Electron Transfer Dissociation of Sodium Cationized Polyesters: Reaction Time Effects and Combination with Collisional Activation and Ion Mobility Separation

The electron transfer dissociation (ETD) characteristics of doubly sodiated polylactide were investigated at varying ion–ion reaction times, ranging from 20 to 220 ms. ETD product ion yields and signal-to-noise ratio maximized at the shortest reaction time. At longer times, the extent of Na/H or H/Na exchange reactions rose, causing spectral crowding; in addition, the sensitivity decreased significantly due to ion losses from neutralization and scattering. ETD in quadrupole ion trap (QIT) and quadrupole/time-of-flight (Q/ToF) mass spectrometers using fluoranthene and p-nitrotoluene reagent anions, respectively, led to similar product ions. The Q/ToF configuration allowed for collisionally activated dissociation (CAD) of the ETD total ion current under mild activation conditions, which gave rise to new and more abundant fragment distributions by mainly depleting residual reduced precursor ions. The ETD and ETD/CAD fragmentation patterns were markedly different from the fragments generated by simple CAD, thereby providing complementary structural information about the analyzed polyester. The collision cross-sections of the major ETD fragment series, determined by ion mobility mass spectrometry (IM–MS) experiments on the Q/ToF instrument, agreed well with the linear architectures expected from radical-induced cleavages at the (CO)-O-alkyl bonds promoted by the electron added in the ETD event