Molecular monodisperse mobility and mass standards from electrosprays of tetra-alkyl ammonium halides

The mobility distributions of electrosprays from solutions of relatively large tetraalkyl ammonium halide salts A+B- produce a series of sharp peaks associated to clusters of the form (A+)z(AB)n, dominated by the bare cation A+. In order to facilitate their use as standards of mass and mobility (in air), these properties are characterized via tandem use of a differential mobility analyzer (DMA) and a mass spectrometer (MS) with an atmospheric pressure inlet. Mobility diameters (based on Millikan's conventional size vs. mobility relation) in excess of 3 nm are obtained from singly and doubly charged clusters. The mobilities of the ions A+(AB)3 are anomalously large for alkyl groups from methyl to butyl, presumably due to their relatively high symmetry.