The effect of cation size (H+, Li+, Na+, and K+) on McLafferty-type rearrangement of even-electron ions in mass spectrometry

Protonation and alkali-metal cation adduction are the most important ionization processes in soft-ionization mass spectrometry. Studies on the fragmentation mechanism of protonated and alkali-metal-cationized compounds in tandem mass spectrometry are essential and helpful for structural analysis. In some cases, it was often observed that a compound attached by different alkali-metal cations (or proton) exhibits similar fragmentation patterns but the relative abundances of product ions are different. This difference was considered to derive from the different electrostatic interactions of alkali-metal cations (or the bonded effect of proton) with the analyte. The alkali-metal cation with a smaller ionic radius shows stronger electrostatic interaction with the molecule because of its higher charge density. In addition, the bonded effect of the proton is stronger than the electrostatic interaction of the alkali-metal cation. In the present study, which used McLafferty-type rearrangements of even-electron ions ([M + Cat](+), Cat = H, Li, Na, K) as model reactions, the effect of cation size in mass spectrometric fragmentation reactions is highlighted. These considerations were also successfully applied to interpret the similar but distinct fragmentation behavior of proton and alkali-metal cation adducts of a synthetic compound (2-(acetamido(phenyl)methyl)-3-oxobutanoate) and a drug (entecavir).