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Using the parallel ion-counting capability of an appropriately configured multi-collector ICP-MS it is possible to simultaneously measure the activity ratios 230Th/234U and 234U/238U 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 230Th/238U and 234U/238U 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.
John Hellström (Wed,) studied this question.
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