Role of photoionization and photochemistry in ionization processes of organic molecules and relevance for matrix‐assisted laser desorption lonization mass spectrometry

Abstract

The ionization in ultraviolet laser desorption was investigated for a large number of small polar organic molecules which have a strong resonance absorption at the laser wavelength. In many cases, both positive-and/or negative-ion mass spectra show strong signals of ion species which deviate from the simple scheme of even-electron quasi-mo\ecular and fragment ion formation commonly expected for desorption techniques. These are radical cations and ion species formed by single and multiple hydrogen cleavage or addition. A model is proposed and discussed which explains these features assuming photoionization as the common initial ionization step followed by ion-molecule reactions to the final product ions. The mass spectra of all compounds proved to function well in matrix-assisted ultraviolet laser desorptionfionization show characteristic features indicative of their photochemical reactivity. This observation substantiates the hypothesis of the essential role of the matrix in analyte ionization

Sodium Cation Affinities of Commonly Used MALDI Matrices Determined by Guided Ion Beam Tandem Mass Spectrometry

“…Because a large number of experimental parameters influence the ionization processes that occur in MALDI, the ionization mechanisms and kinetics are not fully understood despite its widespread usage and popularity. In an attempt to better understand and control MALDI, several groups have focused on studies directed at elucidating the mechanisms that lead to MALDI [18][19][20][21][22][23][24]. The proposed mechanisms are based on empirical observations, but thus far no single mechanism has been proposed that is able to explain all of the experimental observations.…”

“…However, most ionization including cationization is believed to occur in the gas phase [39]. Although MALDI ionization mechanisms are not fully understood, there exists strong experimental evidence that secondary ionization processes occur in the laser plume and are responsible for metal-ligand exchanges [21,40,41]. Sodiated analytes are believed to come from sodium cation transfer reactions that occur in the gas phase [42].…”

Protonation of Glyn Homologues in Matrix-Assisted Laser Desorption Ionization

“…2 The contribution of the different reaction channels in this mechanism heavily depended on the laser wavelength and on the yield of the two possible initiating species.…”

Using fluorescence dyes as a tool for analyzing the MALDI process

“…With regard to analyte ionization, the essential role of protonated matrix species was already discussed in the early MALDI papers proposing that the central reaction of analyte ionization in the positive ion mode is a proton transfer from protonated matrix species to analyte molecules. A so-called "photochemical" ionization model was proposed by Ehring et al [8]; energy pooling processes and resonant two-step absorption were assumed to occur and to facilitate the generation of the primary radical cation matrix species for which more than one UV photon is needed. Karbach et al experimentally determined the ionization potential of 2,5-DHB to be higher than the energy content of two 337 nm UV photons [9].…”