Fluorometric Measurement and Modeling of Droplet Temperature Changes in an Electrospray Plume

Abstract

The evolution of droplet temperatures in an electrospray plume was measured via ratiometric fluorescence. Under typical operating conditions, droplet temperatures decrease ∼30 K over the first 5.0 mm along the spray axis, followed by a slight (∼2-3 K) rewarming. Experimental axial profiles (Z-axis) were fit by use of diffusion-controlled and surface-controlled evaporation models. Both models fit the experimental data well for the cooling portion of the spray (Pearson correlation coefficient R ≥ 0.994), but the surface-controlled model required unrealistic droplet radius values to obtain a good fit. In lateral profiles at a given Z near the emitter tip, temperatures are lower (by 3.0-10 K) in the periphery than on the spray axis. This behavior is consistent with the expected enrichment of the spray periphery with smaller droplets. At longer axial distances, lateral profiles were relatively flat. Droplet temperature as a function of axial displacement fell more rapidly at lower liquid flow rates, possibly attributable to changes in droplet size and/or velocity with flow rate.

Effects of ESI conditions on kinetic trapping of the solution-phase protonation isomer of p-aminobenzoic acid in the gas phase

“…It is not surprising that differences in kinetically trapped structures might be observed between different ESI sources and source conditions, in light of results from spectroscopic investigations of ESI droplet temperatures using “thermometer molecules”; those studies have shown that ESI droplets tend to decrease in temperature,[39] but given sufficiently heated sources, can even rise in temperature. [40] This suggests that sources must be individually characterized to realistically determine droplet temperatures—a feat not readily implemented for most working sources, due to the spectroscopic requirements.…”

Linking molecular models with ion mobility experiments. Illustration with a rigid nucleic acid structure

“…Modelling the ESI process to generate solvent‐free polyatomic ions and the subsequent ion structural changes in the gas phase however encounters three major challenges to be addressed in the future as follows: (1) realistic description of solvent evaporation effects on the species conformation, (2) partial charges location for a given charge state, and (3) the time scale (milliseconds for an IMS experiment). The first two issues have not yet been clarified for all biological molecules, although several attempts been made in this direction, whereas the latter point would require a rather nowadays unaffordable computer time, above all for large systems like biopolymers (to the best of our knowledge, the sub‐milliseconds time scale in a MD simulation in gas phase has been achieved only for a 7‐mer oligonucleotide) …”

“…Modelling the ESI process to generate solvent-free polyatomic ions and the subsequent ion structural changes in the gas phase however encounters three major challenges to be addressed in the future as follows: (1) realistic description of solvent evaporation effects on the species conformation, (2) partial charges location for a given charge state, and (3) the time scale (milliseconds for an IMS experiment). The first two issues have not yet been clarified for all biological molecules, although several attempts been made in this direction, [55,72,74,75,[105][106][107] whereas the latter point would require a rather nowadays unaffordable computer time, above all for large systems like biopolymers (to the best of our knowledge, the sub-milliseconds time scale in a MD simulation in gas phase has been achieved only for a 7-mer oligonucleotide). [77] Finally, although there is still room for improvement in CCS calculation approaches, algorithms and parameterization (especially for gases other than helium), we were pleased to find out that several of the currently available methods, although developed and parameterized for totally different molecules, transpose well to the 24-nucleotide nucleic acid structure studied here.…”

Linking molecular models with ion mobility experiments. Illustration with a rigid nucleic acid structure

“…Modelling the ESI process to generate solvent-free polyatomic ions and the subsequent ion structural changes in the gas phase however encounters three major challenges to be addressed in the future as follows: (1) realistic description of solvent evaporation effects on the species conformation, (2) partial charges location for a given charge state, and (3) the time scale (milliseconds for an IMS experiment). The first two issues have not yet been clarified for all biological molecules, although several attempts been made in this direction, [55,72,74,75,[105][106][107] whereas the latter point would require a rather nowadays unaffordable computer time, above all for large systems like biopolymers (to the best of our knowledge, the sub-milliseconds time scale in a MD simulation in gas phase has been achieved only for a 7-mer oligonucleotide). [77] Finally, although there is still room for improvement in CCS calculation approaches, algorithms and parameterization (especially for gases other than helium), we were pleased to find out that several of the currently available methods, although developed and parameterized for totally different molecules, transpose well to the 24-nucleotide nucleic acid structure studied here.…”