Development of a continuous-flow GC-IRMS method for the simultaneous determination of CO₂ concentration and isotopic composition (δ¹³C_CO2) in volcanic and geothermal gases

This paper describes a simple method for the simultaneous determination of the isotopic composition (δ¹³C_CO2) 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 (δ¹³C_CO2) 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 δ¹³C_CO2 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.