Hydration of gas-phase ions and the measurement of boundary-layer cooling during flame sampling into a mass spectrometer

Abstract

The gas-phase ions present in trace amounts in flames have been observed by continuous supersonic sampling into a mass spectrometer. In the general case, exemplified so far by H₃O⁺ and Li⁺, the extent of hydration of an ion is increased as the sample passes through the relatively cool boundary layer around the sampling orifice and also during the subsequent supersonic and near-adiabatic expansion into the instrument. However, for all other ions studied (e.g. Na⁺, K⁺, CaOH⁺, Cl^–, NO⁺) monohydration in a sample is found to be equilibrated and quenched at the orifice's entrance, where conditions are well defined, because the gas velocity is sonic. This fact has enabled measurements to be made of enthalpies and entropies of monohydration of several such gas-phase ions. In addition, it proved possible to measure the extent of the cooling in the boundary layer of this system, using measurements of the relative concentrations of an ion and its first hydrate.