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

Difficulties in determining the 230Th and 231Pa 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 230Th and 231Pa seawater concentration, and provide important constraints on circulation and mixing rates in the deep ocean. Seawater samples are spiked with 229Th and 233Pa 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 l seawater samples with an internal precision of 2% and a reproducibility of 5% (95% confidence intervals (CI)) in deep water. After correction for procedural blank, 232Th tailing, and 232Th1H interference, the detection limits are 3 fg for 230Th and 0.4 fg for 231Pa. Applied to 20 l volumes, these detection limits correspond to concentrations of 0.15 fg/kg for 230Th and 0.02 fg/kg for 231Pa, 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 230Th and 231Pa concentration consistent with patterns of deep water circulation.