Insight into absorption of radiation/energy transfer in infrared matrix-assisted laser desorption/ionization: the roles of matrices, water and metal substrates

Although the ionization/desorption mechanisms in matrix-assisted laser deso rption/ionization (MALDI) remain poorly understood, there is a clear differ ence between the energy absorption processes in the ultraviolet (UV) and in frared (IR) modes of operation. UV-MALDI demands an on-resonance electronic transition in the matrix compound, whereas results presented here support earlier work showing that a corresponding resonant vibrational transition i s not a requirement for IR-MALDI. In fact, data from the present study sugg est that significant absorption of radiant energy by a potential matrix imp airs its performance, although this observation is at variance with some ot her reports. For example, sinapinic acid, with an IR absorption maximum clo se to the 2.94 mu m wavelength of the Er-YAG laser, has been little used as an IR-MALDI matrix. By contrast, succinic acid, with much lower IR absorpt ion and no history of use in UV-MALDI as it has no UV absorption at the wav elength of common UV lasers, has become widely recognized as a good general purpose matrix for IR-MALDI, Despite reports by others that glycerol is an effective matrix for IR-MALDI, we find that glycerol, which also absorbs s trongly at 2.94 mu m, is useful only if applied as a very thin film. Thus t he cumulative evidence for the role of the matrix in IR-MALDI appears confu sing and often contradictory. Water has been postulated to be a major contr ibutor to the absorption of energy in IR-MALDI, Consistent with this, we fi nd that samples dried from D2O, which does not absorb at 2.94 mu m, give sp ectra of inferior quality compared with the same samples from H2O, Similarl y, samples dried under vacuum, that probably contain less water than those dried in the open laboratory, give weaker and more erratic spectra, Another potential participant in energy absorption and energy transfer is the surf ace of the metal support, an alternative mechanism for IR-MALDI, for which some evidence is presented here. Copyright (C) 1999 John Wiley & Sons, Ltd.