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

Abstract

A hybrid electrostatic analyzer/time-of-flight mass spectrometer was used to examine the matrix-assisted laser desorption/ionization (MALDI) ion kinetic energy distributions for Na + , protonated 7-hydroxy-4-methylcoumarin matrix ions and protonated bradykinin under a variety of source region accelerating electric field conditions. Broad kinetic energy distributions are observed for the three targeted ions with the ions having both positive and negative kinetic energies relative to the full acceleration potential. The positive kinetic energy ions are shown to have flight times in good agreement with flight times calculated from first principles while the flight times of the negative kinetic energy ions diverge positively from the calculated values. Analysis of the flight time shifts for the negative kinetic energy ions allows an entrainment velocity of ∼550 m s -1 for the material to be calculated. A composite picture of the dynamics of MALDI ion formation is presented which combines promptly formed protonated matrix and analyte ions with ions formed/accelerated at delayed times in an expanding, constant velocity plume of laser desorbed material.