A new method to extract ²³²Th, ²³⁰Th and ²³¹Pa from seawater using a bulk-extraction technique with Nobias PA-1 chelating resin

The long-lived radioisotopes of Th and Pa are unique tracers for quantifying rates of biogeochemical processes in the ocean. However, their generally low concentrations (sub-fg/kg for ²³⁰Th and ²³¹Pa and pg/kg for ²³²Th) in seawater make them difficult to measure. Here, we present a new approach to determine ²³²Th and ²³⁰Th using Nobias PA-1 chelating resin following a bulk-extraction technique, and report for the first time the use of this resin to measure ²³¹Pa concentrations. This method has high extraction efficiency (>80%) at pH of 4.4 ± 0.2 and the lowest procedural blanks reported in the literature: 1.0 ± 0.2 pg, 0.10 ± 0.03 fg, and 0.02 ± 0.01 fg for ²³²Th, ²³⁰Th, and ²³¹Pa, respectively, representing 3%, 0.02%, and 0.01% of the total dissolved ²³²Th, ²³⁰Th, and ²³¹Pa found in 5 L of a typical low-concentration surface seawater sample from the subtropical Pacific Ocean. The procedure yields data with high precision for all three isotopes (0.76% for ²³²Th, 0.89% for ²³⁰Th, and 0.96% for ²³¹Pa, 2σ), allowing us to reliably measure Th and Pa in the oceans even at concentrations as low as those found in surface waters of the South Pacific Ocean. The accuracy of this method was confirmed by the analysis of well-characterized standard solutions (SW STD 2010-1 and SW STD 2015-1) and seawater samples collected aboard the FS Sonne (cruise SO245) during the UltraPac cruise in the South Pacific Ocean. Simultaneous and rapid extraction of ²³²Th, ²³⁰Th and ²³¹Pa from seawater, as well as the high precision and accuracy of this method makes it ideal for both spatially and temporally high-resolution studies.