GEMOC ARC National Key Centre

DETERMINATION OF HIGH PRECISION ISOTOPE RATIOS BY LA-MC-ICP-MS

S. E. Jackson¹, W. L. Griffin^1,2, and N. J. Pearson¹

¹GEMOC Macquarie

²CSIRO Exploration and Mining

Introduction: The multi-collector (MC)-ICP-MS makes determination of high precision isotope ratios of wide range of elements routinely possible. Furthermore, the lack of time-dependent differential ionisation of elements allows elemental spikes to be used for mass bias correction and isobaric overlap corrections to be made, which permit effective use of laser ablation (LA) sampling for in situ determination of high precision isotope ratios.

Instrumentation: The instrumentation used was a Merchantek LUV266 Nd:YAG laser ablation sampler operating at a wavelength of 266nm. This is coupled to a Nu Plasma MC-ICP-MS which features an ICP coupled to a double focusing electrostatic/magnetic mass analyser. Novel variable dispersion ion optics focus the ions into a fixed collector array consisting of 12 faraday cups and 3 ion counting detectors. Both instruments were installed at Macquarie University in November 1998.

Methodology: Mass bias correction was effected using two procedures: (1) calculation of mass bias using a stable isotope ratio of the element of interest, where present (e.g., Sr, Hf); (2) where no fixed stable isotope ratio is available (e.g., Cu, Pb, etc.), mass bias was determined from another element, which was introduced, via a T-junction, into the sample carrier gas stream as a "dry" aerosol generated using a CETAC MCN6000 desolvating nebulisation system. For laser analyses, data were acquired using a time resolved analysis acquisition protocol which reports signals 10 times/second, thereby allowing assessment of isotope ratios as a function of ablation time. Here we report several critical observations on isotopic fractionation during laser ablation sampling.

Hf isotopes Hf isotopes were measured on several zircons [1] and LIMA minerals [2]. Mass bias correction was achieved using the stable isotope ratio¹⁷⁹Hf/¹⁷⁷Hf. Significant changes were observed during each ablation analysis in the uncorrected ¹⁷⁹Hf/¹⁷⁷Hf ratios which correlated inversely with total Hf signal intensity. However, corrected ¹⁷⁶Hf/¹⁷⁷Hf showed no relationship with ablation time. Multiple analyses of two zircon standards gave highly precise ¹⁷⁶Hf/¹⁷⁷Hf ratios that were in agreement with TIMS data.

Sr isotopes Sr isotope analyses of several LIMA minerals [3] were made using the stable ⁸⁸Sr/⁸⁶Sr ratio for mass bias correction. As with Hf isotopes, the stable isotope ratio generally showed an inverse relationship with total signal intensity during ablation.

Sb isotopes Sb isotope measurements were made on Sb-bearing native silver and sulphide minerals using Sn introduced via the MCN6000 for mass bias correction. Strong within-run variation (up to 1%) in ¹²³Sb/¹²¹Sb ratios, showing a positive correlation with total Sb signal, were typical while ¹²⁴Sn/¹²²Sn ratios showed minimal or no correlation with signal intensity.

Cu isotopes Cu isotope ratios were measured on chalcopyrite and Cu metal using Zn introduced via the MCN6000 for mass bias correction. Strong within-run variation (up to 1%) in uncorrected ⁶⁵Cu/⁶³Cu ratios were typical, which, like Sb, showed a positive correlation with total signal, while ⁶⁴Zn/⁶²Zn ratios showed minimal or no correlation with signal intensity. LA analyses of chalcopyrite and Cu metal gave ⁶⁵Cu/⁶³Cu ratios that were significantly lower than analyses of the same samples digested and analysed by solution introduction

Discussion: Both lithophile (Hf and Sr) and chalcophile (Sb and Cu) elements showed within-run isotopic variations during laser ablation sampling. For the lithophile elements, the heavy/light ratios showed a negative correlation with total signal intensity, whereas, for the chalcophile elements, the heavy/light ratios showed a positive correlation. Elements introduced via the MCN6000 showed minimal within-run isotopic variations. The different isotopic behaviour of elements derived by laser ablation and the MCN 6000 suggests that observed fractionations cannot be attributed to the MC-ICP-MS and that the laser sampling/transport process can fractionate isotopes to a very significant degree. The isotopically light Cu isotope ratios produced by laser ablation are in accord with a fractionation mechanism involving differential volatilisation and condensation processes during laser sampling/transport.

The relationships between the observed isotopic fractionation patterns and ablation conditions are not yet understood. However, strong elemental fractionation of lithophile and chalcophile elements can occur during laser sampling, suggesting that their ablation mechanisms may be fundamentally different.

Highly precise and accurate Hf isotope data indicate that mass bias correction using a stable isotope ratio of the element of interest adequately corrects laser sampling-related fractionation. However, when mass bias is corrected using an introduced element, laser sampling-related fractionations are detrimental to the precision and accuracy of the technique. Experiments using alternative transport gases and laser cell designs are underway to further investigate and deal with the problem.

References: [1] Griffin W.L. et al. (subm.) Geochim. Cosmochim. Acta. [2] Griffin W.L.et al. (subm.) Ninth Annual V.M. Goldschmidt Conf.