Molybdenum isotope geochemistry |
Determination of mass-dependent molybdenum isotopic variations by MC-ICP-MS: An evaluation of matrix effects
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| 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. |
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Automatrix effects in the MC-ICP-MS. These data were collected over three separate analytical sessions (diamonds, circles and squares) by repeatedly analyzing a Mo solution standard (without Zr). In one case (diamonds), the same solution was analyzed repeatedly on one day. In two other cases (circles and squares), an aliquot of the Mo solution standard was progressively diluted over the course of the day (and run alternately with the original undiluted solution) to determine the nature and magnitude of the automatrix effect. The delta 97/95Mo value (calculated using simple standard-sample bracketing) shows the deviation between the dilute solution compared to the concentrated solution (circles and squares) or every other analysis of a solution of constant Mo concentration (diamonds). The delta 95Mo signal intensity represents the % difference in the signal intensity between the standard treated as an unknown and bracketing standards. Negative values indicate more dilute, low signal intensity solutions.
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Matrix effects in the MC-ICP-MS due to other elements. The correction for instrumental mass bias was performed using both simple (green symbols) and external normalization (blue symbols) standard-sample bracketing. The atomic proportions of the mixture were Na/Mo=0.027, Mg/Mo=0.014, Nb/Mo=0.0090, Cd/Mo=0.045, Sb/ Mo=0.018, W/Mo = 0.0057 and Tl/Mo= 0.0043.
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Mass-dependent Mo isotopic variations for molybdenites. All delta 9X/95Mo values are expressed relative to a Mo solution standard. The dashed lines represent the expected mass-dependent fractionation lines based on the mass differences between the isotopes used for the ratios on each plot.
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