High-accuracy gas analysis via isotope dilution mass spectrometry: carbon dioxide in air

An absolute method, based on isotope dilution mass spectrometry, is described for the determination of atmospheric concentrations of carbon dioxide (CO2) in dry air. In this study, the relative amounts of sample and spike gases are measured manometrically under temperature control before blending. The spike CO2 composition is approximately 0.1 atom % 13C while the oxygen isotopic composition is "normal". Exhaustive assessment of potential error sources leads to accountability of observed imprecision and determination of accuracy confidence intervals (CI). The imprecision interval (95% CI) about the mean is smaller than +/- 0.1% (+/- 0.4 mumol/mol) while the accuracy interval (95% CI) is +/- 0.15% (+/- 0.52 mumol/mol) for air having a CO2 concentration of about 350 mumol/mol. Calculated concentrations of CO2 are statistically indistinguishable from those generated by gravimetry, an independent method of analysis. In this study, the major contributors to uncertainty and imprecision are the predetermination of the gas volume ratio and the measurement of the isotopic composition of the blended CO2, respectively.