[Matrix-assisted laser desorption ionization].

Abstract

Since its invention, matrix-assisted laser desorption ionization (MALDI) has found wide application in mass spectrometry of high molecular weight compounds such as synthetic polymers and biopolymers. Despite widespread application of MALDI, the fundamental processes of ion formation and desorption are still poorly understood. The chemistry of the MALDI process, occurring both during sample preparation and during ionization is reflected in the mass spectrum. As the MALDI technique now stands a low concentration of analyte molecules, which usually exhibit only moderate absorption per molecules, is embedded in matrix crystals consisting of a small, highly absorbing species. In this manner the efficient and controllable energy transfer is retained while the analyte molecules are separated from excessive energy that would lead to their decomposition. The matrix is believed to serve two major functions: adsorption of energy from the laser light and the isolation of analyte molecules from each other. There are 3 major methods for the preparation of samples for analysis which are quite quick and simple: dried droplet, surface and sandwich preparation. Experiments with pH indicator dyes serve as proof that analyte's charge state in the matrix crystals is the same as in solution. Upon laser desorption a sudden and explosive phase transition occurs and a dense plume of desorbed material is formed. The initial velocity of analyte ions in the plume depends only on the matrix used. Initial species formed as a result of laser desorption are tiny clusters. They consist of a matrix, analyte and other ionic species embedded in the matrix crystals all held together by hydrogen bonds and coulombic interactions. The first essential charging and thus ionization process is the statistical occurrence of clusters with a deficit/excess of anions of cations. Very small initial clusters are likely to be highly charged. Highly charged initial clusters cannot survive in the matrix plume and their charge drops. Clusters shrink by evaporation of neutral molecules. This paper presents only those cases leading to analyte ions and compares MALDI and the electrospray ionization technique.