Determination of uranium and thorium in natural waters with a high matrix concentration using solid-phase extraction inductively coupled plasma mass spectrometry

An on-line solid-phase extraction method, linked to inductively coupled plasma mass spectrometry has been developed to overcome the problems of a high sample matrix concentration interfering with the determination of low levels of uranium and thorium in aqueous samples. Using an actinide-specific extraction resin (TRU-spec), in a small column incorporated into the sample introduction manifold, recovery of the analytes (U101-108%, 87Th 115%) was obtained from seawater (NASS-4 seawater CRM, 2.68 ng ml-1 U), ground water (0.1 ng ml-1 U in 3000 mug ml-1 total dissolved solids) and samples containing significant levels of dissolved organic carbon (1 ng ml-1 U/Th, 8 mug ml-1 humic and fulvic acids). This is an improvement on conventional ion exchange and chelating resins which give low recoveries in the presence of dissolved organic carbon. The samples were injected directly on to the column, with no prior dilution. Figures-of-merit obtained were R2: U 0.998, Th 0.999, for 0 to 10 ng ml-1, R.S.D.: U 4.3%, Th 5.8%, and limits of detection: U 0.015 ng ml-1 and Th 0.006 ng ml-1 in a synthetic seawater matrix.