Investigation of matrix effects for Pb isotope ratio measurements by multiple collector ICP-MS: verification and application of optimized analytical protocols

Abstract

Previous studies have demonstrated the potential of multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for precise Pb isotopic measurements using admixed Tl for "external normalization" of instrumental mass discrimination. The Micromass IsoProbe is a new, single-focusing MC-ICP-MS instrument that does not employ an electrostatic sector for energy-focusing. Instead, a hexapole collision cell is used to thermalize the ion beam. This study presents the first in-depth investigation of the application of the IsoProbe to Pb isotope ratio measurements. With a Tl-based mass bias correction, multiple analyses of mixed Pb–Tl standard solutions typically display reproducibilities (±2σ) of about 50 ppm for ²⁰⁷Pb∶²⁰⁶Pb, 100 ppm for ²⁰⁸Pb∶²⁰⁶Pb and ²⁰⁶Pb∶²⁰⁴Pb, 150 ppm for ²⁰⁷Pb∶²⁰⁴Pb and 175 ppm for ²⁰⁸Pb∶²⁰⁴Pb. Using an empirically optimized exponential law normalization, the Pb isotope data show excellent agreement with reference values obtained by thermal ionization mass spectrometry. The cross-calibration between NIST SRM 981 and 982 demonstrates that accurate results can be obtained with this technique over a wide range of Pb isotopic compositions. The Tl-corrected Pb data obtained on the IsoProbe, however, display correlations with the Pb∶Tl ratio of the analyzed solutions. The addition of large amounts of Tl to the samples is unfavorable for the accurate measurement of the low-intensity ²⁰⁴Pb ion beam. Analytical artifacts may also be generated if high concentrations of concomitant elements are present in the sample solutions. Matrix effects and instrumental memory, however, can be readily overcome by the adoption of appropriate analytical protocols and it is demonstrated that sample measurements can achieve the same levels of precision and accuracy as are routinely obtained for analyses of pure standard solutions.