Dole charged residue mechanism

Charged residue mechanism. This was proposed by Dole and suggests that after multiple coloumbic explosions droplets are formed, which contain only one ion [10]. This mechanism is thought to be important for the ionisation of macromolecules. 

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. 

Fernandez de la Mora, J. (2000) Electrospray ionization of large multiply charged species proceeds via Dole s charged residue mechanism. Arud. Chim. Acta, 406, 93-104. 

Two main mechanisms have been proposed for how the resulting droplets yield desolvated ions. Dole proposed the charge residue or solvent evaporation (emission) model in which ion formation is the result of an ion-desolvation process.9,11,12 The droplets, produced by electrostatic dispersion in the liquid at the capillary tip, lose solvent molecules (aided by the curtain or nebulizer gas, usually nitrogen), and eventually produce individual ions (Fig. 3). 

Two mechanisms have been proposed to account for the formation of gas-phase ions from very small and highly charged droplets. The first mechanism, proposed by Dole et al. [10,11], depends on the formation of extremely small droplets which should contain only one ion. Solvent evaporation from such a droplet will lead to a gas-phase ion. Mass spectrometric determinations by Dole and co-workers were by and large unsuccessful, but the charge residue model (CRM) proposed by them survived. A more detailed consideration of, and support for, the mechanism was later provided by Rollgen et al. [25,26]. 

The first applications of ESI in MS date from 1968. Dole et al. [5-6] investigated the possibility to transfer macromolecules from the liquid phase to the gas phase by electrospraying dilute solutions in a nitrogen bath gas. The hypothesis of Dole and cowoikers was that macro-ions can be produced by desolvating the charged droplets produced in electrospray. This ionization mechanism is called the charge residue model. 

The alternative meehanism suggested is based on soft desolvation of the ions by solvent evaporation from small charged droplets produced by either electrohydrodynantic or mechanical instabilities (or both). This mechaitism is similar to the charge residue hypothesis presupposed by Dole [5-6] in their ESI experiments. The desolvation is most effective when the droplets are small and the number of charges on a droplet is small as well. 

Models have been developed on ion formation in ESI, but there is still no consensus on the mechanism by which sample ions are obtained for mass spectrometric analysis. These models rely on the existence of preformed ions in solution i.e., the ions observed in the mass spectra were presumed to be present originally as ionized molecules in solution. According to the charged residue model of Dole et al., the evaporation of solvent from a charged droplet increases the surface field xmfil the Raleigh limit is reached ... 

ESI was first proposed by Malcolm Dole in 1968 who noticed that the Coulombic fission cascade would eventually lead to sufficiently small drops which contained a single solute molecule that retained some of the drop charge such that a fully desolvated gas phase ion would ultimately be left once all the solvent evaporates, this mechanism being known as the charge residue model (CRM). These efforts were, however, largely unsuccessful practically due to the use of an ion-drift spectrometer to which the electrospray was interfaced. It was John Fenn, then at Yale University, who later developed a practical method for electrospray ionization mass spectrometry (ESI-MS) that allowed the identification and structure analysis of biomacromolecules of virtually unlimited molecular weights to an accuracy of 0.01% by averaging... 

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