Determination Of Si Isotopic Compositions Of Geological Samples Using Solution Nebulization High Resolution Multi- Collector Inductively Coupled Plasma Mass Spectrometry

In this paper, the silicon isotope compositions in rock samples were analyzed using a Nu Plasma 1700 multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Rock sample digestion and Si isotope purification procedures were established. The Si isotope analytical method was setup using MC-ICP-MS. Detailed experiments were carried out in order to obtain accurate and precise Si isotopes, including samping method, mass resolution, concentration effect and mass discrimination etc.. The Si isotopic compositions of some international rock standards measured in this work agreed well with the published data within 2s measurement uncertainties.1. Rock powder was mixed with NaOH which is used as flux in double Ag crucible and fused for 10 min in a muffle furnace, then the melt was dissolved using ultra pure water and acidified with HC1. The Si isotope purification procedures was using the AG 50-X12 cation resin chromatography. The blank of total procedure is less than 20 ng and the recovery of Si isotopes was higher than 98%.2.The MC-ICP-MS was run at high-resolution mode (M/?M = 10000 RP) to resolve any polyatomic ion interferences, such as 14N2+?14N21H+?14N16O+ etc.. The Si isotopic compositions were calculated after a standard-sample bracketing calibration method (SSB) to correct for mass discrimination effect. Experiments showed that wet plasma is more stable than dry plasma mode, and ultra pure water is superior to acid in terms of wash out efficiency. The RF power and nebulizer gas flow rate of MC-ICP-MS needed to be optimized carefully due to serious influence on the accuracy of Si isotopes. Experiments also showed that the difference of Si concentration between sample and standard was positively correlated with the silicon isotope ratio, and the concentration difference should remain <20% so as to the diversity of Si isotopic compositions is less than 0.04%o.3.The accurate and precise determination of Si isotopes was achieved using an internal standard (Mg) coupled with SSB method, since the internal standard could calibrate the mass discrimination effects and space-charge effectively. Two magnesium isotopes (25Mg and 26Mg) and three silicon isotopes (28Si,29Si and 30Si) were analyzed in the same data collection cycle to improve the accuracy and reduce the requirements of instrument stability by moving the Faraday cups handly (both the lowest and highest mass side) to their limit end positions. The proposed SSB combine with Mg calibration method reduce the demands of laboratory and instrumental stabilities which affected the accuracy and precision of Si isotope measurement.4. Si isotopic compositions of BHVO-2, IRMM-018a, BCR-2, BIR-1 and AGV-2 were measured using the SSB method devloped in this work. The analytical results agreed well with the published data within 2s analytical uncertainties. The overall precision of S30Si was better than 0.07‰/amu. The Si isotopic compositions of BHVO-2, IRMM-018a by SSB+ Mg calibration method were consistent with the previous data within 2s analytical uncertainties. The precisions of ?3OSi were better than 0.05‰/amu.