Enhanced sensitivity for Os isotope ratios by magnetic sector ICP-MS with a capacitive decoupling Pt guard electrode

A magnetic sector ICP-MS with enhanced sensitivity was used to measure Os isotope ratios in solutions of low Os concentration (similar to 1 ng g(-1) or less). Ratios with Os-192 as the basis were determined, while the geologically useful Os-187/Os-188 ratio was also measured. Sample introduction was via the traditional nebuliser-spray chamber method. A capacitive decoupling Pt shield torch was developed in-house and was found to increase Os signals by similar to 5 x under moderate plasma conditions (1050 W) over that found during normal operation(1250 W). Sensitivity using the guard electrode for Os-192 was similar to 250-350,000 counts s(-1) per ng g(-1) Os. For a 1 ng g(-1) Os solution with no guard electrode, precisions of the order of 0.2-0.3% (Os-189/Os-192 and Os-190/Os-192) to similar to 1% or greater (Os-186/Os-192, Os-187/Os-192 and Os-187/Os-188) were found (values as 1 sigma for n = 10). With the guard electrode in use, ratio precisions were found to improve to 0.2 to 0.8%. The total amount of Os used in the acquisition of this data was similar to 2.5 ng per measurement per replicate. At the higher concentration of 10 ng g(-1), precisions of the order of 0.15-0.3% were measured (for all ratios), irrespective of whether the shield torch was used. Ratio accuracy was confirmed by comparison with independently obtained NTIMS data. For both Os concentrations considered, the improvement in precision offered by the guard electrode (if any) was small in comparison to calculated theoretical values based on Poisson counting statistics, suggesting noise contributions from other sources (such as the sample introduction system, plasma flicker etc). At lower Os concentrations (to 100 pg g(-1)) no appreciable loss of ratio accuracy was observed, although as expected based on counting statistics, poorer precisions of the order of 0.35-3% (1 sigma, n = 5) were noted. Re was found to have a detrimental effect on the precision of Os ratios involving Os-187, indicating that separation of Re and Os samples is a necessary pre-requisite for highly accurate and precise Os isotope ratio measurements.