Measurement of the 13C/12C of atmospheric CH 4 using near-infrared (NIR) cavity ring-down spectroscopy

Abstract

A near-infrared (NIR) continuous-wave-cavity ring-down spectrometry (CW-CRDS) device was developed with the goal of measuring seasonal changes in the isotopic composition of atmospheric CH4 on Earth and eventually on Mars. The system consisted of three distributed feedback laser diodes (DFB-LDs), two of which were tuned to the absorption line peaks of 12CH4 and 13CH4 at 6046.954 cm -1 and 6049.121 cm-1, respectively, and a third that measured the baseline at 6050.766 cm-1. The multiple laser design improved the long-term stability of the system and increased the data acquisition rate. The acquisition frequency was further increased by utilizing a semiconductor optical amplifier (SOA) to initiate cavity ring-down events. The high repetition rate combined with the superhigh reflectivity mirrors yielded precise isotopic measurements in this NIR region, even though the line strengths of CH4 in this region are 200 times weaker than those of the strongest mid-IR absorption bands. The current system has a detection limit of 1.9 × 10-12 cm-1, corresponding to 10 pptv of CH4 at 100 Torr. For ambient air samples that contained 1.9 ppmv CH4, the δ13C of the CH4 was determined to be -48.7 ± 1.7‰ (1σ). © 2013 American Chemical Society.