HR: 0800h
AN: PP31B-1541    [Abstracts]
TI: Development of the Molybdenum Isotopic System for Paleoenvironmental Studies: Evaluation of the MC-ICP-MS Standard-Sample Bracketing Technique for Analyses of Marine Sediments
AU: * Reznik, A D
EM: reznik@uniserve.com
AF: Department of Geological Sciences, San Diego State University, San Diego, CA 92182
AU: Pietruszka, A J
EM: apietrus@geology.sdsu.edu
AF: Department of Geological Sciences, San Diego State University, San Diego, CA 92182
AB: 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; Siebert et al., 2001; G-cubed; v. 2) and external ("zirconium doping" with standard-sample bracketing; Barling et al., 2001; EPSL; v. 193) 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 times better using a double spike. Previously, we conducted a detailed study of the ability of MC-ICP-MS to determine, both precisely and accurately, the isotopic composition of Mo extracted from molybdenite (MoS2) 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 (Pietruszka et al., in press; Chem. Geol.). Based on analyses of molybdenites, the precision of this technique was shown to be similar to published double-spike data (within a factor of 2). In our previous study, all three of the known types of potential matrix effects in the MC-ICP-MS were 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 was found to be either insignificant or controllable. Here we report the discovery of an additional matrix effect. When a high-purity Mo solution standard is passed though a column of anion exchange resin, the isotopic composition of the standard is fractionated by 0.3‰ (lighter than the original standard) on the 97Mo/95Mo ratio even though the recovery of Mo is nearly complete (99.5%). This matrix effect disappears when the sample is run using "wet" plasma mode (as opposed to using a desolvation system, which affords superior precision). We are currently attempting to solve this problem, which appears to result from the partial breakdown of the resin during column chemistry. Once successful, we will evaluate the precision and accuracy of our final technique using analyses of a range of marine sediments that might typically be used for paleoenvironmental studies.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1094 Instruments and techniques
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005