The use of dynamic reaction cell ICP mass spectrometry to facilitate Rb-Sr age determination

By using a set of granitic whole rock samples, originating from the Regensburg Forest (Germany) and dated (approximately 350 Ma old) previously by means of thermal ionization mass spectrometry (TIMS), the capability of dynamic reaction cell (DRC) inductively coupled plasma mass spectrometry (ICPMS) for Rb-Sr age determination was demonstrated. With DRC-ICPMS, chemical separation of Sr from Rb during the sample pretreatment is no longer required as interference-free determination of the (87)Sr/(86)Sr isotope ratio can be accomplished by monitoring the signals of the SrF(+) adduct ions, formed as a result of the selective reaction between the Sr(+) ions extracted from the ICP and the reaction as CH(3)F. The mass discrimination was established to depend strongly on the matrix composition. This drawback could be overcome by using the United States Geological Survey reference material G-2 as an external standard. Results obtained by DRC-ICPMS (age and initial (87)Sr/(86)Sr ratio) showed an excellent agreement with both (a) experimental values obtained by means of quadrupole-based and sector field ICPMS after isolation of Sr via cation exchange chromatography and (b) TIMS literature values. In addition, DRC-ICPMS offers a smaller combined uncertainty on the isotope ratio results as a result of (a) an improved internal isotope ratio precision (< 0.1% RSD when also using Ne as an additional non-reactive collision gas) and (b) the fact that, in contrast to quadrupole-based and sector field ICPMS, no correction for the remaining overlap between the signals of 87 Sr+ and (87)Rb(+) is required.