Review
Signal processing and calibration procedures for in situ diode-laser absorption spectroscopy

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Abstract

Gas analyzers based on tunable diode-laser spectroscopy (TDLS) provide high sensitivity, fast response and highly specific in situ measurements of several atmospheric trace gases simultaneously. Under optimum conditions even a shot noise limited performance can be obtained. For field applications outside the laboratory practical limitations are important. At ambient mixing ratios below a few parts-per-billion spectrometers become more and more sensitive towards noise, interference, drift effects and background changes associated with low level signals. It is the purpose of this review to address some of the problems which are encountered at these low levels and to describe a signal processing strategy for trace gas monitoring and a concept for in situ system calibration applicable for tunable diode-laser spectroscopy. To meet the requirement of quality assurance for field measurements and monitoring applications, procedures to check the linearity according to International Standard Organization regulations are described and some measurements of calibration functions are presented and discussed.

Introduction

Measurements of atmospheric trace gases impose high demands on analytical instrumentation. The great number of gaseous pollutants and their generally low and variable concentrations with large local differences pose challenging requirements to analytical techniques. Thus, sensitive, selective and mobile or even portable instruments with a large dynamic range are needed. Tunable diode-laser spectroscopy (TDLS) is being frequently used for the measurement of trace gases [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] and especially during the last years many new applications have been reported. Most of them are based on indium phosphide [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], antimonide [31], [32], [33] and lead-salt diode-lasers [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58] as well as quantum cascade lasers [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72]. The applications cover the fields of fundamental spectroscopy, atmospheric chemistry, air monitoring applications and field campaigns, airborne systems for tropospheric and stratospheric research and industrial monitoring and process control. Since the recent advances in semiconductor laser based gas monitors have been reviewed in a previous issue [9] and this paper focuses on post-detection signal processing and calibration issues, the TDLAS principles are only briefly summarized here.

In most tunable diode-laser spectrometers a single narrow laser line is scanned over an isolated absorption line of the target species (Fig. 1a–c). To achieve the highest selectivity, analysis is made at low pressure, where the absorption lines are not substantially broadened by pressure. This type of measurement has developed into a very sensitive and general technique for monitoring many atmospheric trace species. The main requirement is that the molecule should have an infrared line-spectrum which is resolvable at the Doppler limit, which in practice includes most molecules with up to five atoms together with some larger molecules. Because TDLS operates at reduced pressure it is not restricted in wavelength to the atmospheric windows such as 3.4–5 and 8–13 μm that are free from H2O and CO2 [73]. Direct absorption measurements have to resolve small changes in a large signal. For typical linestrengths an ambient concentration of 1 ppbv (1ppbv=10−9 volume mixing ratio) produces a line center absorption of only 1 part in 107 or lower over a 10 cm pathlength. Therefore, conventional absorption spectroscopy will not be able to measure such a small absorption. According to Beer’s law one of the most important application of TDLS to atmospheric measurements has turned out to be their use in combination with a multi-pass cell with path lengths of 100 m or more. White [74] and Herriott and coworkers [75], [76], [77] cells achieve the long path by using mirrors to fold the optical path, giving typically 100 passes of a 1 m base-length cell. Additional modulation techniques like derivative spectroscopy using wavelength modulation in the kHz range or high frequency (MHz, GHz) modulation techniques based on single or two-tone modulation concepts are used to reduce the laser noise [9], [78]. Modulation techniques produce a difference signal which is directly proportional to the species concentration (zero baseline technique) and allow the signal to be detected at a frequency at which the laser noise is significantly reduced. In these systems the diode-laser injection current is sinusoidally modulated as the laser wavelength is tuned through an absorption line. In general it is possible to monitor signals at all harmonics of the modulation frequency, although usually only the first and the second-harmonic signals are used. The normal mode of operating a TDLS system is to scan the laser center frequency through the absorption line repeatedly at about 0.1–1 kHz and use a computer-controlled signal averager to accumulate the signal with an amplitude proportional to the species concentration. Repetitive scanning over the line gives increased confidence in the measurement because the characteristic feature of the measured species improves the signal-to-noise ratio (SNR) and unwanted spectral features due to interfering species or étalon fringes can easily be identified and sometimes can be reduced during the averaging process. With these techniques, detection limits of the order of 0.1 ppbv were achieved for many smaller molecules in air. In terms of optical density TDLAS instruments with a multipass absorption cell achieved detection limits on the order of 10−6 with a 1 s integration time. The principal setup of a diode-laser spectrometer is shown in Fig. 1d. A prerequisite for a good signal-to-noise ratio is a low noise current- and temperature-controller for the laser. In many applications the atmospheric sample is pumped through a multipass cell, while a reference cell contains a constant high concentration of the target gas for line locking and diagnostic purposes. The signals from the sample and the reference detector are fed into a preamplifier. Then frequently a phase sensitive detection is applied and after that both channels are digitized and further processed by digital filters. This data post-processing also includes line locking, normalization and calibration procedures.

In the next section (Section 2), a concept for post-detection signal processing to deal with the influence of noise, jitter, drifts and fringes is presented. A strategy for the quantitative measurement of the system stability provides the basis for optimizing the measurement cycle. This in turn helps to obtain an improved system performance. The procedure for the determination of the system performance according to the regulations of the International Standard Organization is summarized in Section 3. Procedures for spectrometer calibration based on standard gas mixtures and permeation based systems are described and finally some measurements of TDL calibration functions are discussed in Section 4.

Section snippets

Noise, jitter, drift and fringes

Trace gas measurements at ambient low levels are usually performed by measuring alternatively an ambient air spectrum and the spectrum of zero air, i.e. ideally ambient air devoid of the target substance, which often is referred to as a background spectrum. A prerequisite for quantitative measurements is a calibration spectrum recorded from a known gas mixture of known concentration in the sample cell. For monitoring of the laser amplitude and frequency fluctuations, a part of the laser beam is

Determination of the system performance

The application of modulation techniques does not allow a simple straightforward calculation of the measured concentration from the measured signal. Additionally, for an accurate calculation of the concentration from the modulated absorption signal from first principles a precise knowledge of the line parameters (line strength, line width, pressure and temperature dependence) along with the modulation waveform and amplitude is necessary. Although many molecules have been carefully investigated,

Standard gas mixtures and permeation devices

Gas analyzers need to be calibrated and periodically checked to ensure system integrity and sensor accuracy. It is important to install stationary sensors in monitoring networks in locations where the calibration can be performed easily. The intervals between calibration can be different from sensor to sensor. Generally, the minimum time interval between calibrations has to be determined for each sensor. However, it is good practice to carefully check the sensor during the first time after

Summary and conclusions

Tunable diode-laser spectroscopy is a powerful tool for analytical applications where high sensitivity, detection speed and the possibility of highly specific and simultaneous in situ measurements of atmospheric trace gases is required. While many improvements in modern TDLS system development focus on optimizing electronics and optical components, much less effort has been put into post detection signal processing and adaptive control. It was the purpose of this paper to discuss post-detection

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