On the chemical resolution of the 87Rb+ (s0)/87Sr+ (s1) isobaric interference: A kinetic search for an optimum reagent

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 CH3F, CH3Cl, N2O, CO2, CS2, SF6, D2O and NH3. Rb+ (s0) is seen to be quite inert with these molecules and reacts either slowly by molecule addition or not at all, while Sr+ (s1) is much more reactive with all these 8 molecules, especially with CH3F, CH3Cl, N2O and SF6. Sr+ reacts with CH3F and SF6 by F-atom transfer, with CH3Cl by Cl-atom transfer and with N2O by O-atom transfer, and the reaction rate coefficients are all quite high, k ≥ 1.4 × 10−11 cm3 molecules−1 s−1. The extreme differences in reactivity with CH3F, SF6, CH3Cl and N2O provide a chemical basis for the separation of isobaric interferences of 87Rb+ and 87Sr+ often encountered in ICP-MS. Among these four molecules, SF6 exhibits the largest difference in reactivity, almost a factor of 104, and so is identified as the kinetically recommended reagent for the chemical resolution of the isobaric interference of 87Rb+ and 87Sr+.