Isotope ratios on transient signals with GC-MC-ICP-MS

ICP-MS instruments with multiple ion collectors (MC-ICP-MS) provide highly precise and accurate isotope ratio measurements. These instruments are easily coupled to chromatography, laser ablation or flow injection devices because of the ICP source, and thus attract general analytical interest for measurement of highly precise element isotope ratios on transient signals. Some few papers have been published today where MC-ICP-MS was employed for isotope ratios on transient signals, and in general, results are encouraging. Isotope ratio precision obtained is usually at least 10-fold superior compared to measurements with ICP-QMS. Nevertheless, most authors describe a drift of isotope ratios during the transient peak. This behaviour was either interpreted as a problem in the sample introduction system, or was related to instrumental mass bias drift, but has to date not been completely evaluated or explained. In this paper, we systematically investigate the drift observed on lead and mercury isotopes during short transient signals obtained from GC coupled to different MC-ICP-MS systems. We can clearly show that neither changes in instrumental mass bias, nor chromatographic fractionation effects are the source of the observed isotope ratio drift. An influence of analyte concentration on the observed drift was as well excluded as a source for this drift, because the slope on isotope ratios show the same values for different concentrations evaluated. In contrast, the peak width was found to influence the extent to which the isotope ratio drifts during peak elution. Thus, the relative intensity change per time seems to be an important factor for the measurements of transient signals with MC-ICP-MS.