Role of three-body recombination for charge reduction in MALDI process

Ions in Matrix-Assisted Laser Desorption/Ionization (MALDI) are predominantly singly charged for small analyte molecules. With the estimated high number density and low temperature of electrons, the three-body recombination mechanism is attractive and should be considered as an important cause for the charge reduction in the plume. Theoretical calculations indicate that the rate coefficient of the three-body recombination is about 50 times higher than that of the two-body recombination if the analyte molecule has insufficient degrees of freedom. Experimental results show that, for small analyte molecules, the ratio of AH22+/AH+ is close to the theoretical 5% value from the three-body recombination modeling and this ratio declines with the increasing electron and matrix molecule number density caused by greater laser irradiance. The ratio of [A + 2]+/[A + 1]+ is higher than the theoretical isotopic value, and the excess [A + 2]+ could exclusively result from the three-body recombination collisions. Further evidence demonstrates that [A + 2]+/[A + 1]+ increases with electron number density, which is in correspondence with the model. All of these theoretical and experimental results indicate that three-body recombination is an essential charge reduction mechanism for small molecules in the MALDI plume.