Laser desorption ionization of gramicidin S on thin silver films with matrix isolation in surface plasmon resonance excitation

Laser desorption ionization (LDI)/laser ablation (LA) time-of-flight mass s pectrometry (TOFMS) studies are performed via back-illumination on thin sil ver films deposited with gramicidin S, a cyclic decapeptide (1141 Da). Expe rimental results indicate an electronic excitation mechanism at low laser f luences and a thermal process at high laser fluences. In a perpendicular or ientation of the thin silver film with respect to the drift tube axis, bett er success of chemical ionization (CI) of neutral silver atoms/clusters and gramicidin S molecules by Ag+ monomer cations within the ablation plume is evident. When these LDI/LA studies are repeated under surface plasmon reso nance (SPR) conditions, the neutral atoms/clusters/molecules undergo SPR LD I/LA via a different laser energy transfer process compared to direct LDI/L A. These results also indicate an electronic excitation mechanism, only wit h a lower laser fluence requirement. Upon addition of a matrix, 2,5-dihydro xybenzoic acid (DHB), CI of gramicidin S molecules by Agi monomer cations i s hindered while the CI of gramicidin S molecules by Na+ cations can occur at an even lower laser fluence. Thus, SPR LDI/LA with matrix isolation is a n excellent technique for the mass analysis of gramicidin S. (C) 1998 Elsev ier Science S.A. All rights reserved.