ISOTOPE RATIOS OF MOLYBDENUM DETERMINED BY THERMAL IONIZATION MASS-SPECTROMETRY FOR STABLE-ISOTOPE STUDIES OF MOLYBDENUM METABOLISM IN HUMANS

Methods were developed to separate and purify Mo from biological sampl es and to measure isotopic ratios in 1 mug of Mo. A magnetic sector, t hermal ionization mass spectrometer was used with simultaneous collect ion of five isotopes. Isotopic ratios were corrected for mass fraction ation by iterative normalization using the 96/98 ratio. Ion beam inten sity was enhanced by using a double-filament configuration, loading sa mples onto evaporation filaments with silica gel and boric acid. A tri ple-isotope-dilution approach was used, so the method could be applied to two-tracer studies of Mo metabolism in human subjects. Mo-94 was a dded to samples prior to purification to quantify the total Mo content of samples and to determine the amounts of enriched Mo-97 and Mo-100 appearing in urine and fecal samples of study participants. The three ratios, 94/98, 97/98, and 100/98, were determined with within-run prec ision of from 0.06 to 0.10% (RSD). Precision of the ratios between rep licates was from 0.05 to 0.08%.