Precise determination of 6Li/7Li isotopic ratio with NaLiBO2+ ion using total evaporation and ion integration by Thermal Ionization Mass Spectrometry(TIMS)

Abstract

Total Evaporation and Ion Integration technique (TE & II) was used for determination of the 6Li/7Li isotope ratio with the NaLiBO2+ (m/z 71–73) and Na2BO2+ (m/z 88, 89) molecular ions emitted from the sodium lithium borate compound on the single filament assembly. Since the ion intensities are simultaneously collected by the multi-collector assembly and integrated till the sample is completely consumed, the measured ion intensity ratio 72I/73I is less affected by fractionation and hence closer to the true value. The fractionation profile for lithium isotopes showed concurrence with the Rayleigh's distillation law since the measured instantaneous ratio of 72I/73I at ∼63% sample consumption was equal to the integrated value at 100% sample consumption. The minimum sample size requirement for molecular ion method decreased to 50 ng of lithium for the TE method of data acquisition compared to 1–2 μg of lithium required for conventional method of limited scans. The relative standard deviation of 6Li/7Li ratio determined from the analyses of six samples of 500 ng each of the two natural Li2CO3 samples namely L-SVEC and high purity Spec-pure lithium was about 0.05% (1σ). Refractory materials Li2TiO3 and LiAlO2 were also successfully analysed by the developed TE method with reduced sample size. The TE method enhanced the benefits of the molecular ion method by further decreasing the effect of fractionation as well as sample size requirement for accurate determination of 6Li/7Li ratio. The method could clearly distinguish between the two natural lithium samples, L-SVEC lithium and Spec pure lithium having a difference of 0.5% in their 6Li/7Li ratio.