An optimized protocol for high precision measurement of Sr/Sr using a Neptune Plus multi-collector inductively coupled plasma mass spectrometer: evaluation of different cone combinations for Sr isotope determination.

Measuring Sr/Sr ratios with MC-ICP-MS is a straightforward technique due to its fast sample introduction. However, excellent accuracy and precision cannot be easily achieved unless careful optimization of plasma running conditions and evaluation of instrumental mass bias are conducted. Here, we developed an optimized protocol for measuring Sr/Sr ratios using a modified cone arrangement (H skimmer cone + Jet sample cone) by carefully examining the effects of plasma working conditions on the performance of Sr isotope analysis with a Neptune Plus™ MC-ICP-MS. The modified cone arrangement significantly enhanced the Sr signal sensitivities by a factor of 2, compared to the standard cone arrangement (H skimmer cone + standard sample cone). For both cone arrangements, the mass bias of Sr isotopes fits the standard exponential law under optimal conditions. However, at non-optimum sample gas flow rates, the corrected Sr/Sr ratios deviated from the reference value, and thus non-linear mass bias was observed. Such mass bias could not be corrected using the standard exponential law. This observation cautioned researchers analyzing Sr ratios at optimized sample gas flow rates when using MC-ICP-MS. With further evaluation of the sample concentration, integration time and interference element correction, Sr/Sr ratio analysis with high accuracy and precision was achieved. Excellent results of the reference materials were obtained using the optimized protocol. Compared to the classical TIMS technique, our method is comparable in precision (∼8 × 10, 2SE) but much faster in operation (14 minutes per analysis), and therefore is a technical advance in Sr isotope geochemistry.