Isotopic composition of silicon measured by multicollector plasma source mass spectrometry in dry plasma mode

Abstract

Precise Si isotopic fractionation in silica (quartz), opal (diatomite, sponges) and standards have been determined employing a Nu Instruments Plasma multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) in dry plasma mode, using a Cetac Aridus desolvating nebulization system. Variations in sample ²⁸Si/²⁹Si ratios are expressed as δ²⁹Si units, which represent deviations in ‰ from the same ratio of the NBS28 standard measured using the standard-sample bracketing technique. We measured Mg isotopes in dynamic mode for adequate correction of instrumental mass bias, applying the fractionation exponential law and external normalisation. The repeatability and the internal precision on the δ²⁹Si measurements of a 1 ppm Si solution in diluted HF/HCl is better than ±0.1‰ (±2 σ). The accuracy has been assessed by cross analyses of an in-house standard with the laser fluorination-isotope ratio mass spectrometry (IRMS). The dry plasma methodology improves the sensitivity more than ten times compared to wet plasma MC-ICP-MS and hence is of great relevance regarding environmental, biogeochemical and geological sciences where Si isotopes open up a range of new studies.