A NOVEL MULTI-PASS CELL SYSTEM FOR MEASUREMENTS OF METHANE CARBON ISOTOPES USING A NEAR-INFRARED TUNABLE DIODE LASER

Abstract

A novel multi-pass cell laser absorption spectrometer to measure δ¹³C diluted with nitrogen using a 1.658 μm near-infrared distributed-feedback laser was developed. The system temperature and pressure were measured with an accuracy of ±11 mK and ±3.2 Pa, respectively. The spectrometer is small, reliable, and compact. To minimize the temperature effect in absorbance ratio measurements, we used two appropriate absorption line pairs with nearly the same temperature dependences. The absorption spectrometer demonstrated high precision (a repetitive precision (reproducibility) of ~0.9‰ (1s) is obtained at 300 ppmv methane concentrations). This concentration ratio is adequate for most CH₄ source characterization studies.