Sample matrix effects on measured carbon and oxygen isotope ratios during continuous-flow isotope-ratio mass spectrometry

摘要

Continuous-flow isotope-ratio mass spectrometry (CF-IRMS) is frequently used to analyze CO_2 found in media such as air, breath, and soil pore space gas with the aid of a sample preparation and transfer device such as a Gasbench II. This study investigated the effect that matrices other than helium (He) have on the measured δ~(13)C and δ~(18)O isotope ratios of CO_2. METHODS: Identical CO_2 was added to sample vials with matrices of pure He, pure N2, or a 21:79 mixture of O_2/N_2 and analyzed by a ThermoFinnigan Delta~(Plus) XP isotope-ratio mass spectrometer coupled to a ThermoFinnigan Gasbench II. Variables such as CO_2 concentration, sample analysis sequence, and sample matrix removal ('blanking') through manipulation of an injection and dilution open split were tested to identify systematic isotope ratio offsets between the different matrix types. RESULTS: The process of blanking induced a δ~(13)C and δ~(18)O offset of ≤0.2‰between otherwise identical populations of CO_2 samples in He. The ~(13)C/~(12)C and ~(18)O/~(16)O isotope ratios of CO_2 sampled from pure N_2 or a mixture of O_2/N_2 were found to be within 0.1 to 0.2‰of those of an identical CO_2 sampled from a He matrix when N_2 or O_2/N_2 was removed prior to transport to the mass spectrometer. The measured oxygen isotope ratios of CO_2 sampled from N_2 and O_2/N_2 varied by as much as 0.6‰ and 4‰, respectively, if matrix gas was not removed prior to ionization. CONCLUSIONS: Sampling CO_2 from matrices similar to air does not significantly affect the measured ~(13)C/~(12)C and ~(18)O/~(16)O isotope ratios of CO_2 when a gas-handling procedure that includes the removal of matrix gas is utilized. This procedure is much preferable to introducing matrix gas and potentially isobaric interference to the ion source.