Species-specific isotope-ratio measurements of volatile tin and antimony compounds using capillary GC-ICP-time-of-flight MS

The analytical performance of an axial inductively-coupled-plasma time-of-f light mass spectrometer (ICP-TOFMS) as a detector for fast transient chroma tographic signals resulting from the coupling to capillary gas chromatograp hy (CGC) was investigated. A cryotrapping GC-ICP-TOFMS method for the deter mination of volatile metal(loid) compounds (VOMs) in gases was used and the suitability of the TOF mass analyzer for multielemental speciation analysi s and multi-isotope ratio determinations was studied in terms of accuracy a nd precision. Isotope ratios Sn-118/Sn-120 and Sb-121/Sb-123 have been dete rmined in in-house gas standard atmospheres in Tedlar bags at two different levels (100 pg and I ng) for different elemental species (SnH4, MeSnH3, Me 2SnH2, Me3SnH, BuSnH3, SbH3, and MeSbH2). A limitation arising from countin g statistics in both detection modes could be shown. A solution containing rhodium (10 ng mL(-1)) and cadmium (40 ng mL(-1)) was introduced simultaneo usly to the GC outlet. Rhodium acts as a continuous internal standard and C d is used for mass-bias correction (by measuring the Cd-111/Cd-113 ratio). The detection system in both pulse counting and analog mode was examined. T he best attainable precision was established for Me2SnH2 (analog mode, 12 r eplicates, 1 ng, RSD 0.34%, accuracy 0.31%) whereas most other species rang ed between 0.4 and 0.5% RSD if higher concentrations were used. The limitat ions of the pulse counting system are clearly seen, with peak heights of mo re than 2000 counts reaching saturation (for an integration time of 100 ms) , which reduces the accuracy of isotope ratio determinations. A dozen VOM c ould be detected in an aged landfill gas sample; several unidentified Sn co mpounds were present. Although their isotope ratios are within the confiden ce value of the standards, it is not yet clear if the acquired precision is good enough to identify isotopic fractionation of metal(loid)s through bio volatilization processes. With the precision achieved, the combination of c ryotrapping GC and ICP-TOFMS is a powerful tool for monitoring volatile mul ti-element species in multi-tracer experiments and isotope dilution methodo logy.