Mass bias: A comparison of solution and laser ablation MC-ICPMS

The development of in-situ isotope ratio measurementsusing LAM-MC-ICPMS has proceeded steadily over recentyears. To date the most widely used technique is the analysisof Hf isotopes in zircon and this application is now undertakenin many laboratories around the world. Other radiogenicisotope systems (e.g, Sr, Nd, Os, Pb) have been successfullymeasured in a variety of minerals but despite the demonstratedsignificance of spatially resolved measurements, these in-situmethods remain relatively restricted in their application. Themain limitations (in accuracy and precision) are the trace-levelabundance of the elements of interest and the magnitude ofisobaric overlap corrections related to parent/daughter ratios.In-situ analysis of ‘non-traditional’ stable isotopes has notprogressed as far as the radiogenic systems and this is mainlydue to a lack of understanding of the processes that contributeto isotopic fractionation during ablation and in the plasma.Recent studies have demonstrated that the origin ofisotopic fractionation in LAM-MC-ICPMS is the result of acombination of laser- and ICP-induced fractionation. Whereasinternal normalization using stable isotope pairs is able toaccount for the effects of these processes for radiogenicsystems, the in-situ analysis of mass dependent stable isotopesrequires careful standard-sample bracketing techniques usingmatrix-matched materials. The isotopic compositions ofdifferent aerosol particle size fractions of Cu metal show anenrichment of up to 0.5 per mil of the lighter 63Cu isotope inthe sub-250 nm particles. Isotopic fractionation is furtherenhanced by preferential ionization of the lighter isotopesfrom incompletely vaporised particles in the ICP. Analysis ofMg isotopes in olivine (Fo 92) indicates that there is nochange in particle size distribution with ablation time andpoints to the influence of the ICP on isotopic fractionation.The aims of this study are to provide a framework tounderstand the parameters and processes that control massbias in the ICP, especially the differences between solution(wet) and laser (dry) plasma. A series of experiments has beencarried out to investigate the contribution of plasma power,extraction voltage, gas flow, torch position, gas composition(Ar±He), sample matrix, and plasma loading on mass bias.Results will be presented for light, middle and heavy massisotopic systems (e.g. Mg, Cu, Sr, Hf).