Comparative evaluation of uranium isotope ratios by peak-jumping and static multi-collector measurements in thermal ionization mass spectrometry

The ²³⁵U/²³⁸U isotope ratios of uranium standard reference materials and an environmental uranium sample recovered from seawater were measured by thermal ionization mass spectrometry using the single-collector peak-jumping and static multi-collection modes. All the ²³⁵U/²³⁸U isotope ratios measured in the peak-jumping mode were systematically lower than the values obtained in the static measurements. The lowering of the measured isotope ratios became more remarkable with the decrease in the ²³⁵U isotopic abundance, resulting in the maximum 2.2‰-lower value comparable to isotope mass fractionation. The lowering of isotope ratios is not due to the apparent loss of ²³⁵U ion currents caused by the time constant of the Faraday amplifier used but attributable to the actual decrease of ion currents by ion-beam changes in the mass switching step of peak-jumping mode. Furthermore, the proper collection of ion beams to the Faraday cup was achieved by allowing sufficient delay time for eliminating the effect of ion-beam changes, which was significantly longer than the time needed for the response of the amplifiers.