Characterization of isomeric organic cations by Esi/MS: 3-alkyl-5-(4-N-methylpyridinium)-1,2,4-oxadiazole cations, 5-alkyl-3-(4-N-methylpyridinium)-1,2,4-oxadiazole cations and 3-perfluoroakyl-5-(4-N-methylpyridinium)-1,2,4-oxadiazole cations, 5-perfluoroakyl-3-(4-N-methylpyridinium)-1,2,4-oxadiazole cations