Optimization of mixed Pb-Tl solutions for high precision isotopic analysis by MC-ICP-MS.

Kamenov, G. Mueller, P. and Perfit, M., 2004, J. Analytical Atomic Spectrometry, v. 19. p. 1262-67.

Abstract

A number of measurements of SRM 981 Pb mixed with Tl for mass bias corrections were conducted on a MC-ICP-MS (Nu Plasma). During the initial stages of the experiments, Pb and Tl from single concentrated stock solutions were mixed in 2%HNO₃ and measured from 48 hours to 60 days after the mixing. The results with these solutions revealed poor overall precision and accuracy with ²⁰⁶Pb/²⁰⁴Pb=16.921 (+/-0.039 2s), ²⁰⁷Pb/²⁰⁴Pb=15.469 (+/-0.052 2s), and ²⁰⁸Pb/²⁰⁴Pb=36.630 (+/-0.160 2s). Similarly, using SRM 981 as a fixed Pb isotopic composition for mass bias corrections of the Tl isotopes yielded large variations in ε²⁰⁵Tl for our Tl standard, ranging from –3.9 to +30.1, relative to NIST-997. Also, measured intensities of Pb and Tl in these aged solutions revealed higher Pb/Tl intensity ratios compared to the known elemental ratios in the original mixture. Analyses of freshly mixed (<1 hour) Pb:Tl solutions yielded highly precise results with ²⁰⁶Pb/²⁰⁴Pb=16.9373 (+/-0.0011 2s), ²⁰⁷Pb/²⁰⁴Pb=15.4907 (+/-0.0012 2s), and ²⁰⁸Pb/²⁰⁴Pb=36.6935 (+/-0.0039 2s). Measured ε²⁰⁵Tl in these “fresh” solutions prepared with our Tl standard was 1.5 (+/-0.8 2s). In addition, no decoupling in the Pb:Tl ratios was observed in the fresh mixtures.

Poor precision and accuracy in the isotope measurements and decoupling of the Pb:Tl elemental abundances versus ion beam intensities are result of interaction of Pb and Tl in solution. Most probably Tl⁺ is transformed into Tl³⁺ after addition of Pb to the analyte mixture. Tl³⁺ is hydrolyzed and forms colloids that exhibit markedly different behavior than the rest of the analyte ions during the desolvation process. It is not clear if the observed large range in ε²⁰⁵Tl in the aged solutions is due to mass-fractionation during the Tl⁺-Tl³⁺ transition or is a result of unknown mass-bias effect in the mass-spectrometer. However, Tl from the aged solutions exhibits different mass-bias behavior than the Pb in the mass-spectrometer. Therefore, most probably this is responsible for the large analytical errors observed during the isotope measurements.

This study demonstrates that highly precise Pb isotopic measurements can be produced using Tl normalization with MC-ICP-MS. However, significant analytical errors, resulting in poor precision and accuracy in the Pb isotope measurements and up to 30-40 epsilon Tl units variation, can be generated during aging of mixed solutions as a result of complex interactions between the analyte ions. Experiments utilizing thallium for mass bias corrections for Pb, and lead for mass bias corrections for Tl isotope measurements and desolvation appear to be most reproducible when conducted on freshly prepared analyte mixtures or in very dilute HNO₃ solutions.

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