Biosynthesis of sulfur-34 labelled yeast and its characterisation by multicollector-ICP-MS

Yeast labelled with ³⁴S (92.82% enriched) has been prepared by yeast growth on a ³⁴S-enriched, specially prepared, culture medium in the absence of natural abundance sulfur. The final product has been characterised both with reference to isotope enrichment and total sulfur concentration by isotope dilution analysis and isotope pattern deconvolution using a MC-ICP-MS instrument. Additionally, two different sulfur spike solutions of ³³S (99.70% enrichment) and ³⁴S (99.37% enrichment) were characterised by MC-ICP-MS with reference to a natural abundance sulfur standard solution. Spectral interferences on the determination of the isotopic composition of all sulfur solutions were also evaluated using a double focusing, single collector ICP-MS instrument. The more serious interferences observed were the presence of ³²SH⁺ and ³³SH⁺, which could be important for the natural abundance sulfur (interference on ³³S) and the ³³S-enriched sulfur (interference on ³⁴S), respectively. Fortunately, those interferences could be eliminated using the pseudo-high resolution mode of the multicollector instrument. The analytical procedure used was validated by the analysis of a blood serum reference material, which contained a certified concentration of sulfur, both using the ³⁴S and the ³³S spikes either separated or in combination using isotope pattern deconvolution. The results found were in agreement with the certified values.