Structure elucidation by ion trap sequential mass spectrometry of radical cations formed in low-energy charge-exchange reactions

Abstract

A low-pressure, low-energy charge-exchange reaction with furan is shown to be a convenient method for generating the requisite intense ion signals from molecular radical cations of dextromethorphan for MS n (sequential, product-ion scans) analysis. Little fragmentation results, concentrating ion current in the molecular species and facilitating MS n probing. Results for the odd-electron species are shown to provide some unique information compared to electrospray ionization MS n of the even-electron, protonated molecule of dextromethorphan. Copyright # 1999 John Wiley & Sons, Ltd. Received 18 February 1999; Accepted 20 February 1999 Previously we have shown the direct structure elucidation of a surfactant in a complex aqueous mixture via sequential product ion analysis (so-called MS n ) using an ion trap. The mixture was analyzed via infusion with electrospray ionization (ESI), using the trap as a gas-phase purification device, followed by structural probing. 1 The main advantages of the MS n experiment compared to triple quadrupole MS/MS analyses were found to be: (1) complete definition of genealogical relationships of fragment ions, (2) cumulative energy deposition over several stages, giving more complete fragmentation, and (3) clear elucidation of informative minor pathways because of enhanced selectivity of later generation (n b 2) experiments.Our experience over the last four years has been that successful structure-elucidation MS n studies out to 6-10 generations are dependent on: (1) the production of an adequate current of pseudomolecular ions (i.e. some species indicative of the intact molecule whose structure is being probed) and (2) little initial fragmentation. Thus, ESI often meets these criteria for MS n studies. For example, when using the ESI positive ion mode, protonated, ammoniated or natriated molecules might be produced, depending on solvent selection. However, these species are all evenelectron ions and fragmentations are often dominated by even-electron, neutral losses. Since all ions can be classified as even or odd electron, it seemed logical that MS n studies of odd-electron species (i.e. molecular ions) might yield unique structural information via different fragmentation pathways (e.g. loss of radicals). Such studies would rely on the production of a high number density of molecular ions with little initial fragmentation (low internal vibrational energy). It is noted here that the term molecular ion most commonly refers to a radical species produced by removal or addition of an electron to a neutral molecule. However, for clarity in this report, the somewhat redundant molecular radical cation will be used.In order to produce the required intense ion signals from molecular radical cations for MS n studies, it is suggested that using a low-energy, charge-exchange reaction (LE-CHER) at low reagent gas pressure is a particularly viable approach for ion traps. The fundamental principles of charge exchange have been discussed by many authors [2][3][4][5] and will not be reiterated here, ...

Ion trap MSn genealogical mapping?approaches for structure elucidation of novel products of consecutive fragmentations of morphinans

“…Indeed, the EI spectrum of dextromethorphan features a prominent ion of m/z 150 (Scheme ) 14. Moreover, we recently performed a selective low‐energy charge exchange reaction (LECHER) experiment on dextromethorphan to produce m/z 271, and upon MS/MS of this species, produced m/z 15015 We were recently able to produce a LECHER MS 6 result for m/z 271 using a modern, bench‐top ion trap with internal ionization and low pressure CI (unpublished results).This experiment will be a useful paradigm for investigating structures of switched, radical ions.…”