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.