Mass Independent Fractionation Correction For Measurements By MC-ICPMS

As one of the most widely used research tools in mass spectrometry analyses,Multicollector Inductively Coupled Plasma Mass Spectrometry?MC-ICPMS?can be used to obtain isotope ratios with high-accuracy and high-precision.Since 1940s,traditional mass dependent fractionation?MDF?correction theory has been used to measure the nuclide mass quantitatively.However,recent researches show that the mass independent fractionation?MIF?phenomenon which exists during the measurements using MC-ICPMS may cause significant effects in terms of choosing the mass bias fractionation correction model.In the use of the traditional mass bias fractionation correction model,the mass independent fractionation cannot be corrected.Among the normal correction models for MC-ICPMS?such as double spike correction model,standard-sample bracketing correction model,internal normalization correction model,regression correction model and gravimetric isotope mixture model?,the SSB model,regression model and?Full?gravimetric isotope mixture model are effective in the MDF correction within the MC-ICPMS itself for obtaining accurate isotope amount ratios.This study focused on the MIF phenomenon within MC-ICPMS itself.Adopting the appropriate isotope fractionation correction model,we have determined the isotope ratios of Pb,Hf and Cr isotopes,the main achievements and conclusions are summarized as follows:1)We have undertaken the first independent isotope ratio measurements of a common lead standard sample HIPB-1 since the seminal work by NIST in the 1960s which has provided widely successful standards SRM 981 and SRM 982 in National Research Council Canada?NRC?using full gravimetric isotope mixture?FGIM?of near-pure lead isotopes.FGIM has been approved by IUPAC to obtain isotopic abundance of unknown sample.The main aim of this method based on the seven gravimetric mixtures of near-pure lead isotopes(²⁰⁴Pb?²⁰⁶Pb?²⁰⁷Pb and ²⁰⁸Pb)and the FGIM correction model is to correct both MDF and MIF and calculate the accurate isotope ratios and abundance of Pb.A cost-effective temporal regression method which utilizes NIST SRM 997thallium isotopic standard as the primary calibrator was also employed.This method based on the simulation of instrument mass bias related to time by different ICP-MS RF power.The measurement efficiency is nearly 20 times higher than the traditional regression correction method.Both calibration methods correct MIF within MC-ICPMS itself and provided highly-consistent results which were used to characterize Pb isotope ratios.This work highlights the strengths of the optimized regression model and will instill further confidence in this secondary isotope ratio calibration method.The uncertainty is evaluated within the absolute determination results.Measurements of HIPB-1 provide calibrated lead isotopic ratios,abundance and a recommended atomic weight of Ar?Pb?=207.1791?1?_k=1.2)We systematically reported the SI traceable full Hf isotopic ratios of the common Hf isotopic standard sample JMC-475 and a NRC reference material HALF-1.The“state-of-the-art regression”model with NIST SRM 989 isotopic rhenium as calibrator was used to correct instrumental MDF and MIF without reliance on other hafnium standards,normalizing isotope ratios,or exponential mass-bias fractionation models.By discussing the fractionation behavior proved the exist of MDF phenomenon during the Hf isotope measurements.We determined the isotopic composition of hafnium in four commercial hafnium reagents.Our work employs MC-ICPMS for isotope measurements which were calibrated using two independent methods,provided high-consistent Hf isotope amount ratio results.Measurements of HALF-1 provide calibrated lead isotopic ratios,abundance and a recommended atomic weight of Ar?Hf?=178.487±0.006.3)Based on the previous research,the Cr isotope analysis procedure was reconstructed by the preparation of the double-spike,the calibration of chemical processes,and the determination of standard samples.Cr isotope data for standard solutions?NIST SRM 979,NIST SRM 3112a,SCP Cr?and geological standards?BHVO-2,JP-1?are in agreement with previously published data within analytical uncertainties.Our Cr isotope data shows that Thermal Ionization Mass Spectrometry?TIMS?gains higher precision than MC-ICPMS.Besides,the Cr isotopic fractionation resulted from MDF phenomenon was discussed by the plot-linear evaluation method.4)The Cr isotopic characteristics study was performed on series of chromitite samples from Luobusha chromite deposit,including massive,disseminated,podiform chromitite ores and their host rocks to seek the possibility for indicating the chromium isotope behavior in high-temperature upper mantle processes.The range of Cr isotope of the Luobusha chromitite is rather limited?-0.11±0.07‰?,which is similar to the the Bulk Silicate Earth?-0.124±0.101‰?.The Cr isotope compositions of chromitite samples displayed a slightly heavier tendency compared to the peridotite and the garnet in the fissure,indicating the possible enrichment of heavier Cr isotope along with the crystallization process of chromitite and which might be related to the migration of Cr³⁺during the crystallization.Part of the disseminated chromitite samples show lighter Cr isotopic composition than some massive chromitite samples,suggesting that there might be Cr isotope fractionation under high oxygen fugacity range during the formation of the chromite.Moreover,compared with chromitite samples,the total serpentinized dunite has a relatively positive Cr isotope ratio,which might be caused by the secondary alteration under the oxidative environments.