Quartz-enhanced photoacoustic spectroscopy of methane isotopologues

We report on the detection of the isotopologues 12CH4 and 13CH4 by employing a quartz-enhanced photoacoustic spectroscopy (QEPAS)-based sensor. By properly selecting the exciting light source and the working conditions, two absorption lines, having a negligible cross-section ratio temperature coefficient of -6.7‰/°C and a cross section ratio of ~ 0.06 for a natural abundance of each isotope, can be targeted. The QEPAS signal of the two isotopologues was acquired for mixtures in nitrogen of methane in natural abundance in a wide range of concentrations (0.02%-20%) showing a non-linear trend with high concentrations and a constant ratio comparable with the cross-section.