Determination of thorium and uranium isotope ratios in low-concentration geological materials using a fixed multi-collector-ICP-MS

Techniques used for the measurement of thorium and uranium isotope ratios i n low-concentration (<0.1 ppm) geological materials on a Nu Instruments" MC -ICP-MS are described. Using an Aridus(R) micro-concentric, desolvating neb uliser sample introduction system, the beam current in a Faraday collector is typically 1.0 nA ppm(-1) Th-232 (total system efficiency = 0.3%). The ab undance sensitivity is less than 100 counts s(-1) at 1 mass unit below a la rge peak with an intensity of 1 x 10(9) counts s(-1), or <0.1 ppm. The Fara day-ion counting gain is stable over a day and the method can provide Th-23 0/Th-232 ratio measurements on ng quantities of total dissolved Th to a pre cision that is better than 1% (95% confidence level). This is comparable to results obtained by thermal ionisation mass spectrometry on samples that a re typically 10 to 100 times larger. Results for our internal laboratory st andards are presented together with our results for standards used in other laboratories to facilitate inter-laboratory comparison.