

Source: [52].

471 thermal dispersion

Procedures whereby an aerosol is produced at a high temperature, for example by sparks, arcs, furnaces, lasers, cathodic sputtering or electron-beam.

Source: [10].

472 thermal volatilization

Process in glow-discharge atomic emission spectroscopy in which sample volatilization occurs by heating due to sample bombardment with argon ions.

1. The process causes evaporation of the sample constituents in accordance to their boiling-points.

2. Although this technique offers highest detection power for most volatile elements its use in routine applications is still limited by the analytical challenges posed by the transient nature of the analyte signal.

Source: [50].

473 time delay in laser induced breakdown spectroscopy

See: gate delay in laser induced breakdown spectroscopy.

474 time-resolved laser-induced breakdown spectroscopy

Measurement method [VIM 2.5] of laser-induced breakdown spectroscopy in which the emitted radiation is monitored with time resolution sufficiently high to resolve different stages in the lifetime of the laser-induced plasma.

Source: [51].

475 total sample consumption system

Liquid-sample introduction system able to achieve 100 % transport efficiency.

Source: [52].

476 transport efficiency of a sample

Mass of analyte entering the atomizer of a flame atomic spectrometer or the plasma of an inductively-coupled plasma spectrometer divided by the mass of analyte entering the dispersion device.

Source: [10].

477 ultrasonic nebulizers

Nebulizers whose aerosol generation principle is based on the transfer of the energy from a vibrating transducer to the liquid sample.

Source: [52].

478 volatilization

Conversion of the solute particles containing the analyte from the solid and/or liquid phase to the vapour phase.

Source: [10].

479 wall atomization in electrothermal atomic absorption spectroscopy

Electrothermal atomic absorption spectroscopy in which the sample is deposited directly onto the wall of the electrothermal atomizer tube, from which it undergoes all the transformations leading to atomization.

Source: [54].

480 wash out time

Time required for the output signal of an inductively-coupled plasma spectrometer for a given analyte to fall back to baseline levels from the end of sampling time and/or after introducing a blank solution.

Source: [52].

481 X-ray emission spectrum

Spectrum comprising the continuum Bremsstrahlung and the most energetic characteristic spectral lines defined by differences in binding energy between electron energy levels.

Source: [59].