Determination of lead isotope ratios in seawater by quadrupole inductivelycoupled plasma mass spectrometry after Mg(OH)(2) co-precipitation

A low blank Mg(OH)(2) pre-concentration method was evaluated for the determ ination of lead isotope ratios (Pb-208/Pb-206, Pb-207/Pb-206) in seawater u sing a quadrupole ICP-MS VG Plasma Quad II +. Possible matrix effects deriv ed from the Mg(OH), co-precipitate were assessed by spiking lead-free seawa ter (PbFS) and 1% (v/v) HNO3 with the certified common lead standard NBS 98 1 to give solutions with concentrations in the lower picogram per millilitr e range. The standard curves for all three masses were linear in both matri ces with minor signal loss (similar to 18%) in the Mg matrix. Mass fraction ation showed similar mass biases (< 2% frac./amu) for Pb-208/Pb-206 and Pb- 207/Pb-206 in both seawater and 1% (v/v) HNO3, indicating that there is no significant matrix influence on the isotope ratio determination. Using the Mg(OH)(2) precipitation method, real seawater samples were pre-concentrated approximately 28-fold, and 1 ml of 5% (v/v) HNO3 end volume was used for t he subsequent measurement. The data acquisition parameters dwell time, repl icates per analysis, and acquisition time were first varied to optimize ana lytical precision and accuracy of the ICP-MS measurements. On the basis of these results, approximately 0.5 mi of pre-concentrated solution was finall y used employing a low-flow, CETAC microconcentric nebulizer to minimize sa mple consumption and extend acquisition time. Analyzing 18 seawater samples from the North Atlantic, the average lo external precision of triplicate m easurements was approximately 0.3% for both ratios, Pb-208/Pb-206 and Pb-20 7/Pb-206, at the level of approximately 20 pg ml(-1) Pb. Blanks accounted f or less than 3% of total lead analyzed for each sample. Six samples were al so measured with TIMS and agreed in average within 0.26% for Pb-207/Pb-206 and 0.37% for Pb-208/Pb-206. Three surface water samples from the Sargasso Sea, collected in 1989, showed ratios in line with previous published ratio s from the western North Atlantic. (C) 2000 Elsevier Science B.V. All right s reserved.