The MH+ ions of N-methylaniline, N,N-dimethylaniline, N-ethylaniline, and N
-N-diethylaniline, when prepared by CH, chemical ionization, show significa
nt elimination of H-2, in both metastable ion and low energy collision-indu
ced fragmentation. In contrast, the MH+ ions of the same species produced b
y fast atom bombardment (FAB) ionization show no elimination of H-2, in eit
her metastable ion or collision-induced fragmentation. Other metastable ion
fragmentation reactions involve loss of CH, for the methylanilines and los
s of C2H4 and C2H5 for the ethylanilines. The MH+ ions prepared using CH,OH
or (CH3)(2)CO as Br phi nsted acid reagents also show H-2 elimination wher
eas protonation using i-C4H9NH2 as reagent produces MH+ ions that show esse
ntially no ii, elimination. It is concluded that FAB ionization leads to th
e N-protonated species whereas protonation in the gas phase using CH4, CH3O
H, or (CH3)(2)CO as reagents leads to substantial formation of the ring-pro
tonated tautomer. Protonation of the ring is endothermic for i-C4H9NH3+ as
reagent ion. Because N-protonation is thermodynamically favoured, the resul
ts provide further evidence for kinetic control in gas-phase Br phi nsted a
cid chemical ionization. (Int J Mass Spectrom 195/196 (2000) 33-43) (C) 200
0 Elsevier Science B.V.