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 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 ???20?000?ppm and δ13C–CO2 values from ?100 up to +30?000?‰ in comparison to VPDB (Vienna Pee Dee Belemnite). 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 +21?740?‰. 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 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.
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