Precise determination of mass-dependent variations in the isotopic composition of molybdenum using MC-ICPMS

We present an analytical approach for the precise determination of mass-dep endent differences in the isotopic composition of Mo between samples and re ference standards using multiple-collector magnetic sector inductively coup led plasma mass spectrometry (MC-ICPMS). Either Zr or Ru "element spikes" a re employed to correct for instrumental mass bias. Differences in Mo-95/Mo- 97 can be determined to a precision of +/- 0.2%o (+/-2 sigma) using 1-10 mu g of Mo. Similar precision is possible for other ratios after correction fo r isobaric interferences from either spike element. This approach facilitat es study of mass-dependent variations in the isotopic composition of Mo in nature and in materials produced by laboratory processes; We observe fracti onation of Mo isotopes of similar to1.5%o/amu during ion-exchange chromatog raphy in the laboratory and a shift of -0.3%o/amu between natural MoS2 and a laboratory standard.