Investigation of the dynamics of matrix-assisted laser desorption ionization ion formation using an electrostatic analyzer/time-of-flight mass spectrometer

A hybrid electrostatic analyzer/time-of-flight mass spectrometer was used t o examine the matrix-assisted laser desorption/ionization (MALDI) ion kinet ic energy distributions for Na+, protonated 7-hydroxy-4-methylcoumarin matr ix ions and protonated bradykinin under a variety of source region accelera ting electric field conditions. Broad kinetic energy distributions are obse rved for the three targeted ions with the ions having both positive and neg ative kinetic energies relative to the full acceleration potential. The pos itive kinetic energy ions are shown to have flight times in good agreement with flight times calculated from first principles while the flight times o f the negative kinetic energy ions diverge positively from the calculated v alues, Analysis of the flight time shifts for the negative kinetic energy i ons allows an entrainment velocity of similar to 550 m s(-1) for the materi al to be calculated. A composite picture of the dynamics of MALDI ion forma tion is presented which combines promptly formed protonated matrix and anal yte ions with Ions formed/accelerated at delayed times In an expanding, con stant velocity plume of laser desorbed material, Copyright (C) 1999 John Wi ley & Sons, Ltd.