Evaporation ion theory

For present purposes this particular series of papers is important because the authors later used a mass spectrometer for the first time (Thomson 1979) to characterize the charged species produced by their spray technique (Figure 5.25). Equally important, the authors introduced the ion evaporation theory for the mechanism whereby single molecular ions are formed from charged droplets (see later). It was soon realized that this... 

Individual properties of specific ions and solvents are included in k, mainly via the AG°qj.j, term. The preceding account does not do justice to the sophistication of the ion evaporation theory or its recent experimental tests (Gamero-Castafio 2000), but wiU suffice for present purposes. 

Figure 2.9 Ionization mechanisms in ESI. (a) Schematic illustration of the single ion in droplet theory and (b) ion evaporation theory...

Figure 1.6 Predictions of the ion evaporation theory [34]. The Rayleigh curve provides the droplet radius R and the number of elementary charges N at w/hich a charged water droplet will be at the Rayleigh limit. Solvent evaporation at constant charge to a smaller radius R will cause a Coulomb fission. Similarly, the curves Cation Cluster and Anion Cluster show the threshold of ion evaporation at a given charge N and droplet radius R. For negatively charged droplets,...

Electrospray ionization mass spectrometry is typically performed in polar solvents such as water, acetonitrile, methanol, or a combination of these. Thus, analytes with significant nonpolar regions should favor the air-solvent interface at the surface of electrospray droplets where these nonpolar regions can be desolvated. Such analytes are termed smface-active. A relationship between response in atmospheric pressure ionization mass spectrometry and analyte surface activity was postulated as early as 1983 by Mbame et al., the original authors of the ion evaporation theory. Such a relationship was also observed by Kebarle and co-workers," " who included a factor related to analyte surface activity in their models. 

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 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. 

Thomson 1979), the charge on droplets of this size that is required for this ion evaporation is lower than that required for Coulomb fission, i.e., ion evaporation replaces Coulomb fission as a means of relaxing the Coulombic repulsion at the droplet surface. The equation predicting the rate constant for ion evaporation from the droplets was derived on the basis of the transition state theory (see any text on physical chemistry) ... 

Iribarne and Thomson came to the conclusion that the formation of abundant high-mobUity gas-phase ions is possible only if a considerable fraction of the charges (i.e., ions) escaped from the droplets before the complete evaporation of the droplets. The theory for this escape process, ion evaporation, was developed in the same paper. ... 

Iribarne and Thomson derived an equation that provided detailed predictions for the rate of ion evaporation from the charged droplets.The treatment is based on transition state theory, used in chemical reaction kinetics. The rate constant ki for emission of ions from the droplets is given by... 

Elucidating the origin of magic numbers has been a problem of long-standing interest, made accessible through the use of the laser-based reflectron TOF technique and evaporative ensemble theory. Three test cases are considered, first protonated ammonia clusters where (NH3)4 NHj has been found to be especially prominent, and then two other cases are considered, one involving water cluster ions and another rare gas clusters. 

Unfortunately, the conflict has not been resolved.36 However from the standpoint of the experimentalist, many of the consequences of the two theories are similar. Both theories require very small droplets to generate gas-phase ions. The time requirement for the evolution to such droplets (see Figure 2) is in the hundreds of microseconds. The ambient gas is essential to provide the thermal energy for the evaporation. Solvents with low vapor pressure and condensation coefficients a may not be suitable or may require higher ambient temperatures. 

Quite recently Raes (1985) applied the classical theory of homogenous nucleation originally developed by Bricard et al (1972) to atmospheres containing SO2, H2O and 218Po ions. Depending on the H2O and SO2 concentrations, ions could grow to a quasi-stable cluster which would evaporate upon electrical neutralization, or to a larger size which would survive neutralization. 

Also so far, the new theory does not consider the possibility of field evaporation as multiply charged ions. The advantage of the new method lies in the possibility of investigating how electronic-charge distribution changes as the atoms are field evaporated from the surface, and also how the binding force changes as the atoms are removed from the surface. 

A practical application coming out of field ion emission is the liquid metal ion source. Ion sources of a wide variety of chemical elements, most of them low melting point metals, can be produced by using either liquid metals131,132 or liquid alloys.133 The idea of extracting charged droplets out of liquid by application of an electrostatic field is perhaps older than field ion microscopy. But the development of liquid metal ion sources from liquid capillaries, from slit shaped emitter modules and from wetted field emission tips, etc., as well as the understanding of the mechanisms of ion formation in terms of field evaporation and field ionization theories,... 

This vapor pressure can be deduced from kinetic theory and the rate of arrival of Cs atoms per square centimeter per second (3,4). This arrival rate. A, can be calculated from the measured value of the saturation positive ion current. Langmuir first showed that if the tungsten is hot enough every cesium atom that strikes the surface evaporates off as a positive ion of cesium. This saturated positive ion current, i.p, can be measured easily with a medium sensitive galvanometer when the collector is negative. It is related to A by the equation... 

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