Use of electrothermal vaporization for volatility-based separation of Rb–Sr isobars for determination of isotopic ratios by ICP-MS

Abstract

A new method of separating Rb and Sr by ETV-ICP-MS is demonstrated. An electrothermal vaporizer (ETV) is used for sample introduction and control of the ETV heating cycle provides temporal separation of the isobars at m/z 87, allowing ⁸⁷Rb and ⁸⁷Sr determination within the same sample. The method utilizes an inductively coupled plasma mass spectrometer (ICP-MS) for ionization and detection. A one or two vaporization stage heating method is used for the ETV separation, and initial methodology and feasibility experiments were conducted on an ETV-ICP-TOFMS. Final measurements were made using a MC-ICP-MS for data collection. The method uses minimal sample preparation and no front end chromatographic separation of Rb and Sr prior to analysis. The technique is applied to the analysis of a potassium feldspar sample (NIST SRM 607) and is demonstrated to be viable as a means of determining isotopic data for the Rb–Sr geochronological technique. The standard was determined to have a ⁸⁷Sr/⁸⁶Sr ratio of 1.1876 ± 0.011, a Sr concentration of 64.1 ± 4.7 µg g⁻¹, and a Rb concentration of 557 ± 6 µg g⁻¹ (95% CI). This corresponds to a model age of 1266 ± 186 million years, statistically similar to the certified value of 1409 ± 14 for the standard. Error analysis is presented for several key calculations and a discussion is presented regarding approaches to improve precision.