Ion–molecule equilibria measurements by high pressure mass spectrometry. Some recent advances in concepts and technique

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Abstract
Two efforts to improve ion–molecule equilibrium measurement methodology and instrumentation are described. The first part deals with dissociation of ions AB+ in the mass analysis system of the mass spectrometer. To measure the equilibrium constant of the association equilibrium A+ + B = AB+ one needs to sample both A+ and AB+. The AB+ ions escaping from the high pressure ion source have a thermal energy distribution. A fraction fex has internal energy which is higher than the dissociation energy for AB+ → A+ + B. This fraction can dissociate before detection. A theoretical analysis for the system K+(H2O)n is made which is based on calculated vibrational frequencies and geometries of K+(H2O)n. From these fn,ex can be evaluated. RRKM calculations also provide rates for the dissociation of the AB+ex. It is found that severe dissociation during mass analysis can occur if the experimental conditions are not selected carefully. Results for H+(H2O)2 and (C6H6+)2 are also presented. Measurements of the association equilibria must be done at low pressures of B in the presence of high pressure of an inert bath gas.By connecting the repeller to a gated radio frequency source, pulses of RF can be applied in the high pressure ion source. These increase the internal energy of the ions. If the reaction system was at equilibrium a displacement of the equilibrium is produced by the RF pulse. The system relaxes to the original state after the RF has been shut off. Kinetic measurements of the relaxation times provide rate constants form which an equilibrium constant not affected by ion mass discrimination can be determined. Fast reaction kinetics also can be studied with this technique.

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