Abstract

It is shown that ion–molecule reactions in a dynamic reaction cell (DRC) allow the ⁸⁷Rb/⁸⁷Sr isobaric interference to be eliminated and that Sr isotope ratios can be determined precisely and accurately by measuring the intensities of the corresponding SrF⁺ ion beams after selective reaction of Sr with CH₃F in the cell. The technique is used for the Rb/Sr age determination of magmatic rocks, without any sample pre-treatment except for the dissolution of the samples. Sr isotope ratios can thus be measured with a precision that is very near to counting statistics and the measured age and initial ⁸⁷Sr/⁸⁶Sr ratio of the rocks fully agrees with reference values of thermal ionisation mass spectrometry (TIMS). The precision of the age is of the same order as the precision of the TIMS results.