Determination of stable silicon isotopes using multi-collector inductively coupled plasma mass spectrometry

An instrumental analysis method for stable silicon (Si) isotope ratios using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) was established in this study. In dry plasma mode, silicon isotopes suffered from the spectral polyatomic interferences of C, N, O and H at medium resolution. The sample gas flow had significant effect on the Si sensitivity and signal stability. Moreover, higher sample gas flow led to higher production of ¹⁴N¹⁶O. Consistent δ²⁹Si and δ³⁰Si within 0.04 ‰ (1σ) was obtained over a mass range of ca. 0.009 amu at the lower mass side of Si at medium resolution. The concentration between the samples and standards was matched within 20% to obtain accurate mass bias correction using standard-sample-bracketing correction (SSB) method. The measurements were not sensitive to the acid molarity and Cl matrix. An internal precision better than 8 ppm (1σ) was achieved for ²⁹Si/²⁸Si and ³⁰Si/²⁸Si by taking an integration time of 8 s. The long-term reproducibility of 0.06‰-0.10‰ (2σ, n = 20) was obtained for δ²⁹Si and δ³⁰Si by repeated analysis of the silicon reference materials, whilst the determined isotopic value for GBW04421 and GBW04422 were in good agreement with the recommended values, suggesting the precision and robustness of this method. The measured silicon isotopic values of fresh waters (river and lake waters), saline waters and seawater demonstrated the capability of this method to trace the biogeochemical processes of silicon in natural waters.