Ion-evaporation model

According to the ion-evaporation model, the droplets become smaller until a point is reached at which the surface charge is sufficiently high for direct ion evaporation into the gas phase to occur. In the case of the charge-residue model, repeated Coulombic explosions take place until droplets are formed that contain a single ion. Evaporation of the solvent continues until an ion is formed in the vapour phase. 

The mechanisms for the formation of gas phase ions from droplets are not fully understood and two therories have been proposed the ion evaporation model (lEV) and the charge residue model (CR) [28]. The lEV model proposes that the ions are directly emitted into the gas phase when, after evaporation and... 

Two models can explain the events that take place as the droplets dry. One was proposed by Dole and coworkers and elaborated by Rollgen and coworkers [7] and it is described as the charge residue mechanism (CRM). According to this theory, the ions detected in the MS are the charged species that remain after the complete evaporation of the solvent from the droplet. The ion evaporation model affirms that, as the droplet radius gets lower than approximately 10 nm, the emission of the solvated ions in the gas phase occurs directly from the droplet [8,9]. Neither of the two is fully accepted by the scientific community. It is likely that both mechanisms contribute to the generation of ions in the gas phase. They both take place at atmospheric pressure and room temperature, and this avoids thermal decomposition of the analytes and allows a more efficient desolvation of the droplets, compared to that under vacuum systems. In Figure 8.1, a schematic of the ionization process is described. 

The alternative mechanism, the ion-evaporation model, was proposed initially by Iribarne and Thomson13 (Fig. 4) and involves desolvation of the droplets, producing an increase in charge density over the droplet surface that causes coulombic explosion and eventually leads to ejection of individual ions. 

In the ion-evaporation model of Iribame and Thomson [11-14], the seqnence of solvent evaporation and electrohydrodynamic droplet disintegration also leads to the production of microdroplets. Gas-phase ions can be generated from the highly-charged microdroplets, at which the local field strength is sufficiently high to allow preformed ions in solution to be emitted into the gas phase (lEV). 

The second theory is based on the ion evaporation model (IBM) proposed by Iribame and Thompson [32]. In IBM, after the radii of the charged droplets have reduced to the order of tens of nanometers, due to solvent evaporation and jet fission, direct ion emission to the gas phase from the droplets becomes possible. The theory states that IBM becomes dominant over jet fission for droplets of radii R < 10 nm [29]. Figure 32.19 illustrates the two different theories of gas phase ion evolution in an electrospray. 

Fig. 32.19 The charged residue model (CRM) and ion evaporation model (lEM) of gas phase ion generation from charged electrospray droplets...

When, 15 years later, the Nobel-prize laureate Fenn and his co-workers continued the work of Dole with other analytes than proteins, they found results not in agreement with the charge-residue model. Therefore, they adopted another model, described in the mid-1970s by two Canadian scientists Iribarne and Thomson. In this so-called ion-evaporation model, it has been thermodynamically demonstrated that ions may escape from a sufficiently small droplet with a sufficient number of charges. This model also requires the analyte to be present as a preformed ion in solution. 

Significant research has been done to prove the correctness of either of these models and/or to falsify one of them. From these studies, it appears that both models contribute to ion formation in ESI. The importance of either effect depends on the analyte. Therefore, ESI is best considered as a mixed-mode ionization. Some effects are more readily explained from the charge-residue model, some other from the ion-evaporation model. An important prerequisite for both models is that analytes should be present as preformed ions in solution. This indicates an analytical strategy to enhance the analyte ionization... 

Although Fenn initially emphasized the importance of ion evaporation in ion formation by electrospray ionization, extensive research has demonstrated that, depending on the analyte and the experimental conditions, the charge-residue model and the ion-evaporation model are both important. In addition, especially in the analysis of small molecules, gas-phase ion-molecule reactions appear to play an important role in ion formation by electrospray ionization. 

When the sprayed solution contains a solute, such as a salt, the continuous evaporation of the droplets will lead to very high concentrations of the salt and finally to charged solid partides - skeletons of the charged droplets that can reveal some aspects of the droplet evolution. Fernandez de la Mora and coworkers [32] have used this approach to study charged droplet evolution. This work is of special relevance to the ion evaporation model and is discussed in Section 1.2.8. 

Mechanism for the Formation of Cas-Phase Ions from Very Small and Highly Charged Droplets. The Ion Evaporation Model (lEM)... 

In summary, the Ion Evaporation Model is experimentally well supported for small ions of the kind that one encounters in inorganic and organic chemistry. However, when the ions become very large, such as polymers, dendrimers or biological supramolecular complexes like proteins and enzymes, the Charged Residue Model (CRM) becomes much more plausible, see Section 1.2.10. Because many applications of ESMS in analytical organometallic and physical organic chemistry involve small ions it is desirable to consider the expected relative sensitivities for these analytes when detected with ESMS. 

The dependence of the sensitivities of ionic analytes on the nature of the analyte, its concentration, and the presence of other electrolytes in solution is of interest to users of electrospray mass spectrometry. The analytes considered in this section are smaller molecules that most likely enter the gas phase via the Ion Evaporation Model (lEM). 

The ion evaporation model [4,5] proposes that as the droplet reaches a certain radius, the field strength at the surface of the droplet becomes large enough to assist the field desorption of solvated ions. 

---