| Cd is a harmful trace element for humans.It has a long biological half-life.Small but continuous uptake can lead to accumulation of Cd in the human body,causing reduced kidney function or weakening of bones.With the development of analytical method based on the application of MC-ICP-MS over several decades,high precision Cd isotope measurement has gained a rapid development.Current studies in Cd isotope geochemistry indicate that Cd isotopes may be used as a proxy to trace contaminant sources in the environment.However,the Cd content in most of soil and rock samples in nature is very low,and the current Cd isotope analyses are limited to determine the isotope compositions of low Cd samples.In addition,there is a lack of Cd isotope data for international soil and igneous rock reference materials,which hinders the application of Cd isotopes in the high-temperature magmatic and low-temperature supergene processes.In this thesis,based on the previous studies,we develop a high precision Cd isotope measurement method for low Cd content samples using MC-ICP-MS with double spike correction.By using one step micro-column with anion exchange resin and adding double spike before the chemical purification,Cd was efficiently separated from matrix elements and the potential mass-dependent isotopic fractionation induced by chemical purification procedure was well corrected.Compared with previous methods,our new purification procedure improves time efficiency and avoids the use of expensive TRU Spec resin.Cd isotope data were determined by MC-ICP-MS employing double spike method to calibrate mass-dependent isotopic fractionation induced by instrument.Parameters that could affect the precision and accuracy of Cd isotope measurement have been tested,which include the potential effects of acid molarities and concentration mismatch,and the variation in double spike and sample ratio.The effects of matrix element,molecular interferences and isobaric interferences on Cd isotope measurement were also quantitatively evaluated.When the measured solution has Ag/Cd?5,Zn/Cd?0.02,Mo/Cd?0.4,Zr/Cd?0.001,Pd/Cd ?5Χ10-5 and In/Cd ?10-3,the measured Cd isotope data were not significantly affected by these interferences.The effect of Sn on the measured Cd isotope values can be corrected using an exponential law even when the Sn/Cd ratio is as high as 0.5.Based on the long-time repeated analyses,the Cd isotope ratios of three pure reference solutions(BAM 1012 Cd,Munster Cd and AAS Cd)relative to NIST SRM 3018 pure Cd standard solution(defined as ?114/110Cd=[(114Cd/110Cd)Sample/(114Cd/110Cd)SRM 3108-1]Χ 1000)are-1.325±0.043(2 SD,n=74),4.455±0.047(2 SD,n=71)and-0.691±0.041(2 SD,n=57),respectively.The long-time external precision of pure reference solutions is better than ±0.05(2 SD)for ?114/110Cd.The mean ?114/110Cd of synthetic standards,which are made by mixing NIST SRM 3108 with matrix elements,are in agreement with the recommended value with the limit of error.Based on the analytical method established in this thesis,I determined the Cd isotope compositions of fifteen international reference materials,including the soils with different content of Cd contamination,the igneous rocks with different properties and one manganese nodule.The ranges of Cd mass fractions in these analysed reference materials vary from?0.07 to?54.1 ?g g-1.The ?114/110Cd value of these reference materials are giving below:SRM 2709:-0.251±0.048(2 SD,n=10);SRM 2709a:-0.218±0.053(2 SD,n=8):SRM 2710:-0.180±0.063(2 SD,n=10):SRM 2710a:-0.195±0.045(2 SD,n=4);SRM 2711:0.632±0.035(2 SD,n=14);SRM 2711a:0.551 ±0.051(2 SD,n=8);GSS-1:0.110±0.050(2 SD,n=8);BCR-2:0.018±0.067(2 SD,n=14);BIR-1:0.037±0.051(2 SD,n=12);BHVO-2:0.039±0.047(2 SD,n=8);W-2:0.027±0.046(2 SD,n=8);AGV-2:0.090±0.035(2 SD,n=8);GSP-2:-0.196±0.068(2 SD,n=7);COQ-1:0.098±0.052(2 SD,n=8);NOD-P-1:0.163±0.040(2 SD,n=8).The ?114/110Cd values of soil standards relative to NIST SRM 3108 are in the range of-0.251?0.632,the ?114/110Cd values of rock standards vary from-0.196to 0.098.The large variations in Cd isotopes in soils and igneous rocks indicate that Cd isotopes can be more widely used to study magmatic and supergene processes. |
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