Absorption and dispersion: In search of a versatile spectroscopic technique

Abstract

This paper investigates three spectroscopic techniques — Tunable Diode Laser Absorption Spectroscopy (TDLAS), Wavelength Modulation Spectroscopy (WMS), and Heterodyne Phase Sensitive Dispersion Spectroscopy (HPSDS) — to evaluate their performance in detecting carbon monoxide in nitrogen in a 31 m Herriot cell using an Interband Cascade Laser emitting at 4559 nm. With the developed spectrometer, we can switch between the techniques with minimal hardware modifications. We compare the results when using the different techniques with respect to linearity, minimum detectable value, standard deviation of the method, coefficient of variation of the method, limit of detection, and Allan-Werle deviation analysis. The results show significant differences in the performance of the three techniques when establishing calibration curves based on peak-to-peak evaluation of the recorded spectra. However, when establishing model-based calibration curves a more uniform performance of the investigated techniques was found. The results demonstrate that TDLAS is a straightforward and robust technique but has a limited measurement range caused by absorption. WMS offers a wider linear range if an appropriate spectral model is applied, and a baseline-free operation but requires careful calibration. HPSDS stands out for its wide linear range and immunity to power fluctuations. However, its operation requires a complex mathematical model and radio frequency (RF) components. • Comparison between spectroscopy techniques reveals similar performance. • HPSDS achieves a wide linear range and LOD comparable to absorption techniques. • WMS shows a broad linear range but requires careful calibration and 2 f signal simulation. • TDLAS excels in unknown environments but suffers from peak saturation in transmittance. • All three spectroscopic methods achieve sub-ppb LOD for carbon monoxide and 0.999 linearity.

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