Determining Biogenic Content of Biogas by Measuring Stable Isotopologues 12CH4, 13CH4, and CH3D with a Mid-Infrared Direct Absorption Laser Spectrometer

Abstract

A tunable laser absorption spectrometer (TLAS) was developed for the simultaneous measurement of δ13C and δD values of methane (CH4). A mid-infrared interband cascade laser (ICL) emitting around 3.27 µm was used to measure the absorption of the three most abundant isotopologues in CH4 with a single, mode-hop free current sweep. The instrument was validated against methane samples of fossil and biogenic origin with known isotopic composition. Three blended mixtures with varied biogenic content were prepared volumetrically, and their δ13C and δD values were determined. Analysis demonstrated that, provided the isotopic composition of the source materials was known, the δ13C and δD values alone were sufficient to determine the biogenic content of the blended samples to within 1.5%.

Absolute 13C/12C isotope amount ratio for Vienna PeeDee Belemnite from infrared absorption spectroscopy

“…Those biases can be mitigated through a high degree of temperature control 17,18 , or by studying pairs of transitions with nearly identical lower-state rotational energy 19 . While these prior proposals and proof-of-principle demonstrations collectively suggest that accurate optical measurements of R( i E/ j E) sample may be possible without calibrations, each prior work was limited by either the aforementioned temperature biases or by uncertainties in the chosen absorption cross-sections.…”

“…C,18 O,17 O,15 N, 2 H, and34 S. For stable carbon isotope ratios, our own efforts to ascertain more accurate values for13 C 16 O 2 transition intensities in the 2 μm wavelength region using gravimetrically prepared, potentially isotopically enriched gas samples with traceability to the kilogram are just beginning. Accurate measurements of analogous 12 C 16 O 2 transitions could also reduce uncertainty in the 13 C 16 O 2 ab initio predictions (Supplementary Information, Section S2).…”

Mid-infrared broadband sensing and temperature dependence of trace gas molecular transitions

“…From the figure, it can be said that reasonable results for TIPS were shown at low temperatures, which can be attributed to the vibrational state structure of methane isotopologues [17]. With that understanding, the main isotopologue 12 CH4 was shown to be more ideal than 12 CH3D at higher temperatures. Furthermore, to better understand how these values are quantified and calculated in HaPi and enable the ability to evaluate partition sums from a transition-by-transition basis, we developed a similar process of calculation in MATLAB for line strength and partition sums.…”

“…Isotopic detection: Several isotopes of CO2, CH4, N2O, and Sulfur are studied to monitor stable isotopic ratios to help improve understanding and partition sources and sinks of CO2 and CH4. Flux measurements allow one to measure physical and biological interactions in the environment that alter the isotopic abundances [6][7][8][9][10][11][12]. In addition, isotopic ratios and their fractional change are useful in fast, real-time, and accurate temperature measurements because many of these isotopic lines have minimal temperature dependence.…”

A TC/WMS-TDLAS mid-infrared detection method for ultra-low concentration carbon isotope methane

“…B. Mortazavi et al 25 used a tunable laser direct absorption method to detect carbon isotope methane gas in the near-infrared band and added a temperature control system to the instrument case to reduce the effect of temperature on the detection. T. Kaariainen et al 26 used a tunable laser direct absorption method to detect 13 CH 4 in the mid-infrared band. He controlled the temperature of the gas cell to reduce the effect of temperature changes on the detection.…”