METHYL-IODIDE - AN INTERESTING REAGENT FOR POSITIVE-ION CHEMICAL-IONIZATION

Methyl iodide (CH3I) has been used as a chemical ionization reagent ga s and the ion/molecule reactions of formed reagents ions with a number of nucleophiles were studied in a chemical ionization source of a con ventional mass spectrometer, Generalizing, charge exchange reactions d ominate for compounds with lower ionization energy than CH3I, while th e others preferred protonation and methylation reactions. For ketones both protonation and charge exchange reactions were thermodynamically possible and the prevailing reaction was dependent on the compound. Su rprisingly, all the isomeric C-5 ketones gave rise to clearly differen t spectra. Alcohols, having higher ionization energies than CH3I and a lower proton affinity than that of CH2I., also reacted interestingly. Ionization through hydroxide abstraction prevailed for C-3-C-6 alkano ls. The formation of direct adduct ions was rare but, in contrast, sub stitution product ions [M - OH + CH3I](+) and [M - H2O + I](+) were ob served for alkanols. (C) 1997 by John Wiley & Sons, Ltd.