Formation of Ions from Charged Droplets

The process of droplet jet fission starts on a macroscopic scale and eventually leads to states that might be regarded as large clusters or multiply solvated ions. The final step of the creation of isolated gas phase ions from these multi-molecular entities is not addressed by this model, however. 

Example The cleavage of disulfide bonds by reduction with 1,4-dithiothreitol causes the unfolding of the protein. This exposes additional basic sites to protonation, and therefore results in higher average charge states in the corresponding positive-ion ESI spectrum (Fig. 12.19) [105]. 

In contradiction to lEM, the electric field strength locally necessary to evaporate ions from a droplet cannot be attained, because the droplet would be crossing the Rayleigh limit before [25,107]. 

More recent work revealed the importance of gas-phase proton transfer reactions [108-111]. This implies that multiply charged peptide ions do not exist as preformed ions in solution, but are generated by gas-phase ion-ion reactions (Chap. 12.4.4). The proton exchange is driven by the difference in proton affinities PA, Chaps. 2.12 and 9.18.3) of the species encountered, e.g., a protonated solvent molecule of low PA will protonate a peptide ion with some basic sites left. Under equilibrium conditions, the process would continue until the peptide ion is saturated with protons, a state that also marks its maximum number of charges. 

Note There is a continuing debate about ion formation in ESI [89,90,104]. In summary, it may be assumed that CRM holds valid for large molecules [20] while the formation of smaller ions is better described by lEM [89,90]. 

The elder model of ion formation, the charged-residue model (CRM), assumes the complete desolvation of ions by successive loss of all solvent molecules from droplets that are sufficiently small to contain just one analyte molecule in the end of a cascade of Coulomb fissions. [9,42,84] The charges (protons) of this ultimate droplet are then transferred onto the molecule. This would allow that even large protein molecules can form singly charged ions, and indeed, CRM is supported by this fact. [23] 

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