Impact of Sample Preparation History on Isotopic Fractionation: Isotope Ratios of Lead SRM 981

Abstract

Several batches of NIST SRM 981 dissolved in 0.15 mol/kg HNO₃ were used for the determination of K factors of isotope ratios ²⁰⁴Pb/²⁰⁸Pb, ²⁰⁶Pb/²⁰⁸Pb, and ²⁰⁷Pb/²⁰⁸Pb applying MC-ICP-MS. Pure solutions of the primary standard (reference) material NIST SRM 981 (w(Pb) ≈ 70 ng/g to 100 ng/g) were compared with those treated with a chromatographic separation using a crown ether resin, and additionally with those treated with microwave digestion followed by chromatographic separation. The procedural blank influence was studied and potential interferences of ²⁰⁴Hg on the ²⁰⁴Pb signal were corrected. An improved chromatographic separation protocol demonstrates the suitability of Pb separation. Mass bias correction factors (K factors) were determined for each ratio and compared for the three procedures. The respective uncertainty budgets for the first time gave direct quantitative indications of differences and uncertainty contributions of the respective digestion and column separation processes compared to untreated solutions. To derive the impact of the preparation steps alone (digestion, column separation), pure SRM® solutions with Pb (no Pb containing matrix samples) were investigated. For K(²⁰⁴Pb/²⁰⁸Pb) an impact of a sample pretreatment cannot be observed and isotopic fractionation can be mainly deduced from the mass spectrometric measurement. In case of the ratios ²⁰⁶Pb/²⁰⁸Pb and ²⁰⁷Pb/²⁰⁸Pb, the K factors of the digested and column separated as well as only column separated samples can be distinguished from the non-pretreated samples for k = 1, which suggests a small, but observable impact of the sample history on the K factor although the initial samples were matrix-free.