Cepstral analysis for baseline-insensitive absorption spectroscopy using light sources with pronounced intensity...

Area of Science:

• Analytical Chemistry
• Spectroscopy
• Physical Chemistry

Background:

• Absorption spectroscopy is crucial for gas analysis but is limited by baseline light intensity (I₀) errors.
• Traditional methods struggle to isolate molecular signals when baseline and spectral responses overlap, especially with tunable lasers.

Purpose of the Study:

• To develop a novel data-processing technique for improved accuracy and precision in absorption spectroscopy measurements.
• To address limitations of existing methods in separating molecular absorbance signals from baseline intensity variations.

Main Methods:

• The study introduces a method using cepstral analysis and least-squares fitting of a simulated molecular free-induction decay (m-FID) signal to measured data.
• This technique is robust against errors in estimated I₀, which vary slowly with optical frequency.
• Scanned-wavelength direct-absorption spectroscopy with a distributed-feedback quantum cascade laser (DFB QCL) was employed for CO measurements.

Main Results:

• The new m-FID-based method demonstrated superior accuracy in determining gas temperature and CO concentration compared to traditional techniques.
• Measurement precision was improved by 1.5 to 10 times over traditional methods.
• The technique was validated in both a gas cell and a diffusion flame environment.

Conclusions:

• The presented data-processing technique significantly enhances the accuracy and precision of absorption spectroscopy.
• It offers a robust solution for analyzing spectral data, particularly when using light sources with pronounced intensity tuning.
• This advancement has broad applicability for precise gas property measurements in various environments.