Precise isotope-ratio determination by CGC hyphenated to ICP-MCMS for speciation of trace amounts of gaseous sulfur, with SF6 as example compound

Capillary gas chromatography coupled to an inductively coupled plasma mass spectrometer with multiple-collector detection (GC-ICP-MCMS) has been used to assess the precision and instrumental mass bias in sulfur isotope-ratio determination for the gaseous sulfur species SF6. The isotopic composition of the compound was certified by the institute for reference materials and measurements (IRMM, Belgium) and is available as PIGS 2010. Integration of the peaks (peak half-width 1.4 s) was performed using a special peak-integration method based on definition of the integration area by assessment of a uniform isotope-ratio area within the chromatographic peak. Instrumental mass bias was determined to be approximately 12% per mass unit and proved to be stable in the concentration range measured. Replicate injections of 2, 10, 20, and 30 ng (as S) SF6 diluted in argon gave precision for the S-32/S-34 ratio from 0.6% RSD for 2-ng injections to 0.03% RSD for 30-ng injections. The S-32/S-33 and S-33/S-34 isotope-ratio precision was better than 0.4% RSD for injections of 10 ng (as S) and higher. Detection limits were in the absolute pg range for all measured sulfur isotopes.