MALDI. A Very Useful UV Light-Induced Process....That Still Remains Quite Obscure

ERRA BALSELLS, ROSA

Fundamentals of Mass Spectrometry

Springer-Verlag

Lugar: New York; Año: 2013; p. 173 - 197

The matrix-assisted laser desorption ionization (MALDI) process constitutes a complex mixture of events, involving optical and mechanical phenomena, as well as thermodynamic and physicochemical processes of phase transition and ionization. The experiment is based on irradiation of the surface of a solid sample that is a mixture of a photosensitizer material (the matrix) and analyte(s) with a short-pulse UV laser. A successful MALDI analysis involves a number of crucial steps, namely sample preparation, UV excitation of the matrix (photosensitizer)?analyte sample and disintegration of the condensed phase, generation and separation of charges and ionization of analyte and matrix molecules, and, finally, in the analysis step, ion separation according to the mass-to-charge ratio in the mass spectrometer, and detection. Despite the rapid adoption of the technique in various areas of science (i.e., chemistry, biology, physics, biomedicine, synthetic polymers, etc.) after its introduction almost 25 years ago, [1, 2] for a long time its mechanisms have not been fully understood, and a more comprehensive picture has only recently begun to emerge. One of the most straightforward experimental tools for gaining insight into mechanisms is the systematic variation of one or more of the relevant input parameters and consequently a large number of fundamental studies into MALDI have followed this method. The roles of the different relevant irradiation (laser) parameters in the desorption/ ionization process, as well as those of the matrix and the preparation protocol, have been addressed in these investigations. These included, for example, the roles of the laser parameters, such as wavelength, pulse duration, and fluence (laser energy per pulse and unit area), the nature of materials, the type of (co-)crystallization and the incorporation of analyte molecules into matrix crystals [3, 4 and references therein]. Other works have addressed the dynamical parameters of the expanding MALDI particle ?plume?: the initial kinetic energies and energy distributions of molecules and ions, and the composition of the plume (ion-to-neutral ratio and ejection of particles and clusters versus the emission of molecular constituents) [3,4 and references therein]. Several researchers have developed theoretical models for the desorption?ablation, as well as for the ionization processes. By far the largest part of experimental investigations has naturally been focused on the MALDI ions as detected in a mass spectrometer. In interpreting these data, it should be kept in mind that ion formation is generally the combined result of three processes, i.e., electronic excitation, desorption?ablation and ionization, and this fact may easily conceal relevant details of any of the three processes [3?5]. The present chapter deals with the fundamental principles for understanding the basics of the electronic excitation process, its deactivation pathways and the current knowledge on desorption?ablation.