foxides VIII, X, XVII, and XXIII, the latter affording M? ions having <15% relative abundance. The variations in the abundance of this ion are not surprising because plausible processes leading to its formation (including charge transfer to M, loss of H. from MH+, and direct ionization by electron impact) are sensitive to slight variations in temperature, pressure, and chemical entities in and near the source.
Fragmentation processes involving the trimethylene bridge appeared to be common to the spectra of 11-V, VII-XV, and XVIII-XXIX (phenothiazine-CH&HzCHrbase) and to XVI and XVII, producing weak ions that corresponded in mass to the (substituted) phenothiazin-10-yl nucleus (Ion A) and to its methylene homolog (Ion B). Generally more abundant ions, corresponding in mass to the terminal methylene unit plus the attached aliphatic nitrogen atom and its substituents (Ion C, CH2-base or CHSCH-base) and to the ethylene homolog (Ion D, CaHB-base), were observed at m/e 113 and 141 (XVIII-XXIII) or 143 and 171 (XXIV-XXIX), respectively. Although the corresponding ions in the spectra of 111-XVIII had m/e values <1M) and were, therefore, not measured in this study, the chemical ionization (methane) mass spectra of XI1 and two other hydroxylated homologs of XI1 were found (6) to exhibit prominent ions at m/e 58 and 86, corresponding to [CHzN(CH&]+ and [C3HeN(CH3)2]+, respectively, in accord with the idea that these form by processes general for compounds of this type.
Molecules in this study having polar substituents (halogen or hydroxyl groups) also generally appeared to undergo loss of a hydrogen atom plus the substituent from the MH+ species. Fragments resulting from such processes were relatively abundant in the chemical ionization (methane) spectra but minor or not observed in the corresponding chemical ionization (isobutane) spectra. The derivatives of 2-chlorophenothiazine (I-XII, XVIII-XXI, and XXIV-XXVI) also exhibited an MH+ -34 ion that did not have a satellite ion heavier by two mass numbers; this resulted from contamination by the respective halogen-free homologs.
In summary, derivatives of phenothiazine generally form very stable MH+ and M? ions under conditions of chemical ionization. Although no simple correlation is obvious to account for the variations in relative intensity of fragment ions formed by different compounds in this study, decomposition appears to be limited mainly to cleavage of exocyclic carbon-carbon single bonds, plus loss of water or hydrogen halide in appropriately substituted derivatives. The prominence of these stable proton-capture ions in the chemical ionization spectra affords a measurable property that is not only characteristic of a given drug or metabolite but also exquisitely sensitive for its detection. These two conditions suggest that selective ion monitoring techniques (12) in conjunction with chemical ionization are potentially useful for bioanalytical studies of phenothiazines.