234Th determination
Immediately after sampling, the 20-liters of seawater were passed through a filter (Millipore membrane: diameter 142 mm, 0.45 µm pore size) to separate dissolved from particulate phases. Within two weeks after the collection, particulate 234Th (234ThP) was directly measured on the filter with a low background-high efficiency g detector (Schmidt and Reyss, 1996). 234Th activities were measured from its 63.2 and 92.4-92.8 keV gamma rays, and decay corrected to the time of sample collection (Schmidt and Reyss, 2000). Because of the short half-life of 234Th and to avoid significant ingrowth corrections, separation of dissolved 234Th (234ThD) from its 238U parent was carried out on board within 24 hours after seawater collection (Schmidt and Reyss, 2000). For analysis, 229Th yield tracer and 150 mg Fe (as FeCl3) were added to the dissolved sample after acidification to pH 2. After spike equilibration, Fe(OH)3 was precipitated by addition of NH4OH to pH 7. After recovery of the precipitate, the separation of 234Th and 238U was obtained by passage through an anion exchange column (Dowex 1x8, 100-200 mesh) that had been pre-conditioned with 8N HCl. In the laboratory ashore, purification of thorium was achieved by a second passage through an 8N HNO3 column. After elution, Th was extracted with 1-(2-thenoyl)-3,3,3-trifluoracetone in toluene at pH 3 and then evaporated onto an aluminium foil. A first a-counting of this foil allowed the determination of 229Th for chemical yield (between 20 to 60%); the following g-counting allowed the measurement of 234Th. As a result precision estimates are variable, reflecting the count rate of each sample (dependent on the chemical efficiency and the decay) and the detector used.
Schmidt, S., Reyss, J.-L., 1996. Radium as internal tracer of Mediterranean Outflow Water. Journal of Geophysical Research, 101: 3589-3596.
Schmidt, S., Reyss, J.-L., 2000. Improvement in radiochemical and g-counting procedures for the determination of 234Th in seawater. Radiochimica Acta 88, 459-463.