The Evolution of Electrospray Generated Droplets is Not Affected by Ionization Mode

Abstract

Ionization efficiency and mechanism in ESI is strongly affected by the properties of mobile phase. The use of mobile-phase properties to accurately describe droplets in ESI source is convenient but may be inadequate as the composition of the droplets is changing in the plume due to electrochemical reactions occurring in the needle tip as well as continuous drying and fission of droplets. Presently, there is paucity of research on the effect of the polarity of the ESI mode on mobile phase composition in the droplets. In this paper, the change in the organic solvent content, pH, and droplet size are studied in the ESI plume in both ESI+ and ESI- ionization mode. We introduce a rigorous way - the absolute pH (pH) - to describe pH change in the plume that takes into account organic solvent content in the mobile phase. pH enables comparing acidities of ESI droplets with different organic solvent contents. The results are surprisingly similar for both ionization modes, indicating that the dynamics of the change of mobile-phase properties is independent from the ESI mode used. This allows us to conclude that the evolution of ESI droplets first of all proceeds via the evaporation of the organic modifier and to a lesser extent via fission of smaller droplets from parent droplets. Secondly, our study shows that qualitative findings related to the ESI process obtained on the ESI+ mode can almost directly be applied also in the ESI- mode. Graphical Abstract ᅟ.

Replacing H+by Na+or K+in phosphopeptide anions and cations prevents electron capture dissociation

“…Changes in pH of the evaporating droplets from ESI, operated in either positive or negative ion mode, are relatively small, on the order of half a pH unit. 95 – 97 Thus in a pH ∼ 9.0 solution, and also in the evaporating droplets from ESI of that solution, the phosphopeptide can form a variety of salt bridges between the protonated sites and even remote (in sequence) deprotonated D and E sidechains (or the C terminus) that should be preserved in the (M – 2H) 2– ions because of the high stability of salt bridge interactions in the gas phase. 44 , 53 Moreover, these salt bridges can constitute the basis of networks of electrostatic interactions that include ionic hydrogen bonds between protonated sites and backbone amide oxygen such as that in Scheme 2 .…”

Symmetric Potentiometric Cells for the Measurement of Unified pH Values

“…Reproducibility of the results was improved by making the salt bridge using a single piece of glass (Figure 1B) [20]. A salt bridge support made of polytetrafluoroethylene (PTFE) was tested, but it was found unsuitable.…”

“…An ionic liquid salt bridge can also be above the solutions [22,23]. [16], (B) cell used in [20], (C) cell used in [21], (D) cell used in 2018 and in [24] and (E) cell used in 2020. Arrows show the positions of the used auxiliary electrode in (A-C,E).…”

“…Cells used at University of Tartu (UT) with the salt bridge above (A and B) and below (C,D,E) the measurements compartments. (A) Cell used in [16], (B) cell used in [20], (C) cell used in [21], (D) cell used in 2018 and in [24] and (E) cell used in 2020. Arrows show the positions of the used auxiliary electrode in (A-C,E).…”

“…Such symmetric setups have already found use for pH H 2 O abs measurements in nonaqueous solutions [21] and aqueous-organic mixtures [16,20,24]. However, comprehensive validation of their performance and investigation of influencing factors has not been carried out.…”