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

Abstract

A method was devised for analysing small discrete gas samples (50mL syringe) by cavity ring-down spectroscopy (CRDS). Measurements were accomplished by inletting 50mL syringed samples into an isotopic-CO2 CRDS analyser (Picarro G2131-i) between baseline readings of a reference air standard, 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 > 20000ppm and δ13C-CO2 values from -100 up to +30000≈ in comparison to VPDB (Vienna Pee Dee Belemnite). Throughput was typically 10 samplesh-1, with 13h-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 2109ppm xCO2 and δ13C-CO2 levels varying from -27.3 to +21740≈. Repeatability tests demonstrated that method precision for 50mL samples was ca. 0.05% in xCO2 and 0.15≈ in δ13C-CO2 for CO2 compositions from 300 to 2000ppm 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 application 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.