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

Previous studies have demonstrated the potential of multiple-collector indu ctively coupled plasma mass spectrometry (MC-ICP-MS) for precise Pb isotopi c measurements using admixed Tl for "external normalization" of instrumenta l mass discrimination. The Micromass IsoProbe is a new, single-focusing MC- ICP-MS instrument that does not employ an electrostatic sector for energy-f ocusing. Instead, a hexapole collision cell is used to thermalize the ion b eam. This study presents the first in-depth investigation of the applicatio n of the IsoProbe to Pb isotope ratio measurements. With a Tl-based mass bi as correction, multiple analyses of mixed Pb-Tl standard solutions typicall y display reproducibilities (+/-2 sigma) of about 50 ppm for Pb-207:Pb-206, 100 ppm for Pb-208:Pb-206 and Pb-206:Pb-204, 150 ppm for Pb-207:Pb-204 and 175 ppm for Pb-208:Pb-204. 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-calibrat ion 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 correl ations with the Pb:Tl ratio of the analyzed solutions. The addition of larg e amounts of Tl to the samples is unfavorable for the accurate measurement of the low-intensity Pb-204 ion beam. Analytical artifacts may also be gene rated if high concentrations of concomitant elements are present in the sam ple solutions. Matrix effects and instrumental memory, however, can be read ily overcome by the adoption of appropriate analytical protocols and it is demonstrated that sample measurements can achieve the same levels of precis ion and accuracy as are routinely obtained for analyses of pure standard so lutions.