Discrete sample introduction module for quantitative and isotopic analysis of methane and other gases by cavity ring-down spectroscopy

Carbon dioxide (CO₂) and methane (CH₄) are natural and anthropogenic products that play a central role in the global carbon cycle and regulating Earth’s climate. Applications utilizing laser absorption spectroscopy, which continuously measure concentrations and stable isotope ratios of these greenhouse gases, are routinely employed to measure the source and magnitude of atmospheric inputs. We developed a discrete sample introduction module (DSIM) to enable measurements of methane and CO₂ concentrations and δ¹³C values from limited volume (5–100 mL) gas samples when interfaced with a commercially available cavity ring-down spectroscopy (CRDS) analyzer. The analysis has a dynamic range that spans six orders of magnitude from 100% analyte to the lower limit of instrument detection (2 ppm). We demonstrate system performance for methane by comparing concentrations and δ¹³C results from the DSIM-CRDS system and traditional methods for a variety of sample types, including low concentration (nanomolar CH₄) seawater and high concentration (>90% CH₄) natural gas. The expansive concentration range of the field-portable DSIM-CRDS system can measure enhances analytical performance for investigating methane and CO₂ dynamics and, potentially, other gases measured by laser absorption spectroscopy.