Fig. 1. The complex, multi-phase processes present during ESI-MS. (a) Droplets are formed from a capillary emitter tip by applying a voltage (ESI/nESI) and/or a high-pressure coaxial gas flow (ESSI); droplets are formed with a certain size distribution. (b) Rapid solvent evaporation causes a decrease in the sizes of the droplets. In this schematic, reactants are considered non-volatile. In the condensed phase, the concentration of reagents can increase because of solvent evaporation and the amount of product formed can be a function of droplet size (Section 4.2); condensed-phase reactions can occur in the bulk (kbulk) or at the surface (ksurface) regions of the droplets. (c) Charged ESI droplets undergo repeated fission events (charge residual model, CRM) that produce smaller progeny droplets, and reagent ions can be directly emitted into the gas-phase (ion emission model, IEM). Gas-phase reactivity can occur with a certain kgas. (d) Gas-phase species and droplets reach the MS inlet. The colour scales qualitatively represent the concentration of reagents A and B (cyan/blue) and of product C (purple/red) in the ESI droplets. (e) and (f) Imaging of 1 : 1 MeOH/H2O ESSI microdroplets plumes (120 psi N2 gas flow, 5 μL min−1 solvent flow rate, +5 kV applied to the emitter tip) formed with two capillaries with 250 μm and a 50 μm inner diameters (i.d.) and 350 μm and a 150 μm outer diameters (o.d.), respectively.4 Adapted with permission from ref. 4. Copyright (2020) American Chemical Society.
