On the chemical resolution of the 87Rb + (s 0)/ 87Sr + (s 1) isobaric interference: A kinetic search for an optimum reagent

Abstract

Room-temperature reactions of the atomic cations Sr + and Rb + have been surveyed systematically with a variety of gases using an Inductively-Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients and product distributions have been measured in He buffer gas at 0.35 Torr and 295 K for reactions of Sr + and Rb + with CH 3F, CH 3Cl, N 2O, CO 2, CS 2, SF 6, D 2O and NH 3. Rb + (s 0) is seen to be quite inert with these molecules and reacts either slowly by molecule addition or not at all, while Sr + (s 1) is much more reactive with all these 8 molecules, especially with CH 3F, CH 3Cl, N 2O and SF 6. Sr + reacts with CH 3F and SF 6 by F-atom transfer, with CH 3Cl by Cl-atom transfer and with N 2O by O-atom transfer, and the reaction rate coefficients are all quite high, k ≥ 1.4 × 10 −11 cm 3 molecules −1 s −1. The extreme differences in reactivity with CH 3F, SF 6, CH 3Cl and N 2O provide a chemical basis for the separation of isobaric interferences of 87Rb + and 87Sr + often encountered in ICP–MS. Among these four molecules, SF 6 exhibits the largest difference in reactivity, almost a factor of 10 4, and so is identified as the kinetically recommended reagent for the chemical resolution of the isobaric interference of 87Rb + and 87Sr +.