Pulsed Nano-ESI: Application in Ion Mobility-MS and Insights into Spray Dynamics

We have previously shown that pulsed nano-ESI offers direct ion introduction into an AP-IM cell in the absence of conventional gates and desolvation. Here, we further characterize this ion injection method and utilize it to gain insights into nano-ESI pulsed spray dynamics. We demonstrate that a pulsed nano-ESI operated at 20 Hz with ion generation pulses of 170-510 μs offers reproducible ion arrival times (0.09-0.21% RSD). Arrival times are then translated to effective collision cross sections (CCSs) using tetraalkylammonium ions as CCS internal standards. For ions with low solvent affinity, effective CCS values match those reported for fully desolvated ions. For amino acids and a series of alkylamine homologues, the effective CCS values are higher than those for fully desolvated ions and correlate with solvent affinity, suggesting that ions with high hydration affinities traverse the mobility cell as hydrated ions. Notably, hydrates are not observed in the MS spectra due to ion activation during the transport into vacuum. Using these observations as a framework to interpret effective CCS values, we investigate the impact of nano-ESI pulse duration on ion properties. We observe that longer pulse durations lead to the enhancement of ion abundance for low-ionization-efficiency analytes and a reduction in clustering. However, effective CCSs are not significantly altered by spray pulse duration, implying that similar ion structures emerge rapidly at all investigated pulse durations. Ion abundance results suggest a temporal evolution of droplets in pulsed nano-ESI where droplets emitted later in the spray formation appear to be smaller, providing enhanced ionization.