Study of Ion—Molecule Reactions and Collisionally-Activated Dissociation of Dopamine and Adrenaline by an Ion Trap Mass Spectrometer with An External Ionization Source

Study of the reaction mechanisms for ion–molecule reactions and for collisionally-activated dissociations (CAD) of dopamine and adrenaline has been performed using an external chemical ionization source quadrupole ion trap mass spectrometer. This work demonstrates the possibility of applying an external 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 with relative intensity as low as 0–2%, formed as ion–molecule products of dopamine and adrenaline, have been successfully performed. Study of some fragment ions of M^+• and [M + H]⁺, observed in the chemical ionization (CI) spectra, by CAD techniques, permits elucidation of a series of mechanisms for the sequential dissociations of the M^+• and [M + H]⁺ ions. Thus, the structural information obtained from this method is similar to that which would have been obtained if MS ⁿ had been performed for M^+• and [M + H]⁺ ions. From the proposed CAD reaction mechanisms and the semi-empirical calculations, 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=C₂H⁺ ion addition, dopamine is either on the amino group or on the phenyl ring. However, adrenaline is only on the benzylic hydroxyl group. Temperature effects on the formation of the ion–molecule products were also investigated. It was shown that the best source temperature for formation of [M + H]⁺ and [M + 13]⁺ ions of dopamine is 200°C. Information about use of dimethyl ether (DME) as the reagent gas in the external chemical ionization of an ion trap mass spectrometer is provided.