Abstract

The potential of ICP-MS for ultra-trace analysis using isotope dilution (ID) and chemical separation has been assessed and exemplified by the determination of Th and U at low levels in silicate geological materials. A selective separation of the analytes was carried out with very high recoveries and satisfactory procedural blanks using the TRU.Spec solid-phase extraction material, following dissolution with hydrofluoric acid. The advantages of combining ID with chemical separation prior to ICP-MS measurements are three-fold: (1) quantitative separation and sample handling are no longer required; (2) dilution factors (analysed solution mass/initial sample mass) as small as 5 can be routinely used; and (3) only weights and isotope ratios need to be measured, making the method relatively insensitive to instrumental drifts. Based on this beneficial combination, external reproducibilities of ca. 1% (RSD) can be achieved down to the low-ppb range in silicate rocks.