OMS Letters
To The Editor-in-Chief Sir,
Charge-remote and Charge-reversal Competition
The analytical utility of charge-remote fragmentations (1 ) has generated considerable interest in the mechanistic aspects of the fragmentation (2). In compounds containing long-chain alkyl groups, these fragmentations have been proposed to involve expulsion of H, and an alkene through a cyclic transition state which does not involve the charge. Other types of fragmentations have also been attributed to charge-remote fragmentation (1). Evidence for the absence of charge involvement was threefold: 1) Alkenes and unsaturated fatty acids were observed in the pyrolysis MS of neutral fatty acid esters (H, was not specifically looked for) (3). 2) Fragmentation in cholesterol sulfate, in which interactions between the charge-bearing sulfate group and the sidechain were sterically prohibitive, was observed to occur primarily from the side-chain.
- No randomization of the hydrogens in labelled fatty acids was observed as might be expected if the charge migrated down the hydrocarbon chain.
Cooks and co-workers have recently published two papers in which several valid concerns are raised regarding the inference that there is no charge involvement in these fragmentations (43). These concerns include the observation that the energy required to obtain spectra containing charge-remote fragmentation is dependent on the nature of the charge-bearing group. The authors stated that this group dependence may be due either to charged-site intervention in the charge-remote fragmentation or to competitive fragmentations. An additional concern is that the critical energy for these reactions can be as low as 1.3-1.9 eV (6.7) while laboratory collision energies in the keV range are optimum for observing the phenomenon.
The apparent inconsistency between low critical energy and the importance of high laboratory collision energy is most apparent for reactions involving long-chain alkyl groups under negative ion conditions. This led us to consider which factors, such as competitive reactions, might be important under negative ion conditions that might not be operative in the positive ion mode. In this letter we are reporting some observations that are consistent with the existence of reactions competitive with charge-remote fragmentations under negative ion conditions.
The 8 keV collision spectrum of the oleate anion (Fig. 1) contains few ions (eg. m/z 58, 71 , 86, and 100 ) that might arise from reactions competitive with the chargeremote fragmentations. In the CAD spectrum of the stearate anion obtained at 400 eV on a hybrid instrument by Fraley and Lawrence (8) these charge-proximate ions have increased in abundance relative to the charge-remote fragmentation. These reactions might be considered as potential competitive reactions. Under 30 eV multiple collision conditions in a triple quadrupole, however, only a small amount of water loss is observed while the CAD spectrum of stearyl sulfate contains both chargeremote and charge-proximate fragmentations under 30 eV multiple collision conditions (4). In contrast, under single collision conditions at 30 eV, Cooks and co-workers observed no charge-remote fragmentations. These authors, however, did observe charge-remote fragmentation at 30 eV (single collisions) for the n-dodecylbenzenesulfonate anion (4). This fragmentation to give CH,=CHC,H,SO;mi ght involve a charge-proximate reaction, however, through a resonance stabilized anion. From these data, one must conclude that, if there is a reaction competitive with the chargeremote fragmentations, it is also competitive with the charge-proximate reactions and is not observable in the negative ion spectra. One process suggested by Gross and co-workers (2,9) which may fulfill both requirements is electron stripping. Loss of one electron will produce a neutral which cannot be observed. Loss of a second electron