CBNM - determination of Li in BCR reference materials RM 303 and RM 304 lyophilized serum by isotope dilution mass spectrometry (IDMS)

Summary

Lithium was determined in two BCR Candidate Reference Materials 303 and 304 by Isotope Dilution Mass Spectrometry using the state-of-the-art performance of isotope-specific methods gained during previous certifications of ⁶LiF reference targets used for the determination of the neutron lifetime [1]. After reconstitution of the serum, four aliquots of each of the two candidate materials from four different bottles were spiked with a previously characterized enriched ⁶Li spike [2] which is now available as CBNM IRM-615 and has a certified ⁶Li/⁷ Li ratio of 21.78±0.12 and a certified lithium concentration of 4.001±0.028 μmol/g solution. The serum aliquots were digested in an HNO₃/H₂O₂ mixture and after evaporation of the acid, the lithium was separated on a cation exchange column, eluted with 0.3 mol/L HCl and used as LiCl for mass spectrometric measurement on an NBS type thermal ionization mass spectrometer. Similarly an unknown sample BCR X, provided by BCR to check the performance of the certifying laboratories, was analyzed. In addition the chemical preparation method was controlled by assaying NBS (NIST) SRM 909. The chemical blank was determined by IDMS using ⁶Li enriched CBNM IRM-615. The measurements were corrected for isotopic fractionation using the Isotopic Reference Material CBNM IRM-016 chemically prepared in the same way as the samples. The CRM samples as well as the BCR X sample and the NBS SRM 909 were also analyzed for isotopic composition to verify whether they had indeed natural isotopic composition. The final results have an overall uncertainty of 1.2 and 1.5%, respectively. This overall uncertainty (on a 2s basis or an estimate thereof) takes into account all uncertainty contributions of statistical as well as of systematic nature (uncertainties on used reference materials, density and blank determinations). The final results compare favorably with the values proposed by BCR for certification, but have a smaller (better) uncertainty: CRM 303: (0.517 4±0.005 7) mmol/L, CRM 304: (0.987±0.014) mmol/L