Study of ion-molecule reactions and collisionally-activated dissociation of dopamine and adrenaline by an ion trap mass spectrometer with an externalionization source

Study of the reaction mechanisms for ion-molecule reactions and for collisi onally-activated dissociations (CAD) of dopamine and adrenaline has been pe rformed using an external chemical ionization source quadrupole ion trap ma ss spectrometer. This work demonstrates the possibility of applying an exte rnal source ion trap instrument to perform selective ion-molecule reactions in the gas phase, due to its high sensitivity and low detection limits in mass spectrometry/mass spectrometry (MS/MS) mode. CAD experiments on ions w ith relative intensity as low as 0-2%, formed as ion-molecule products of d opamine and adrenaline, have been successfully performed. Study of some fra gment ions of M+. and [M + H](+), observed in the chemical ionization (CI) spectra, by CAD techniques, permits elucidation of a series of mechanisms f or the sequential dissociations of the M+. and [M + H](+) ions. Thus, the s tructural information obtained from this method is similar to that which wo uld have been obtained if MSn had been performed for M+. and [M + H](+) ion s, From the proposed CAD reaction mechanisms and the semi-empirical calcula tions, the favored reactive sites for formation of the adduct ions could be determined. The reactive site for protonation of dopamine is on the amino group, but for adrenaline, it is on the benzylic hydroxyl group. As to the reactive site for the CH3O=CH2+ ion addition, dopamine is either on the ami no group or on the phenyl ring. However, adrenaline is only on the benzylic hydroxyl group. Temperature effects on the formation of the ion-molecule p roducts were also investigated,It was shown that the best source temperatur e for formation of [M + H](+) and [M + 13](+) ions of dopamine is 200 degre esC, Information about use of dimethyl ether (DME) as the reagent gas in th e external chemical ionization of an ion trap mass spectrometer is provided .