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area. In addition, the amount of available stationary phase on a bonded phase can be modified by adjusting the molecular size (chain length) of the bonded material. The chain length of the bonded material can be critical when separating proteins as dispersive interactions between the alkane chains and the dispersive (hydrophobic) groups of the protein can be strong enough to cause structural deconformation; (i.e., the protein becomes denatured). Reducing the chain length of the bonded material, the dispersive forces can be reduced significantly and the deconformation diminished. In practice, carbon chains only two or four carbon atoms long are among those most commonly used for separating labile proteins. Stationary Phase Limitation by Chiral Selectivity. The extent to which an enantiomer can interact with the stationary phase depends on how close it can approach the molecules of the stationary phase. If the stationary phase is also chiral in nature

matter on which the stationary phase is bonded or coated. The mobile phase (which may be a gas or a liquid) passes under pressure through the column to elute the sample. The column form may also be a long, small-diameter open tube that has the stationary phase coated or bonded to the internal surface. Alternatively, the chromatographic system may take the form of a plate (usually glass) the surface of which is loaded with particulate matter to which the stationary phase is coated or bonded. The mobile phase (a liquid) is arranged to percolate up the plate (usually by surface tension forces) to elute the sample. The sample is injected into the mobile phase stream just before the front of the columns. The column is designed to allow two processes to take place that will produce the separation. Firstly, as a result of different forces between each molecular type and the stationary phase, each solute is retained to a different extent and, thus, the more

nbsp; Vancomycin is a very stable chiral stationary phase, has a relatively high sample capacity, and can be covalently bonded by multiple linkages to the silica gel surface. It can be used with mobile phases with a high water content, as a reversed phase, or with a high solvent content, as a largely polar stationary phase. For example, when used as a reversed phase THF–water mixtures are very effective mobile phases. Conversely, when used as a polar stationary phase, n-hexane–ethanol mixtures are often employed. Vancomycin has a number of ionizing groups and thus can be used over a range of different pH values (pH 4.0 to 7.0) and exhibit a wide range of retention characteristics and chiral selectivities. Ammonium nitrate, triethyl-ammonium acetate and sodium citrate buffers have all been used satisfactorily with this stationary phase. The effect of the chosen buffer has little or no effect on chiral selectivity only on pH control. An example of

for about 5 hours and the product is then filtered on a sintered glass filter, washed sequentially with toluene, tetrahydrofuran (THF), methanol, methanol water (50:50 v/v) and finally with methanol and dried under suction. The bonded phase now needs end-capping; that is, any unreacted silanol groups are treated with a small molecular weight silanizing reagent to react with those hydroxyl groups that were stearically unavailable to the larger reagent due to exclusion. To end-cap the product, thebonded phase is refluxed for two hours in a mixture of 100 ml of toluene and 25 ml of hexamethyldisilazane. The product is again filtered free of the reaction liquid mixture and washed sequentially with, toluene tetrahydrofuran, methanol, methanol water (50:50 v/v) and finally with methanol and then dried under vacuum. It should be pointed out that end-capping cannot eliminate the hydroxyl groups that are stearically hindered by the bonded moiety, or at best, only a small proportion of them will

bonded phase is often used to determine the efficacy of bonding, but its value must be used in conjunction with a knowledge of the surface area of the native silica in order to arrive at a meaningful conclusion. The amount of material bonded to the silica will depend, not only on the efficiency of the reaction, but also on the number of silanol groups that were available with which it could react; ipso facto it will also depend on the surface area of the parent silica. The carbon content of the bonded phase is usually determined by micro-analysis and the result expressed as %w/w of the combined bonded organic material and the silica gel. Consider a bonded phase where the carbon content is (y)%w/w coated with a hydrocarbon moiety having (n) carbon atoms per aliphatic chain (e.g., for the dimethyl octyl brush phase, n=10). Then, the concentration of aliphatic chains in mols. per gram of silica (m')will be,                                          Consequently, if the surface

The mobile phase passes through several stationary columns which contain the stationary phase which may be silica gel, or a bonded phase, or, in GC a suitable coated support. There are, also, a number of different ports, one for the mobile phase and one for the return of the mobile phase. In addition there is a central feed port and two take-off ports. These ports can, by appropriate valve programming (usually by means of a computer) connect, sequentially, each column to its neighbor. In modern simulated moving bed systems, disc valves are no longer employed and have been replaced by sets of individual valves. It is clear that the apparent counter current movement of the stationary phase, relative to the mobile phase, is achieved