Lead isotope analysis of marine carbonates and seawater by multiple collector ICP-MS

A consistent method for stable lead isotope analysis of marine carbonates and seawater is presented, utilizing multiple collector ICP-MS (MC-ICP-MS). This study presents new observations of the large (0.7% amu(-1)), time-dependent mass bias determined by thallium normalization, including preferential light ion transmission induced by the acceleration potential and plasma interface (beta = -1.3 to 0.9). These experiments show equivalent results for three empirical correction laws, and the previously proposed beta(Pb)/beta(Tl) correction does not improve isotope ratio accuracy under these conditions. External normalization to SRM-981 following the thallium correction provides one simple alternative, and a rationale is provided based on secondary bias effects. With current intensities less than 1.5 x 10(-12) A, external isotope ratio precision better than 250 ppm for SRM-981 Pb-207/Pb-206 and Pb-208/Pb-206 ratios is observed (2sigma). From reconstructed lead isotopic variability in the North Atlantic, this instrumental precision results in a signal-to-noise ratio greater than 100. Matrix effects are significant with concomitant calcium in SRM-981 (-280 ppm at 257 muM [Ca]). With the appropriate corrections and minimal concomitants, MC-ICP-MS can reliably determine Pb-207/Ph-206 and Pb-208/Ph-206 ratios of marine carbonates (30 mg) and seawater (160-200 g). (C) 2003 Elsevier B.V. All rights reserved.