A Versatile Multiple-Pass Raman System for Industrial Trace Gas Detection

13CH4/12CH4 sensing using Raman spectroscopy

The paper presents a technique for measuring the concentration of 13CH4 in natural methane using Raman spectroscopy. The peak positions and the relative scattering cross-sections of the Q-branches for the most intense vibrational bands of 13CH4 are determined. Features of the 13CH4/12CH4 ratio measurement methods using Q-branches of the ν1 and ν3 bands were considered. It was shown that the 13CH4/12CH4 ratio can be determined by simulation of the ν3 bands of these molecules without the use of experimental spectra. In our experiments the measurement error of δ13C value was 10 ‰ using the 100-s exposure spectrum at a gas pressure close to 1 atm recorded on the developed Raman spectrometer. In addition, the Raman spectra of alkanes (up to n-hexane) in the range of 2850-3050 cm-1 at a resolution of 0.4 cm-1 are presented, and their integrated intensities in the ranges of the characteristic bands of 13CH4 and 12CH4 are provided. The data obtained make it possible to expand the capabilities of Raman gas analyzers in the mud gas logging industry.

Trace Analysis of C4F7N Insulating Gas Mixtures by Spontaneous Raman Spectroscopy and Gas Chromatography

2,3,3,3-Tetrafluoro-2-(trifluoromethyl) propanenitrile (C4F7N) is being researched as an alternative to sulfur hexafluoride (SF6) for applications in gas-insulated switchgear. We independently assessed the effectiveness of gas chromatography-mass spectrometry (GC-MS) and a novel method of feedback-assisted multipass cavity spontaneous Raman spectroscopy (SRS) for the trace quantification of impurities in C4F7N and its related byproducts. A total of 14 gases were identified with estimated concentrations as low as 20 ppm (ppm) for C3F6 using GC-MS and 7.4 ppm for CH4 using SRS and as high as 500 ppm for CF4 using GC-MS and 1430 ppm for CO using SRS. While GC-MS is highly effective in selectively detecting and quantifying trace contaminants, it necessitates separate detectors for various gases, such as CH4 and H2. SRS succeeded in detecting CF4 and C2F6 at concentrations of 465 and 100 ppm, respectively, and in placing an upper bound of several hundred ppm for the other analytes. Crucially, SRS holds potential for portability-and thus for field applications-in gas-insulated switchgear equipment diagnostics.

Natural Gas Analysis Using Polarized Raman Spectroscopy

Raman spectroscopy is a promising tool for measuring the composition of natural gas. However, to obtain high measurement accuracy, it is necessary to take into account changes in the spectral characteristics of methane, since its spectrum overlaps the characteristic bands of other species. In this study we present a technique for natural gas analysis based on polarized Raman spectroscopy. It is shown that the use of only isotropic components of spectra simplifies the procedure for extracting concentrations and improves the measurement accuracy of components whose spectral bands are significantly overlapped in conventional Raman spectra. The presented technique will be very useful both in the field of analysis of various multicomponent gas mixtures and in the field of measuring the isotopic composition of molecules.