87 Sr/86 Sr isotope ratios in rocks determined using inductively coupled plasma tandem mass spectrometry in O2 mode without prior Sr purification.

Abstract

An inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) instrument can be developed to determine 87 Sr/86 Sr ratios with an external precision better than 0.05% relative standard deviation (RSD) in "mass shift" mode without prior Sr purification. Previous studies suggested using CH3 F, N2 O, and SF6 as reaction gases for this method because a better reaction rate can be achieved with Sr+ than with O2 in the reaction cell. However, these gases are not commonly used in general chemistry laboratories, and processes using these gases are difficult to implement quickly due to regulations. We aim to develop a rapid method that can be applied to many samples for the accurate determination of 87 Sr/86 Sr isotope ratios with precision below 0.1% RSD (or approximately to the fourth decimal place). We evaluated the accuracy and precision of 87 Sr/86 Sr ratios in certified reference materials and different rock types determined using ICP-MS/MS with O2 as the reaction gas in comparison with those determined using the multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) method. This study showed that by using the ICP-MS/MS method, the 87 Sr/86 Sr ratios of BCR-2 and BHVO-2 do not vary significantly with and without prior Sr purification; when the Sr concentration of the measured solution is within the range of 60-350 ng/mL, there is no significant effect on the measured 87 Sr/86 Sr ratios. The results also showed that the 87 Sr/86 Sr ratios of 23 different rock types measured by ICP-MS/MS and MC-ICP-MS methods agree very well. The precision of the 87 Sr/86 Sr ratio measured using ICP-MS/MS varies between 0.0001 and 0.0019 (2SD). This precision is less than that of the MC-ICP-MS method but is sufficient for certain applications, such as identifying 87 Sr/86 Sr ratios in different rock types. These results suggest that the developed ICP-MS/MS method has the potential for future studies involving the identification of Sr sources.