Abstract
Silicon isotope analysis traditionally uses a standard-sample bracketing (SSB) method that relies upon greater instrument stability than can be consistently expected. The following proposed method reduces the level of instrumental stability required for the analysis process and provides a valid solution for high-precision and accurate studies of Si isotopic compositions. Rock samples were dissolved by using alkali fusion and acidification. Silicon isotopes were purified with an ion exchange resin. Interfering peaks for isotopes were separated by using a Nu Plasma 1700 multi-collector inductively coupled plasma mass spectrometry (MS) system in high-resolution mode (M/ΔM > 8000 RP). Two magnesium isotopes (25Mg and 26Mg) and three silicon isotopes (28Si, 29Si, and 30Si) were analyzed in the same data collection cycle. Mg isotopes were used as an internal standard to calibrate the mass discrimination effects in MS analysis of Si isotopes in combination with the SSB method in order to reduce the effects of MS interference and instrumental mass discrimination on the accuracy of measurements. The conventional SSB method without the Mg internal standard and the proposed SSB method with Mg calibration delivered consistent results within two standard deviations. When Mg was used as an internal standard for calibration, the analysis precision was better than 0.05 ‰ amu.
Highlights
Silicon is the second-most abundant element in Earth’s crust; it makes up the skeleton of rocks and has stable chemical properties
Si isotopic composition is normally determined by using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), whose accuracy and precision are mainly affected by interfering mass spectrometry (MS) peaks for isobaric heterotopes, instrumental mass discrimination effects, and the stability of the sample injection system
Rock samples were dissolved by alkali fusion and acidification, and silicon isotopes were purified with ion exchange resin for a recovery rate above 98 %
Summary
Silicon is the second-most abundant element in Earth’s crust; it makes up the skeleton of rocks and has stable chemical properties. Fluctuations in instrumental conditions that affect the accuracy of the isotopic ratio analyses are mainly caused by changes in the sensitivity of the sample injection system or in the mass discrimination effects in the MS system. These are usually calibrated using two isotopes of elements with similar atomic weights (e.g., adding 203Tl and 205Tl for isotopic analysis of Pb). Because of previously experienced hardware limitations, they analyzed Mg and Si isotopes in two different data collection cycles during the measurement process; simultaneous analysis of Mg and Si isotopes could not be achieved, and so the changes in the measured Si isotopic composition caused by variations in the instrumental conditions were not effectively calibrated. Exchange resin (Catalog # 142-1651, Bio-Rad Laboratories Inc., CA, USA)
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