Chromatographic purification for the determination of dissolved silicon isotopic compositions in natural waters by high-resolution multicollector inductively coupled plasma mass spectrometry

A procedure is described for accurate Si isotope ratio measurements by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Dissolved silicon was preconcentrated and separated from other elements present in natural surface waters using anion-exchange chromatography. The optimized procedure provides virtually complete elimination of major inorganic constituents while maintaining Si recovery in excess of 97%. High-resolution capabilities of MC-ICPMS used in this study allow interference-free measurements of Si-28 and Si-29 isotopes using conventional solution nebulization sample introduction without aerosol desolvation. Owing to the magnitude of polyatomic ion contributions in the region of mass 30, mostly from (NO+)-N-14-O-16, measurements of the Si-30 isotope can be affected by tailing of the interference signals, making exact matching of analyte and nitric acid concentrations in all measurement solutions mandatory. Isotope abundance ratio measurements were performed using the bracketing standards approach and on-line correction for mass-bias variations using an internal standard (Mg). Uncertainties, expressed as 95% confidence intervals, for replication of the entire procedure are better than +/- 0.18 parts per thousand for delta Si-29 and 0.5 parts per thousand for delta Si-30. For the first time with MC-ICPMS, the quality of Si isotope abundance ratio measurements could be verified using a three-isotope plot. All samples studied were isotopically heavier than the IRMM-018 Si isotopic reference material.