Development of a continuous-flow GC-IRMS method for the simultaneous determination of CO2 concentration and isotopic composition (δ13CCO2) in volcanic and geothermal gases.

This paper describes a simple method for the simultaneous determination of the isotopic composition (δ13CCO2) and concentration of CO2 in volcanic and geothermal gases using a continuous flow mass spectrometry system (CF-IRMS). The instrumental configuration includes a Thermo Fisher Scientific Delta Q mass spectrometer, connected through a Conflo IV to a Thermo Trace 1610 gas chromatograph (GC) physically connected at the module Isolink II. The performance of the developed GC-IRMS method was evaluated through a comprehensive series of tests assessing the precision and accuracy of the δ13C values and the simultaneous determination of CO2 concentrations. Specifically, we evaluated instrumental precision and isotopic data reproducibility (δ13CCO2) under various gas chromatographic operating conditions (split ratio) using gas mixture and certified isotopic standards; concentration calibration curves were specifically developed at different split ratios (3, 20, 35, and 45) using certified gas mixtures to quantitative CO2 determination with a wide concentration range (from 0.06-100 % vol.). The concentration data were compared with those obtained with traditional analytical techniques (GC and IR). Subsequently, the entire system performance, for both isotopic composition and CO2 concentration, was validated on volcanic and geothermal gases. These samples, including fumaroles, bubbling pools, and soil gas emissions, were selected to test the simultaneous determination capability under real-world conditions. The results confirm that the achieved operating configuration ensures optimal performance for the simultaneous determination of δ13CCO2 and CO2 concentrations, guaranteeing a precision of ± 0.08 ‰ and an accuracy within ± 0.1 ‰ for isotopic data and a quantitative accuracy typically within ± 5 % for concentration data.