The myth of data acquisition rate
Graphical abstract
Introduction
The quality of digitized signals has been widely studied. Most often the effect of data acquisition rate is studied in terms of minimum data points per peak required for the accurate quantitation.
Different studies based on the accuracy of the numerical integration of peak area state that at least 10 data per σ are required when a Gaussian peak is digitized [1], [2], [3]. Some authors claim that 6–7 points per peak, or one data per σ is sufficient for signal analysis [4], which latter density is, of course, in accordance with the Nyquist frequency, but such a low sampling rate is obviously improper when dealing with real signals where baseline noise is omnipresent [5], [6].
The influence of data acquisition rate is often revisited as faster detectors become available. The need for high-frequency data acquisition is sometimes illustrated using fast detectors intentionally set to a low-frequency acquisition rate. In those instances the authors of scientific papers [7] or instrumentation brochures [8] claim that peak shapes become distorted and broad when the signal is sampled with a less-than-optimum sampling rate.
The aim of this study is to shed light on this misconception that accompanies data acquisition.
Section snippets
Sampling theorem
When merely the theory of digital signals is considered, the Nyquist or Shannon sampling theorem suggests that at least two samples for the highest frequency present in the continuous signal must be collected [5]:where Δt is the sampling time1 and fmax is the highest frequency to be preserved after digitalization. Obviously, in order to accurately determine the location of peak maxima or the peak area, a higher
Misleading experimental data
In order to reproduce the effect of sampling frequency on peak width, first we carried out the separation of a toluene–ethylbenzene mixture on an Ascentis Express C18 (50 × 2.1 mm, 2.7 μm) column using an Agilent 1100 instrument. The mobile phase was MeOH:H2O = 80:20 (v/v%). The chromatograms recorded at 80 Hz and 1.25 Hz sampling rates are plotted in Fig. 1.
When the sampling rate is 80 Hz, the number of theoretical plates is N = 1078 for toluene and it is N = 1344 for ethylbenzene. The resolution factor
Conclusions
Data acquisition rate has no influence at all on band broadening and hence resolution. What some researchers observe is not the effect of sampling rate but it is the consequence of undocumented software features. Loss of efficiency or resolution may indeed be observed when a modern detector is set to a slow sampling rate. The loss of efficiency, however, occurs when instrument manufacturers think that a good signal-to-noise ratio is more important than faithful representation of the original
Acknowledgments
This research was realized in the frames of TÁMOP 4.2.4.A/2-11-1-2012-0001 “National Excellence Program – Elaborating and operating an inland student and researcher personal support system.” The project was subsidized by the European Union and co-financed by the European Social Fund.
The research infrastructure was supported by the Hungarian Scientific Research Fund (OTKA K 106044).
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2019, TrAC - Trends in Analytical ChemistryCitation Excerpt :For short path lengths, the flow profiles at the inlet and outlet of the outlet of the detector start to make out the majority of the dispersion rather than the cell volume and θdet < 1 can easily be observed [95,96]. In some cases, also the dispersion caused by the electronics of the detector needs to be taken into account [97,98]. This dispersion becomes apparent whenever the detector sampling frequency is too low, or the detector rise time is set at a too high value, or the digital filter used to artificially smoothen has too coarse settings.
Comparison of integration rules in the case of very narrow chromatographic peaks
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2016, Journal of Chromatography ACitation Excerpt :In this section, we address two questions: What should be our sampling frequency and what response times (rise times) should we select in the software? The question of sampling frequency, which dictates the number of data points collected across a peak, has been discussed for at least four decades without a consensus [26,86,87]! The most popular number discussed in textbooks as well as all over the web is 20 data points per peak [88].
Sampling frequency, response times and embedded signal filtration in fast, high efficiency liquid chromatography: A tutorial
2016, Analytica Chimica ActaCitation Excerpt :Given the drastic decrease that we observe above in the peak efficiencies, one may well wonder why a recent publication stated: “When the data acquisition frequency is too low, only a few points per peak are recorded. We will miss the peak apices, …, but peaks remain as sharp as they were at the highest sampling frequency” [21]. The reason is that these authors used a column with 2000–3000 plates only, and in such a case, it may not matter.