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A quadrupole, quistor, quadrupole tandem mass spectrometer allowing selected ion/selected molecule reactions was built. The quistor will be used as a reaction chamber for the study of organic ion-molecule reactions. Ions are generated in a differentially pumped ion source, quadrupole mass selected and injected into the quistor. The ions are trapped in the quistor by the combined action of a deceleration lens, the presence of helium buffer gas and the quistor RF field. As a first test of the performance of the instrument, the reaction rate constant of the reaction of 0;. with methane was measured to be 5 x lo-'' cm3 s -I . This is in good agreement with literature values, indicating that the ions are near thermal equilibrium after a few milliseconds of trapping time.

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The ion-molecule reactions between protonated acetaldehyde and methanol were explored on a quadrupole, quistor, quadrupole tandem mass spectrometer. In addition to proton transfer to methanol and subsequent methanol ionmolecule chemistry, the protonated acetaldehyde-methanol adduct and methylated acetaldehyde could be detected. Experiments with labelled counterparts showed that the formation of methylated acetaldehyde corresponds to substitution of the oxygen-bound hydrogen of protonated acetaldehyde by the methyl group of methanol. The reaction rates involved and H-D exchange rates were determined.

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