Rapid determination of 230Th and 231Pa in seawater by desolvated micro-nebulization Inductively Coupled Plasma magnetic sector mass spectrometry

Difficulties in determining the Th-230 and Pa-231 concentration of seawater have hindered rapid progress in the application of these unique natural tracers of particle scavenging and ocean circulation. In response, we have developed an ICP/MS analytical procedure combining a degree of sensitivity, precision and sample throughput that can facilitate the systematic measurement of basin-scale changes in Th-230 and Pa-231 seawater concentration, and provide important constraints on circulation and mixing rates in the deep ocean. Seawater samples are spiked with Th-229 and Pa-233 and equilibrated before pre-concentration using conventional methods of Fe oxyhydroxide co-precipitation and anion exchange. Isotopic ratios are measured using a Finnigan MAT Element magnetic sector Inductively Coupled Plasma mass spectrometer (ICP/MS) equipped with a desolvating micronebulizer. Measurements are done on 10-20 1 seawater samples with an internal precision of similar to 2% and a reproducibility of similar to 5% (95% confidence intervals (CI)) in deep water. After correction for procedural blank, Th-232 tailing, and (ThH)-Th-232-H-1 interference, the detection limits are similar to 3 fg for Th-230 and similar to 0.4 fg for Pa-231. Applied to 20 1 volumes, these detection limits correspond to concentrations of 0.15 fg/kg for Th-230 and 0.02 fg/kg for Pa-231, which are 5-15 times lower than typical concentrations in surface water. The capability of this method is illustrated by two seawater profiles from the Equatorial Atlantic region that show systematic variations in Th-230 and Pa-231 concentration consistent with patterns of deep water circulation. (C) 2001 Elsevier Science B.V. All rights reserved.