Detection of CH4 and SF6 in small volumes with infrared photoacoustic spectroscopy: A comparison with direct absorption spectroscopy

Infrared (IR) spectroscopy has been applied to monitor gases in various circumstances; however, the IR spectral technique has limitations in detecting small volumes or trace amounts of gas. To evaluate this issue, we deliberately measured the IR spectra of methane and sulfur hexafluoride using the photoacoustic spectroscopic (PAS) technique for gaseous volumes of 1 mL or less. For comparison, the IR spectra of these gases were also determined by the direct absorption spectroscopy (DAS) method for the same quantities. As results, we found that the IR-PAS technique can compete with DAS with advantages including better baseline, scan time saving, and less distortion from the background for the same quantities of small gaseous samples. The calibrations of the intensity vs. absolute amount of gas and the optical path length of the cell were engaged in this work; the results show the relationships are linear, conforming to the successful quantitative analysis for small volumes or minute quantities of gas with the IR-PAS technique. In addition, the IR-PAS spectra of the gases were investigated with various construction materials of sample cells. In this manner, our investigation may further motivate the development of a better means or scheme in the IR-PAS technique for monitoring minute amounts of gas in the future.