Accurate Analysis Of Lithium Isotopic Ratios In Geological Samples With High Precision

With the extensive application of MC-ICP-MS and the huge breakthrough of isotopic analysis methods,the research of non-traditional stable isotopes has gained great development.Lithium,an emerging tracer tool,has gained more and more researchers' attention.There are two aspects in getting Li isotopic ratios: bulk analysis and in situ microanalysis.In this research,we make a review of the research status of the Li isotopic analysis.Based on these knowledge,we have built the method about accurate measurements of Li isotopic ratios by SN-MC-ICP-MS and microanalysis of Li isotopic ratios by LA-MC-ICP-MS.1.Accurate determination of Li isotopic ratios by SN-MC-ICP-MS with low-blank,low-memoryFor the accurate determination of Li isotopic ratios,there are two main problems: chemical purification and isotopic ratios analysis by MC-ICP-MS.The purification of Li was achieved using a one-step column separation method,in which 2.7 ml of AG50W-x8 cation exchange resin?200-400 mesh?and a boron-silicate glass column?0.6 cm ID×21.5 cm bed height,10 mL reservoir?is used.And 1 mol L-1 HNO₃ in 80% v/v CH3 OH is used as eluant.The purification method are routinely calibrated by applying a series of silicate standards.The elution curves cannot be changed with the sample type and Mg:Li ratio of samples.For the purification method,the recovery is as high as 100%.In addition,the concentration of residual elements are measured,major elements/Li ratios of the solution collected in the elution interval are less than 1.5 for all measured elements,and the majority of them are far away from 1.The results of doping tests show no detectable matrix-induced mass discrimination on Li isotopic measurement even though the matrix element/Li ratio was up to 10.Thus,the sample digestion and chemical purification method have been built for samples.For the determination of Li isotopic ratios on MC-ICP-MS,the accuracy and precision of Li isotopic ratios is generally limited by the high Li instrumental blank and serious memory effect.In order to solve the problem,we found that Li blank of 2% HNO₃ can be magically reduced by a factor of 15-70 by the addition of a 5% NaCl rinse step.With the elimination of Li instrumental blank and memory effect,there is not a large deviation even though the Li concentration and HNO₃ concentration of sample and standard are not strictly matched.Based on this conclusion,a non-strict matrix-matched method is built to analyze Li isotopic ratios.We apply the method to analyze Li isotopic ratios of six silicate samples?AGV-1,AGV-2,BHVO-2,GSP-2,JG-2 and RGM-2?,a Li-carbonate sample?IRMM-016?and a seawater sample?NASS-6?.The measured data is consistent with the refered ratios within the uncertainty,and the external precision of this method is better than ±0.25‰?2SD?for ?⁷Li,which is suitable for the identification of small fractionation of Li isotopes occurring in geological processes.Moreover,the low Li instrumental background and negligible memory effect can pave the way for the accurate and precise measurement of Li isotopic ratios in low-Li samples.2.Accurate measurement of Li isotopic ratios by LA-MC-ICP-MS with non-matrix matched calibration methodThe application of ns-LA-MC-ICP-MS to analyze Li isotopic ratio is limited by the low Li signal intensity,⁷Li/⁶Li stability and non-matched of Li signal intensity.Comepared with the Jet + X cone combination,the use of standard + X cone combination can improve the signal / background ratio by 4-fold.Combined with the distribution of Li ions in plasma,the ⁷Li/⁶Li ratio stability can be improved from 4-to 13-fold by “sacrificing” 5% of signal intensity when shallower sampling depth or higher sample gas flow rate are applied.In the analysis of Li isotopic ratios,the intensity mismatch effect can be reduced when ⁷Li intensity and signal/background ratio were higher than 0.2 V and 214,respectively.But for the low-Li content geological samples,it is difficult to achieve ⁷Li signal intensity above 0.2 V.On this condition,Li signal intensity matching is relatively essential.BCR-2G with moderate Li concentration and Li isotopic ratio was selected as the bracketing standard,and ⁷Li intensity difference between samples and BCR-2G should be kept within the range of ±40% by changing laser repetition rates.With the optimized instrumental parameters and the reduction of the effect of intensity mismatch,Li isotopic ratios of BHVO-2G,ATHO-G,T1-G,StHs6/80-G,GOR132-G,GOR128-G,ML3B-G and KL2-G with different Li contents can be accurately measured,demontrating the importance of optimizing of those operating parameters of 193 nm excimer LA-MC-ICP-MS to accurate analyze Li isotopic ratios.