Ion evaporation mechanism

When buffer is present in the mobile phase and the analytes are largely ionic in nature or give preformed ions in solution, the ion-evaporation mechanism is... 

Iribame and Thompson proposed the ion-evaporation mechanism. This suggests desorption of the ions from the droplets when they are less than 10 nm in size [14]. This is thought be the dominant mechanism for small molecules. 

The breakthrough of TSP was partly due to the introduction of a new ionization technique [58], based on the ion-evaporation mechanism (Ch. 6.2-3). Ammonium acetate or another volatile buffer is assumed to assist in the process. However, in the vast majority of the applications, TSP is best considered as a solvent-mediated Cl method. Four modes of ionization in TSP LC-MS can be distinguished, i.e., two liquid-based ionization modes (applied in 60% of the applications), ion-evaporation, and thermospray buffer ionization, and two electron-initiated ionization modes (applied in 40% of the applications) filament-on ionization, and discharge-on ionization. 

Figure 1.3. Mechanisms proposed for the formation of ions from small charged droplets ion evaporation mechanism (IEM) and charge residue mechanism (CRM).

For the formation of the gaseous analyte, two mechanisms are discussed. The charged residue mechanism (CRM) proposed by Cole [58], Kebarle and Peschke [59], and the ion evaporation mechanism (lEM) postulated by Thomason and Iribarne [60]. In CRM, the droplets are reduced as long as only one analyte in the microdroplets is present, then one or more charges are added to the analyte. In lEM, the droplets are reduced to a so-called critical radius (r < 10 nm)... 

It is generally accepted that low molecular weight ions are liberated into the gas phase through the ion evaporation mechanism [5], while larger ions form by charged residue mechanism [8]. The third model appears well demonstrated in the ionization of lipid species. 

Although the ion evaporation mechanism is the most popular view on the thermospray ionization mechanism, the ionization characteristics imder typical operating conditions, and for most analytes, are best understood in terms of chemical ionization. 

In summary, the ion evaporation mechanism is experimentally well-supported for small inorganic and organic ions, but development of quantitative predictions based on this model remains a considerable challenge for the future. 

Turning to non-metallic catalysts, photoluminescence studies of alkaline-earth oxides in dre near-ultra-violet region show excitation of electrons corresponding to duee types of surface sites for the oxide ions which dominate the surface sUmcture. These sites can be described as having different cation co-ordination, which is normally six in the bulk, depending on the surface location. Ions on a flat surface have a co-ordination number of 5 (denoted 5c), those on the edges 4 (4c), and dre kiirk sites have co-ordination number 3 (3c). The latter can be expected to have higher chemical reactivity than 4c and 5c sites, as was postulated for dre evaporation mechanism. 

There are alternative explanations for the actual mechanism by which these ions are produced, e.g. the ion-evaporation [11] and charge-residue models [12], and these have been debated for some time. 

Ion evaporation One of the two mechanisms used to account for the production of ions by electrospray ionization. 

The MC-ICP-MS consists of four main parts 1) a sample introduction system that inlets the sample into the instrument as either a liquid (most common), gas, or solid (e.g., laser ablation), 2) an inductively coupled Ar plasma in which the sample is evaporated, vaporized, atomized, and ionized, 3) an ion transfer mechanism (the mass spectrometer interface) that separates the atmospheric pressure of the plasma from the vacuum of the analyzer, and 4) a mass analyzer that deals with the ion kinetic energy spread and produces a mass spectrum with flat topped peaks suitable for isotope ratio measurements. 

Electrospray ionization (ESI) refers to the overall process by which an intense electric field disperses a sample liquid into a bath gas as a fine spray of highly charged droplets. Evaporation of those charged droplets produces gas-phase ions by mechanisms that remain the subject of much argument and debate. The ESI is a complex of independent component processes, the two most important of which are electrospray dispersion, the electrostatic dispersion of sample liquid into charged droplets, and ionization, the transformation of solute species in those droplets to free ions in the gas 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. 

With TSP, ammonium acetate has emerged as the best general-purpose electrolyte for ionizing neutral samples. Improved ionization can be obtained by the use of a filament or discharge electrode to generate reactive ions for CI (87, 88). The processes involved in filament or discharge-assisted ionization must be used when operating in the absence of a buffer with nonaqueous eluents. With ionic analytes, the mechanism of ion evaporation is supposed to be primarily operative since ions are produced spontaneously from the mobile phase (89). Ion evaporation often yields mass spectra with little structural information in order to overcome this problem, other ionization modes or tandem MS have been applied (90). 

From the above discussion of the characteristics of the spectra and ion formation mechanisms, it is obvious that, though there can be no doubt about the usefulness of laser mass spectrometry for a large variety of analytical tasks, more research is needed for a better understanding. This is particularly true for the transition from thermal evaporation to desorption and the desorption mode itself. In the following, a few first results of such experiments, conducted recently in the author s group, will be reported. 

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