Determination of mass-dependent molybdenum isotopic variations by MC-ICP-MS: An evaluation of matrix effects

Several analytical techniques are currently used to determine mass-dependent molybdenum isotopic variations in natural materials using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), including different methods for the separation of Mo from the sample and the correction for instrumental mass-dependent isotopic fractionation (instrumental mass bias). Both internal (“double-spiking” using two enriched Mo isotopes) and external (“zirconium doping” with standard-sample bracketing) techniques have been used in previous studies to deal with the effects of instrumental mass bias. The results of these studies have indicated that the precision for Mo isotopic analyses of natural (matrix-bearing) samples is a factor of ∼4–7× better using a double spike. Here we present a detailed study of the ability of MC-ICP-MS to determine, both precisely and accurately, the isotopic composition of Mo extracted from molybdenite using a low blank, high yield two-column procedure for Mo separation and a simple standard-sample bracketing approach to correct for instrumental mass bias. Based on analyses of molybdenites, the precision of this technique is shown to be similar to published double-spike data (within a factor of ∼2). All three of the known types of potential matrix effects in the MC-ICP-MS are also evaluated: automatrix effects, matrix effects due to Zr doping and matrix effects due to elements in the sample other than Mo and Zr. Each of these matrix effects is found to be either insignificant or controllable. Analyses of five molybdenites of hydrothermal origin reveal a range in their Mo isotopic composition that is a factor of ∼4 greater than the previous range reported for such samples. More detailed work is required to elucidate the origin of these mass-dependent Mo isotopic variations in molybdenites.