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 CH(3)F, CH(3)Cl, N(2)O, CO(2), CS(2), SF(6), D(2)O 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(3)F, CH(3)Cl, N(2)O and SF(6). Sr(+) reacts with CH(3)F and SF(6) by F-atom transfer, with CH(3)Cl by Cl-atom transfer and with N(2)O by O-atom transfer, and the reaction rate coefficients are all quite high, k > or = 1.4x10(-11) cm(3) molecules(-1) s(-1). The extreme differences in reactivity with CH(3)F, SF(6), CH(3)Cl and N(2)O provide a chemical basis for the separation of isobaric interferences of (87)Rb(+) and (87)Sr(+) 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 (87)Rb(+) and (87)Sr(+).