Species selectors

The synthesis of zeolite A, mixtures of A and X, and zeolite X using batch compositions not previously reported are described. The synthesis regions defined by triangular coordinates demonstrate that any of these materials may be made in the same area. The results are described in terms of the time required to initiate crystallization at a given reaction temperature. Control of the factors which can influence the crystallization time are discussed in terms of "time table selectors" and "species selectors . Once a metastable species has preferentially crystallized, it can transform to a more stable phase. For example, when synthesis conditions are chosen to produce zeolite A, the rate of hydroxysodalite formation is dependent on five variables. These variables and their effect on the conversion of zeolite A to hydroxysodalite are described mathematically. 

These seven factors which determine the time required for crystallization can be divided into timetable selectors and species selectors. Rate of addition, anion activation, source of Si02 and reaction temperature can all be considered "timetable selectors". 

In summary, the particular zeolite species or mixture of species which crystallizes can be controlled and is predictable within the synthesis conditions studied. This control can be achieved by applying the concept of a continuum between the species HS-A-X-Y, utilizing the timetable selectors to establish a fixed frame of reference, and using the species selectors to selectively produce a desired zeolite. Once the desired species has crystallized, it is then possible to regulate and predict its conversion to the species which precedes it in the continuum. 

Selection of the species can be even automated by a computer. Such a device, called the species selector, or STYRE, was first introduced by Silldn [12]. From the list given one species after another is added to the starting chanical model, and a new species is accepted if p/a(P) < and rejected if p/o(p) < or if p has no physi-... 

Another cycle of the species selector, giving previously rejected parameters (species) one more chance, is started. 

Aguilar and Valiente [73] studied extraction of hydrochloric acid by THA and in this very detailed study the applied graphical methods [9] and MESAK evalnation of average p,q values and LETAGROP-DISTR program with species selector STYRE all together, 19 models were examined. The best one found was including species (HCl)p(THA), with (p,q) values (1,1) and (3,3). 

The use of a charged chiral selector is probably the best solution to improve the classical PET when CE is hyphenated with MS. Better solubility, additional electrostatic interactions, and improvement of the stereoselective separation power afforded by the self-mobility of the chiral additives into the BGE are among the numerous advantages of these charged selectors. When electromigration of the chiral species and the analytes are opposite (PFT-countercurrent approach), the mobility difference between free and complexed analytes is increased, leading to a higher resolution than with a neutral chiral selector. In optimized countercurrent... 

There are three possible approaches to the separation of chiral species by CE (1) addition of chiral selectors to the buffer, (2) use of a chiral stationary phase, and (3) precolumn derivatization. These correspond to the approaches in HPLC, and the separation mechanisms are described in Section 2.8. In the first approach, additives are added to CZE, CGE, or MECC buffers to effect the separation. In the second approach, chiral selectors can be immobilized on the capillary wall, although that is a difficult process. Alternatively, capillaries filled with enantiospecific packings can be employed for CEC. In the third approach, enantiomers are derivatized with chirally specific reagents prior to CZE or MECC. Addition of chiral selectors to the buffer is the most common approach. 

Additional substances (buffer additives) are often added to the buffer solution to alter selectivity and/or to improve efficiency, and the wall of the capillary may be treated to reduce adsorptive interactions with solute species. Organic solvents, surfactants, urea and chiral selectors are among the many additives that have been recommended (table 4-24). Many alter or even reverse the EOF by affecting the surface charge on the capillary wall, whilst some help to solubilize hydrophobic solutes, form ion-pairs, or minimize solute adsorption on the capillary wall. Chiral selectors enable racemic mixtures to be separated by differential interactions with the two enantiomers which affects their electrophoretic mobilities. Deactivation of the capillary wall to improve efficiency by minimizing internet ions. with solute species can be achieved by permanent chemical modification such as silylaytion or the... 

CE enantioseparations are commonly performed in the direct additive mode. The chiral selector is added to the background electrolyte (BGE) and undergoes stereoselec-tively complexation with the charged SA enantiomers. Different equilibrium constants of (/ )- and (S)-enantiomers and different mobilities of free and complexed solute species under the influence of the electric field are the basis for the differences in the observed migration times of the enantiomers. The indirect approach has only a little practical significance in this context. 

A diastereoisomeric interaction is always required for the resolution of enantiomeric substances. This interaction occurs between the enantiomers of interest and a second enantiomeric species often referred to as the chiral selector. The diastereoisomeric interaction between the enantiomers and the chiral selector may involve a covalent bond or other less stable non-cova-lent associations. In the example below, a mixture of R and S isomers associates with the R isomer of the chiral selector to yield two diastereoisomeric products ... 

These macrocyclic ethers assume a crown-like shape in solution with a central cavity capable of containing a small solute. They bind to small cationic species through association with the electron-rich oxygens of the ether linkage. Chiral crown ethers (Fig. 31) serve as selectors for enantiomeric amines in the protonated state. They have been used as mobile-phase... 

However, surfactants incorporated into the electrolyte solution at concentrations below their critical micelle concentration (CMC) may act as hydrophobic selectors to modulate the electrophoretic selectivity of hydrophobic peptides and proteins. The binding of ionic or zwitterionic surfactant molecules to peptides and proteins alters both the hydrodynamic (Stokes) radius and the effective charges of these analytes. This causes a variation in the electrophoretic mobility, which is directly proportional to the effective charge and inversely proportional to the Stokes radius. Variations of the charge-to-hydrodynamic radius ratios are also induced by the binding of nonionic surfactants to peptide or protein molecules. The binding of the surfactant molecules to peptides and proteins may vary with the surfactant species and its concentration, and it is influenced by the experimental conditions such as pH, ionic strength, and temperature of the electrolyte solution. Surfactants may bind to samples, either to the... 

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