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

A low blank Mg(OH)2 pre-concentration method was evaluated for the determination of lead isotope ratios (208Pb/206Pb, 207Pb/206Pb) in seawater using a quadrupole ICP-MS VG Plasma Quad II+. Possible matrix effects derived from the Mg(OH)2 co-precipitate were assessed by spiking lead-free seawater (PbFS) and 1% (v/v) HNO3 with the certified common lead standard NBS 981 to give solutions with concentrations in the lower picogram per millilitre range. The standard curves for all three masses were linear in both matrices with minor signal loss (18%) in the Mg matrix. Mass fractionation showed similar mass biases (<2% frac./amu) for 208Pb/206Pb and 207Pb/206Pb 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 the subsequent measurement. The data acquisition parameters dwell time, replicates per analysis, and acquisition time were first varied to optimize analytical precision and accuracy of the ICP-MS measurements. On the basis of these results, approximately 0.5 ml of pre-concentrated solution was finally used employing a low-flow, CETAC microconcentric nebulizer to minimize sample consumption and extend acquisition time. Analyzing 18 seawater samples from the North Atlantic, the average 1? external precision of triplicate measurements was approximately 0.3% for both ratios, 208Pb/206Pb and 207Pb/206Pb, at the level of approximately 20 pg ml?1 Pb. Blanks accounted for less than 3% of total lead analyzed for each sample. Six samples were also measured with TIMS and agreed in average within 0.26% for 207Pb/206Pb and 0.37% for 208Pb/206Pb. Three surface water samples from the Sargasso Sea, collected in 1989, showed ratios in line with previous published ratios from the western North Atlantic.