Controlling the mass bias introduced by anionic and organic matrices in silicon isotopic measurements by MC-ICP-MS

Abstract

One of the most widely used sample preparation methods for Si isotopic analyses (δ³⁰Si and δ²⁹Si) is based on cationic chromatography, which does not remove anions from samples. Although it was first thought that the presence of anions in natural concentrations does not distort the isotopic analyses, it has recently been shown that the presence of sulfate can induce a significant shift in isotopic ratio measurements above SO₄²⁻/Si ratios (wt) of 0.02. Here, we show that dissolved organic matter can also induce a major Si isotopic bias when analysing river waters. To overcome these non-spectral matrix effects we propose fast and reliable ways, tested on natural freshwater and rock digestion solutions. The sulfate matrix effect is solved by adding to both sample and bracketing standard sulfuric acid in large excess compared to the naturally occurring SO₄²⁻. The organic matrix is mineralized by the combined action of UV-C and ozone. We also provide the first δ³⁰Si signature measurements of two common geostandards: SGR-1 (δ³⁰Si = +0.03‰) and FeR-1 (δ³⁰Si = −0.20‰).