Silicon Isotope Compositions of Soil and Sediment Reference Materials Determined by MC-ICP-MS

This study presents a method for high-precision Si isotope measurement of soil and sediment reference materials using multi-collector inductively coupled plasma-mass spectrometry with HF-free digestion. Potential impacts on instrumental measurement, including acid concentration, Si concentration mismatch and matrix effects, were rigorously evaluated. The acidity mismatch caused a significantly negative shift in delta Si-30 values when the HCl concentration ratios of the sample/standard were higher than 2. The Si concentration mismatch could result in an obvious deviation in delta Si-30 values when the sample/standard concentration ratios are > 1.5 or < 0.7. Matrix elements in solution did not affect Si isotope measurements when Na/Si <= 2, Mg/Si <= 2, Al/Si <= 0.5, K/Si <= 2, Ca/Si <= 1 and Fe/Si <= 2. Increasing the loading volume from 1 ml to 6 ml during chemical separation can still enable effective purification of Si and is more efficient for low Si content samples. The long-term measurement precision of three igneous rock reference materials (BHVO-2, BCR-2 and AGV-2) was better than +/- 0.06 parts per thousand (2s) for delta Si-30 values. The delta Si-30 values of eighteen soil and sediment reference materials were identical in two laboratories. The values varied from -1.65 parts per thousand to -0.13 parts per thousand, indicating that Si isotopes can be more widely used to study supergene processes.

Automated summary

This study introduces a method for high-precision Si isotope measurement of soil and sediment reference materials using HF-free digestion and multi-collector inductively coupled plasma-mass spectrometry. It rigorously evaluates potential impacts on instrumental measurement, such as acid concentration, Si concentration mismatch, and matrix effects. The results show that acidity and Si concentration mismatch can cause significant deviations in delta Si-30 values, highlighting the importance of careful sample preparation for accurate Si isotope measurements.