System for δ13C-CO2 and xCO2 analysis of discrete gas samples by cavity ring-down spectroscopy

Measuring discrete gas samples on an isotopic laser-absorbtion gas analyser

A method was devised for analysing small discrete gas samples (50 ml syringe) by cavity ring-down spectroscopy (CRDS). Measurements were accomplished by inletting 50 ml syringed samples into an isotopic-CO2 CRDS analyser (Picarro G2131-i) between baseline readings of a standard reference air, which produced sharp peaks in the CRDS data feed. A custom software script was developed to manage the measurement process and aggregate sample data in real-time. The method was successfully tested with CO2 mole fractions (xCO2) ranging from <0.1 to >20000 ppm and δ13C-CO2 values from -100 up to +30000 ‰ vs VPDB. Throughput was typically 10 samples h-1, with 13 h-1 possible under ideal conditions. The measurement failure rate in routine use was ca. 1 %. Calibration to correct for memory effects was performed with gravimetric gas standards ranging from 0.05 to 2109 ppm xCO2 and δ13C-CO2 levels varying from -27.3 to +21740 ‰. Repeatability tests demonstrated that method precision for 50 ml samples was ca. 0.05 % in xCO2 and 0.15 ‰ in δ13C-CO2 for CO2 compositions from 300 to 2000 ppm with natural abundance 13C. Long-term method consistency was tested over a 9-month period, with results showing no systematic measurement drift over time. Standardised analysis of discrete gas samples expands the scope of applications for isotopic-CO2 CRDS and enhances its potential for replacing conventional isotope ratio measurement techniques. Our method involves minimal set-up costs and can be readily implemented in Picarro G2131-i and G2201-i analysers or tailored for use with other CRDS instruments and trace gases.