Rapid and accurate U/Th dating using parallel ion-counting multi-collector ICP-MS

Using the parallel ion-counting capability of an appropriately configured multi-collector ICP-MS it is possible to simultaneously measure the activity ratios Th-230/U-234 and U-234/U-238 in a single solution. A method has been developed which fully internalises the measurement of elemental fractionation, instrument mass bias and ion counter gain, allowing on-line calculation of U/Th ages on a cycle-by-cycle basis during data acquisition. Because of the high efficiency of this technique very small sample sizes can be used, with permil-level precision possible on both Th-230/U-238 and U-234/U-238 from analyses of a few tens of nanograms of uranium. As U and Th fractions do not need to be collected and analysed separately sample throughput is very high, using a simple TRU-Spec ion exchange resin procedure. Repeat analyses of Harwell Uraninite, NIST SRM 960 and two carbonate age standards show excellent agreement with results from other laboratories.