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-dependent differences in the isotopic composition of Mo between samples and reference standards using multiple-collector magnetic sector inductively coupled plasma mass spectrometry (MC-ICPMS). Either Zr or Ru "element spikes" are employed to correct for instrumental mass bias. Differences in ^95Mo/^97Mo 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 for isobaric interferences from either spike element. This approach facilitates study of mass-dependent variations in the isotopic composition of Mo in nature and in materials produced by laboratory processes. We observe fractionation of Mo isotopes of ~1.5% amu during ion-exchange chromatography in the laboratory and a shift of ~0.3%o/amu between natural MoS2 and a laboratory standard.