Determination of 232Th and 230Th in seawater using a chemical separation procedure and thermal ionization mass spectrometry

A method for the determination of 232Th and 230Th in seawater has been developed by substantial modifications of available techniques mainly used for other matrixes. We use a well calibrated 229Th spike and iron oxyhydroxides as a carrier phase for the precipitation of Th from about 3.5 L filtered seawater. The chemical separation and purification of Th from the Fe carrier comprise three ion chromatography steps including AG 1-x8 (200–400 mesh, Cl− form), TRU Resin, and UTEVA Resin. The separated Th fraction is loaded with a mixture of graphite/phosphoric acid onto zone-refined single rhenium filaments. These are analyzed in peak-jumping mode on a thermal ionization mass spectrometer. Due to the low Th concentration in seawater, the 232Th blank constitute a significant problem for the chemical separation of Th from the seawater matrix and also for the filament loading. By applying a detailed cleaning scheme including a HF-HCl mixture, the total procedural blank could be substantially reduced to approximately 0.4 pg. This is generally < 1% of a typical 232Th concentration in surface seawater, which ranges from about 0.01 to 1 picomol/kg. The analysis of standards, artificial seawater, and a series of seawater samples obtained from the GEOTRACES intercalibration cruises in 2008 and 2009 demonstrate that this method can yield a precision for 232Th concentration in seawater of about 1% to 2% (2σ) and 230Th concentration of about 2% to 5% (2σ).