Effects of ionization mode on charge-site-remote and related fragmentationreactions of long-chain quaternary ammonium ions

Comparison of collisionally activated fragment spectra of long-chain quater nary ammonium ions, formed by liquid-assisted secondary ion mass spectromet ry (LSIMS) and electrospray ionization (ESI), shows the latter are dominate d by radical cations while the former yield mainly even-electron charge-sit e-remote (CSR) fragments, similar to the report for different precursors by Cheng et al., J. Am. Soc. Mass Spectrom. 1998, 9, 840. Here, mixed-site fr agmentation products (formal loss of a radical directly bonded to the nitro gen plus a radical derived from the long chain) are of comparable importanc e for both ionization techniques. These observations are difficult to under stand if the CSR ions are formed by a concerted rearrangement-elimination r eaction, since precollision internal energies of the ESI ions are much lowe r than those of the ions from LSIMS. Alternatively, if one discards the con certed mechanism for high-energy CA, and assumes that the even-electron fra gments are predominantly formed via homolytic bond cleavage, the colder rad ical cations from ESI survive to the detector while the more energized coun terparts from LSIMS preferentially lose a hydrogen atom to yield the CSR io ns, as proposed by Wysocki and Ross (Int. J. Mass Spectrom. Ion Processes 1 991, 104, 179). The present work also attempts to reconcile discrepancies i nvolving critical energies and known structures for neutral fragments. (J A m Soc Mass Spectrom 2001, 12, 571-579) (C) 2001 American Society for Mass S pectrometry.