Desolvation step, chemical

In the case of some ion-transfer reactions the chemical desolvation step controls the rate of the overall process and the currents observed are lower than those expected for the process limited solely by the mass transport rate. The formation of such less-hydrated species was attributed [210] in the case of the electroreduction of nick-el(II) in water to a slow exchange of water molecules from the first solvation sphere of Ni(II) under the influence of the crystal field stabilization. A similar mechanism was found for Ni(II) and Co(II) in methanol [211]. 

It is evident that with the discrete cycles of the non-flame atomizers several reactions (desolvation, decomposition, etc.) which occur simultaneously" albeit over rather broad zones in a flame (due to droplet size distributions] are separated in time using a non-flame atomizer. This allows time and temperature optimization for each step and presumably improves atomization efficiencies. Unfortunately, the chemical composition and crystal size at the end of the dry cycle is matrix determined and only minimal control of the composition at the end of the ash cycle is possible, depending on the relative volatilities and reactivities of the matrix and analyte. These poorly controlled parameters can and do lead to changes in atomization efficiencies and hence to matrix interferences. 

The mechanism of the reduction of cadmium ions at DME in NaCl04 solutions with varied water activity was also studied [26]. In these solutions, the electrode process of the Cd(II)/Cd(Hg) system was described by the mechanism that includes (1) fast loss of 12.5 water molecules in a preceding equihbrium, (2) a slow chemical step, which is not a desolvation, (3) slow transfer of the first electron. 

The relative importance of bulk diffusion, desolvation, and integration depends on the solid-state properties and solution properties. These processes are analagous to a reaction pathway, similar to a homogeneous chemical reaction pathway (Bennema 1969). Pictorially this is represented in Figure 3.8. (Davey et al. 2000). Although the volume diffusion step may be analyzed in a classical manner, the quantification of the surface diffusion steps requires consideration of the structure of the interface as well as the physical and chemical nature of the adsorption and diffusion process. The impact of the strength of adsorption on the surface diffusion process is discussed in Section 3.6. 

The fundamental function of atomizers in AAS is simply to produce atoms. The basic steps carried out continuously in flame atomization are (i) Nebulization (conversion of sample into droplets leaving small particles) (ii) Desolvation (removal of solvent) (iii) Atomization (thermal or chemical breakdown of solid particles) (iv) Measurement (interaction with radiation) ... 

In a particle-beam interface (Figure 3A), the column effluent is pneumatically nebulized into an atmospheric-pressure desolvation chamber. This is connected to a momentum separator where the analytes are transferred to the MS ion source while the low molecular mass solvent molecules are efficiently pumped away. The analyte particles hit the heated source surface, evaporate and can be ionized by electron or chemical ionization. The evaporation step obviously limits the application range of the interface to not-too-polar analytes. 

As shown in Fig. 7, the first step involved in the decarboxylation of an alpha-keto acid by thiamine pyrophosphate is the addition of the carbonyl by the carbanion nucleophile, followed by decarboxylation of the acid. As reviewed by Jencks (1975), investigations of the role of thiamine pyrophosphate analogs in the pyruvate dehydrogenase-catalyzed decarboxylation of pyruvate have provided evidence for rate enhancement by a desolvation effect. This effect comes about because of the removal of both the carboxylate group of the substrate and the cationic nitrogen of the coenzyme from an aqueous environment to the hydrophobic active site of the enzyme. The bulky and apparently chemically nonfunctional pyrophosphate and pyrimidine moieties of the... 

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