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.