Kinetic energy measurements of molecular ions ejected into an electric field by matrix-assisted laser desorption.

Measurements of kinetic energy distributions of molecular ions ejected into an extraction field by matrix-assisted laser desorption are reported. The measurements were made in a time-of-flight mass spectrometer with an electrostatic mirror by measuring the reflected signal as a function of the difference between the accelerating voltage and the voltage applied to the mirror. The molecular ions were found to have less kinetic energy than the extraction field alone would normally provide, i.e., we observed an energy deficit. Under conditions typical for a matrix-assisted laser desorption experiment, the deficit is about 24 eV for molecular ions of insulin. The size of the deficit increases with the intensity of the molecular ion signal, and the molecular weight of the protein; it is also larger for negative molecular ions than for positive molecular ions.