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 in low-concentration ( < 0.1 ppm) geological materials on a Nu Instruments® MC-ICP-MS are described. Using an Aridus® micro-concentric, desolvating nebuliser sample introduction system, the beam current in a Faraday collector is typically 1.0 nA ppm^-1 232Th (total system efficiency = 0.3%). The abundance sensitivity is less than 100 counts s^-1 at 1 mass unit below a large peak with an intensity of 1 × 10⁹ counts s^-1, or < 0.1 ppm. The Faraday-ion counting gain is stable over a day and the method can provide ²³⁰Th/²³²Th ratio measurements on ng quantities of total dissolved Th to a precision that is better than 1% (95% confidence level). This is comparable to results obtained by thermal ionisation mass spectrometry on samples that are typically 10 to 100 times larger. Results for our internal laboratory standards are presented together with our results for standards used in other laboratories to facilitate inter-laboratory comparison.