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⁺.