Field desorption mass spectra of the cyclic depsipeptide valinomycin: Energy deficits in ion formation

The formation of ions of the cyclic depsipeptide valinomycin by field desorption has been investigated in detail. The ratio of molecular ions M+˙ to protonated molecules [M + H]+ has been found to be independent of emitter heating current and the nature of particular additives to the solution. In contrast, the abundance of sodiated [M + Na]+ ions compared with the molecular ions and protonated molecules was strongly dependent on the heating current. The energy deficits for M+˙ and [M + H]+ ions were similar, but were significantly larger than the energy deficits for [M + Na]+ ions. There appeared to be no relationship between the abundance of cationized species generated by field desorption and the selectivity of valinomycin for monovalent cations in solution. The implications of these observations for the mechanism of field desorption are discussed.