Isotope dilution-chemical separation: a powerful combination for high precision analysis at the ultra-trace level using ICP mass spectrometry; example of U and Th determination in silicate rocks

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