Certain analytes appear in laser desorption/ionization or matrix-assisted laser desorption/ionization (MALDI) mass spectra as radical ions, rather than deprotonated, protonated or cationized molecules. Radical ion intensity ratios are found to be consistent with matrixŐanalyte and analyteŐanalyte electron transfer (ET) reactions in the desorption/ablation plume. As predicted by the two-step model of ultraviolet MALDI, both suppression of matrix by analytes, and one analyte by another are observed. The dependence of these effects on mixing ratios, laser fluence and matrix choice parallel suppression phenomena previously observed with proton or cation transfer reactions are discussed. The driving force for ET, as reflected in differences of gas-phase molecular ionization potentials (IPs) or solution oxidation potentials (Eox), is usually the dominant factor determining relative ion intensitites. In positive polarity, low IP (Eox) analytes suppress signals of high IP (Eox) analytes. However, for large IP (Eox) differences and at high matrix-to-analyte mole ratios, relative intensities were found to invert. This is tentatively ascribed to a rollover in the rate of matrixŐanalyte electron transfer at high exoergicity.

Keywords: MALDI ionization, electron transfer, ionization potential, oxidation potential