Laser desorption ionization (LDI)/laser ablation (LA) time-of-flight mass spectrometry (TOFMS) studies are performed via back-illumination on thin silver films deposited with gramicidin S, a cyclic decapeptide (1141 Da). Experimental results indicate an electronic excitation mechanism at low laser fluences and a thermal process at high laser fluences. In a perpendicular orientation of the thin silver film with respect to the drift tube axis, better 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 resonance (SPR) conditions, the neutral atoms/clusters/molecules undergo SPR LDI/LA via a different laser energy transfer process compared to direct LDI/LA. These results also indicate an electronic excitation mechanism, only with a lower laser fluence requirement. Upon addition of a matrix, 2,5-dihydroxybenzoic acid (DHB), CI of gramicidin S molecules by Ag+ monomer cations is 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 an excellent technique for the mass analysis of gramicidin S.

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Keywords Chemical ionization, Electronic excitation mechanism, Laser desorption ionization, Matrix isolation, Silver film substrate, Surface plasmon resonance, Time-of-flight mass spectrometry
Journal Journal of Photochemistry and Photobiology A: Chemistry
Owega, S. (Sandy), Lai, E. P, & Mullett, W.M. (Wayne M.). (1998). Laser desorption ionization of gramicidin S on thin silver films with matrix isolation in surface plasmon resonance excitation. Journal of Photochemistry and Photobiology A: Chemistry, 119(2), 123–135.