JOURNAL OF MASS SPECTROMETRY, VOL. 30, 1742 1746 zyxwvuts (1995) zyxwvuts Decomposition Channels Alternative to Remote-charge Fragmentations Observed in Mass Spectrometry of Alkyltrimethylammonium Cations Massimo Barnbagiotti-Alberti, Silvia A. Coran and Fabrizio Benvenuti Piero LoNostro Dipartimento di Chimica, Universita di Firenze, Via G. Capponi 9,1-50121 Firenze, Italy Silvia Catinella, Donata Favretto and Pietro Traldi? Dipartimento di Scienze Farmaceutiche, Universita di Firenze, Via G. Capponi 9,1-50121 Firenze, Italy CNR Area di Ricerca, Corso Stati Uniti 4,1-35020 Padova, Italy A series of alkyltrimethylammonium cations, generated by fast atom bombardment from 1-dodecanaminium-, 1-tetradecanaminium- and zyxwvutsr 1-hexadecanaminium-N,N,N-trimethyl bromides, were studied by high-energy collisions and comparison with N,N,N-trimethyl-d, labelled analogues. The results show that, together with remote-charge fragmentation processes, alkyltrimethylammonium cations exhibit further decomposition pathways which may be explained by interaction between the alkyl chain and the methyl group linked to the nitrogen atom. The sharp interruption of remote-charge fragmentation at the C-5 atom was observed for all compounds and explained as due to the formation of an energetically favoured six-membered cyclic structure. INTRODUCTION In the last decade, numerous papers have appeared on a unique type of decomposition pathway, called remote- charge fragmentation,' typical of closed-shell ions with a long hydrocarbon chain and consisting of the primary loss of zyxwvutsrq CnH2n+Z neutral species arising from the alkyl terminus. This type of decomposition, first observed in 1983 by high-energy collision of carboxylate anions arising from fatty acids and their methyl esters,' has been the object of many investigations on various sub- strates such as alkylsulphates, steroids and alkyl- sulphonate anions3 as well as alkylammonium and alkylphosphonium cation^.^ Protonated molecules of alkylamines, amides, carboxylic acid pyrrolidides and picolinyl esters also show the same behaviour.' Further studies showed that remote-charge fragmentation is not characteristic of high-energy collision alone but also of slow decomposition, as studied by metastable ion spectra.6 The mechanism of these processes has been proposed to be similar to gas-phase thermolysis. The internal energy required to obtain spectra exhibiting remote-charge fragmentation processes has been determined' and found to be compound dependent. In a recent paper, a series of quaternary ammonium halides were studied by fast atom bombardment mass spectrometry (FAB/MS). Interestingly, remote-charge fragmentation products were detected in simple FAB spectra, in perfect analogy with what is observed in high-energy collisions.8 As an example, peaks corre- sponding to losses of CH,, C,H,, C3H,, etc., were t Author to whom correspondence should be addressed detectable in low abundances (1-5%) but with a signal- to-noise ratio in the region of 10: I, making these peaks diagnostically significant from the analytical point of view. However, on that occasion it was observed that CnH2n+2 losses are not operative for each zyx n value, but that there is an n value above which the remote-charge fragmentation processes do not take place. The object of this study was to investigate this behaviour. This was performed by collisionally induced decomposition studies and deuterium-labelling experiments on a series of quaternary ammonium halides, i.e. 1-dodecanaminium-N,N,N-trimethyl bromide (I), 1-tetradecanaminium-N,N,N-trimethyl bromide (2). 1-hexadecanaminium-N,N,N-trimethyl bromide (3) (usually named dodecyltrimethyl- ammonium, tetradecyltrimethylammonium and hexadecyltrimethylammonium bromide, respectively) and N,N,N-trimethyl-d, labelled analogues (la -3a). EXPERIMENTAL All mass spectrometric measurements were performed under positive-ion FAB9 ionization conditions (8 keV xenon atoms bombarding glycerol solutions of the sample) on a VG ZAB2F" (VG Analytical, Fisons, Manchester, UK) double-focusing mass spectrometer. Collisionally activated dissociation mass-analysed ion kinetic energy (CAD-MIKE) spectra were obtained using air as collision gas." Alternatively, when a higher mass resolution was required, the collisions were per- formed in the first field-free region and the product ions were detected by BIE = constant linked scans.I2 CCC 1076--5 174/95/12 1742 05 zyxwvuts ((3 1995 by John Wiley & Sons, Ltd. zyxwvutsr Received 7 July 1995 Accepted 6 September 1995