Baseline-free quantitative absorption spectroscopy based on cepstral analysis

Abstract

The accuracy of quantitative absorption spectroscopy depends on correctly distinguishing molecular absorption signatures in a measured transmission spectrum from the varying intensity or 'baseline' of the light source. Baseline correction becomes particularly difficult when the measurement involves complex, broadly absorbing molecules or non-ideal transmission effects such as etalons. We demonstrate a technique that eliminates the need to account for the laser intensity in absorption spectroscopy by converting the measured transmission spectrum of a gas sample to a modified form of the time-domain molecular free induction decay (m-FID) using a cepstral analysis technique developed for audio signal processing. Much of the m-FID signal is temporally separated from and independent of the source intensity, and this portion can be fit directly with a model to determine sample gas properties without correcting for the light source intensity. We validate the new approach in several complex absorption spectroscopy scenarios and discuss its limitations. The technique is applicable to spectra obtained with any absorption spectrometer and provides a fast and accurate approach for analyzing complex spectra. AbstractThis document provides supplementary material for "Baseline-free Quantitative Absorption Spectroscopy Based on Cepstral Analysis." Here, we include further details of the spectral model used to fit the broadband spectrum of ethane and methane. We also compare the two sources of absorption cross section data used to generate and fit a simulated spectrum of four broadly absorbing compounds in the mid-infrared. We give a full table of fit results for the simulated MIR spectrum to accompany an abbreviated version in the full text.

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