Measurement Of Cr Isotope Compositions By Double-spike MC-ICP-MS

This research made an attempt to develop a procedure for separating of Cr from the sample matrix by cation resin and a method for determining Cr isotopes with MC-ICP-MS.The 50Cr-54 Cr Double-spike is used to correct the mass dependent fractionation of stable isotopes during the sample separation and MC-ICP-MS analysis procedure.The element Cr in the dunite,basalt,shales can be separated by AG50W-X8 cation exchange resin with two columns.The recovery percentages of the first column and the second column are about 90%,98%,respectively.The total recovery is about 90%.The elements Al and Ti of basalt and shales are overloaded in the first column,and then will be wiped out in the second column.After two column step separation the ratio of Ti/Cr,V/Cr,Fe/Cr is lower than 2.5×10-4,2.4×10-4 and 2.4×10-4,respectively.The isobaric experiment shows the residual Ti,Fe,V have no effects to the analysis of Cr isotopes.According to the simulation of the The double-spike tool box program,the optimal composition of the double-spike 50Cr/54 Cr is 1.41.The optimal msample/mdouble-spike ratio is 28.33:71.67.The actual composition of the double-spike is approximately 1.74.The long-term reproducibility of the NIST SRM979 was ?⁵³Cr?‰?= 0.00±0.06?2SD,N=20?.The ?⁵³Cr?‰?of NIST3112 a relative to NIST SRM979 is-0.05 0.07 2SD,N 8.The result is in agreement with the published data and with similar precision.With respect to NIST SRM979,the ?⁵³Cr?‰?of basalt standard BIR-1a and BHVO-2 are-0.16 ± 0.11?2SD,N=5?and –0.19±0.11?N=10,2SD?,respectively.Both of the result is in agreement with the published data with similar precisions.The ?⁵³Cr?‰?of dunite standard DTS-2b is –0.05±0.11?N=4,2SD?,which is in agreement with that of terrestrial igneous silicates data.The ?⁵³Cr?‰?of the in-house black shales standard CAGS-BS is 0.20± 0.11?N=6,2SD?.The results indicate the method for determining Cr isotopes is suitable for basalt,dunite and black shales.