Physical coating

Chiral stationary phases in tic have been primarily limited to phases based on normal or microcrystalline cellulose (44,45), triacetylceUulose sorbents or siHca-based sorbents that have been chemically modified (46) or physically coated to incorporate chiral selectors such as amino acids (47,48) or macrocyclic antibiotics (49) into the stationary phase. 

Many applications have been found for reversed-phase chromatography in HPLC. The composition of the stationary phase is more easily controlled than with the TLC methods, and thus provides more reproducible separations. The use of bonded non-polar phases enables gradient elution to be carried out in a reversed-phase system. This approach has been useful for the analysis of polar compounds and gives improved separations compared with normal-phase HPLC. These methods usually involve separation with systems consisting of Carbowax, C -polymer or similar phases bonded or physically coated on the support. 

The physically coated support may require a pre-column to ensure establishment of equilibrium, as described above.) The mobile phase is usually water-methanol in various ratios, or, in the case of bonded phases, a gradient proceeding from water to methanol. A list of some chemically bonded reversed phases is given in Chapter 3. 

The stationary phase can be bound to the tubing either as a physical coating on the wall, or can be chemically immobilized. The former type phases are called nonbonded phases, while the chemically bound phases, cross-linked within the tubing are known as bonded phases. The latter is preferred because it can be used at high temperatures with less bleeding and can be rinsed with solvents to remove nonvolatile substances that accumulate on the column. 

One of the first steps in modifying the performance of capillary electrophoresis was the deactivation of silica groups of the capillary column by physically coating the capillary wall with methylcellulose (58,59), as well as via silane derivatization (10,44,60). Presently, many other changes have been carried out either to the capillary surface or addition of chemical agents to the separation buffer (see Table II), including manipulation of... 

The separation of enantiomers is especially important in the pharmaceutical field, because drag enantiomers may produce different effects in the body. Enantiomer separations by chromatography require one of the components of the phase system to be chiral. This can be achieved by (a) the addition of a chiral compound to the mobile phase, which is then used in combination with a nonchiral stationary phase, or (b) the use of a chiral stationary phase in combination with a nonchiral mobile phase. The chiral phase can either be a solid support physically coated with a chiral stationary phase liquid or a chemically bonded chiral phase. For mobile-phase compatibility reasons, a chiral stationary phase is preferred in LC-MS. However, most chiral stationary phases have stringent demands with respect to mobile-phase compositiorr, which in turn may lead to compatibility problems. Three types of phase systems are applied in LC-MS ... 

Capillary Gel Electrohoresis. In this mode, molecules are separated according to size as they migrate through a polymer matrix. The polymer can be in solution, physically coated on the capillary wall, or chemically bonded to the capillary wall. This mode is primarily used for the separation of large molecules like proteins, peptides, and DNA species. 

Mechanistic considerations (e.g., the extensive work published on brush-type phases) or the practitioner s experience might help to select a chiral stationary phase (CSP) for initial work. Scouting for the best CSP/mobile phase combination can be automated by using automated solvent and column switching. More than 100 different CSPs have been reported in the literature to date. Stationary phases for chiral pSFC have been prepared from the chiral pool by modifying small molecules, like amino acids or alkaloids, by the deriva-tization of polymers such as carbohydrates, or by bonding of macrocycles. Also, synthetic selectors such as the brush-type ( Pirkle ) phases, helical poly(meth) acrylates, polysiloxanes and polysiloxane copolymers, and chiral selectors physically coated onto graphite surfaces have been used as stationary phases. 

Early in the development of modern liquid chromatography, attempts were made to adjust the stationary-phase chemistry in a fashion analogous to gas chromatography that is, viscous organic liquids were physically coated onto an inert support. This proved highly frustrating for routine users and researchers alike. While there were many commercially available liquids for use as phases, the inability to keep a constant amount of the liquid coated on the support led to extremely poor reproducibility. Even when the mobile phase was presaturated with the or-... 

The anode and cathode should be stable in the electrolysis medium, allow the desired oxida-tion/reduction reactions at the highest possible rates with miiumal by-product formation, and be of reasonable cost. In actuality, the electrodes may corrode or undergo physical wear during reactor operation, which may limit their lifetime. Often, if an expensive electrode material is needed for a given reaction, it can be plated or physically coated on a less costly, inert, and electronically conducting substrate. Common anode and cathode materials are listed in Table 26.8. 

In the second approach, the stationary phase is physically coated with a chelating reagent and then conditioned to remove unstable reagent prior to use with a reagent-free mobile phase (precoated stationary phases) [458,465,466]. Alternatively, the... 

Chiral stationary phases, chemically grafted on a silica matrix, can also be used for quite a long period of time. The same holds true for the very versatile physically coated cellulose and amylose phases if they are treated with care. 

Gonzalez, N., Elvira, C., San Roman, J., and Cifnentes, A., New physically coated adsorbed polymer coating for reprodncible separations of basic and acidic proteins by capillary electrophoresis,... 

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