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Determination of 232 Th and 230 Th in seawater using a chemical separation procedure and thermal ionization mass spectrometry
Author(s) -
Andersson Per S.,
Schöberg Hans
Publication year - 2012
Publication title -
limnology and oceanography: methods
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.898
H-Index - 72
ISSN - 1541-5856
DOI - 10.4319/lom.2012.10.296
Subject(s) - seawater , chemistry , mass spectrometry , artificial seawater , analytical chemistry (journal) , geotraces , chromatography , geology , oceanography
A method for the determination of 232 Th and 230 Th in seawater has been developed by substantial modifications of available techniques mainly used for other matrixes. We use a well calibrated 229 Th 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 232 Th 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 232 Th 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 232 Th concentration in seawater of about 1% to 2% (2σ) and 230 Th concentration of about 2% to 5% (2σ).