Correction of spike contribution for strontium isotopic measurement by thermal ionization mass spectrometry: a test for spike-standard mixed solutions

Background: The Rb-Sr isotope system has long been used for radiometric dating and petrogenetic investigation. The concentrations of Rb and Sr could be precisely measured by isotope dilution thermal ionization mass spectrometry combined with chemical purification of these elements. For the simultaneous measurement of Sr isotopic composition and isotope dilution, the contribution from the added spike should be carefully corrected. Findings: Reliable ⁸⁷Sr/⁸⁶Sr ratios of the spike-standard mixed solutions were obtained using the new mass bias factor calculated on the basis of measured Sr isotopic ratios. This correction yielded reasonable ⁸⁷Sr/⁸⁶Sr ratios for overspiked standard solutions whose spike fractions reaching 25 wt.%. Conclusions: The correction procedure described in this study shows that the simultaneous measurement of Sr isotopic composition and isotope dilution is available for the case that the spike proportion in the sample-spike mixture is significantly high (⁸⁴Sr/⁸⁶Sr > 3.7). The principle of this correction protocol can also be applied to other isotope systems such as Sm-Nd and Lu-Hf pairs.