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

A tunable laser absorption spectrometer (TLAS) was developed for the simultaneous measurement of δ 13 C and δ D 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 δ 13 C and δ D values were determined. Analysis demonstrated that, provided the isotopic composition of the source materials was known, the δ 13 C and δ D values alone were sufficient to determine the biogenic content of the blended samples to within 1.5%.