Accurate Determination of Lithium Isotopic Compositions in Geological Samples by Multi-collector Inductively Coupled Plasmon-Mass Spectrometry

Accurate determination of Li isotopic composition in natural geological samples is the basis for Li isotope geochemical studies. In this study, a method was presented for the preparation of geological materials (water and rock) and accurate determination of Li isotopic composition was set up. Separating Li from water and rock samples was implemented by a single column containing 1.5 mL Bio-Rad AG 50W-X12 (200 similar to 400 mesh) resin, and 0. 40 mol/L HCl and 1.0 mol/L HCl were used as the eluents. Only 8.5 mL and 14 mL of eluent were used to separate Li from water and rock samples 14 this method , respectively. Blank signal of the operation procedure was 2. 4 +/- 0. 1 mV, which was almost same as the 2. 3 mV of the 2% HNO3 signal used in this study. Results from experiments showed that significant Li isotopic fractionation during leaching process and the difference in delta(7) Li values in samples with incompletely recovered Li reach up to similar to 50%o. Lithium isotopic ratios were determined by multi-collector ICP-MS ( Nu Plasma II ) using the sample standard bracketing ( SSB ) method. L-SVEC standard with similar Li concentration to samples ( similar to 80 ng/mL ) was used in this study. The external precision ( 2 sigma ) of this technique, determined by repeated measurement of pure Li standard solutions and seawater was < +/- 0. 8 parts per thousand. The measured delta(7) Li values of seawater and rock standards AGV-2 , BCR-2 and GSP-2 were +31. 4 parts per thousand +/- 0. 7 parts per thousand (n = 18 ) , +7. 2 parts per thousand +/- 0 . 2 parts per thousand ( n=4), +3. 7 parts per thousand +/- 0. 7 parts per thousand(n= 8) and -0. 10 parts per thousand +/- 0. 18 parts per thousand ( n = 4) , respectively, similar to previously published values. This method can be used to accurately determine Li isotopic composition of various types of geological samples such as waters and rocks. The advantage of this method was that the amount of resin and reagent is reduced to 50% or less of the previous studies , thereby significantly improving the work efficiency and reducing the operation procedure blank.