Abstract
We evaluate the spectral resolution and the detection thresholds achievable for a photoacoustic spectroscopy (PAS) system in the broadband infrared wavelength region driven by a continuous wave optical parametric oscillator (OPO) with . The absorption spectra, , for diluted propane, ethane and methane test gases at low concentrations () were measured for ∼1350 discrete wavelengths . The spectra were then compared to the high resolution cross section data, , obtained by Fourier Transform Infrared Spectroscopy published in the HITRAN database. Deviations of 7.1(6)% for propane, 8.7(11)% for ethane and 15.0(14)% for methane with regard to the average uncertainty between and the expected reference values based on were recorded. The characteristic absorption wavelengths can be resolved with an average resolution of . Detection limits range between ppb (ethane) to ppb (methane). In an additional step, EUREQA, an artificial intelligence (AI) program, was successfully applied to deconvolute simulated PAS spectra of mixed gas samples at low limits of detection. The results justify a further development of PAS technology to support e.g., biomedical research.
Highlights
Hydrocarbons and other volatile organic compounds (VOCs) are important substances in day-to-day life with regard to e.g., their environmental impact, the exploration of natural gas resources and a manifold of medical applications
The modulated beam had an intensity of IA ∼ 0.46 · II and was directed to the gas cell which is hermetically closed with two calcium fluoride CaF2 windows, transmitting 90% of the incoming light intensity and allowing the constant measurement of the remaining idler wave’s intensity after passage through the cell, IM, with a resolution of 3%
The red abscissa on the right side refers to the cross section σFTIR and is for guidance only
Summary
Hydrocarbons and other volatile organic compounds (VOCs) are important substances in day-to-day life with regard to e.g., their environmental impact, the exploration of natural gas resources and a manifold of medical applications. By measuring the amplitude as function of the wavelength provided by e.g., a tunable laser system, a broadband absorption spectrum can be derived This allows the identification and quantitative measurement of low concentrations of the specific molecule within the sample if the initial energy of the light source is of adequate magnitude to supply a strong enough signal and the spectral resolution of the PAS system suffices. Choosing a light source with a centre-frequency matching λres makes frequency tuning expendable and the laser’s repetition rate can be adjusted to the resonance frequency of the photoacoustic cell leading to an optimized single line detection system Such a single line system is too limited in resolution to allow a quantitative measure of complex mixtures of gases. The work was seen as a first step towards the creation of a validated reference database for broadband PAS absorption spectra which could complement the existing data sets for VOC chemicals which are already characterized by high precision IR studies [16]
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