The initial-ion velocity as a marker for different desorption-ionization mechanisms in MALDI

Abstract

While the high, mass-independent and matrix-determined initial velocity of matrix-assisted laser desorption ionization (MALDI) ions has been proposed to be a meaningful characteristic feature of the MALDI process and indicative for the generation of a jet of clusters, considerably lower values as obtained for some low-mass non-protein analytes and certain preparation protocols pointed to a possible second and different process of ion generation. In this paper it is shown that only low-mass neutral compounds can be ionized exhibiting initial velocities considerably lower than that determined by the matrix. New results presented in this paper show that there is a gradual increase of the initial velocities with increasing mass for neutral oligosaccharides and technical polymers to the high level characteristic for peptides and proteins which is also obtained for a small oligosaccharide by introduction of a charged functional group via derivatization. This indicates that typical MALDI analytes need incorporation and cluster generation to be detected whereas an evaporation and gas-phase cationization is viable for small neutral analytes. The directionality of the emission of ions is investigated for different preparation protocols and crystallization states of the matrix. In a further experiment, the correlation between a high initial velocity and the softness of the MALDI ion generation is substantiated.