Separation factor resolution

The examples shown in Table 7.2 do not show a particularly high enantioselectiv-ity. However, with an enantiomeric excess of 50% (75/25), approximately optically pure material may be obtained in five cycles, and for chromatographic resolutions separation factors a > 1 are sufficient for efficient resolution processes132,331. Nickel(II) complexes of the type shown in Table 7.2 have been used to modify ion-exchange resins which were used for racemate separations1321, and derivatives of (S),(S)-ppm with functional groups that may be fixed to supports are readily available. ... 

Three separate factors affect resolution (1) a column selectivity factor that varies with a, (2) a capacity factor that varies with k (taken usually as fej). and (3) an efficiency factor that depends on the theoretical plate number. 

The separating power of a chromatographic process arises from the development of many theoretical plates to achieve adsorption equiUbrium within a column of moderate length. Even though the separation factor between two components may be small, any desired resolution may be achieved with sufficient theoretical plates. 

The heating effect is the limiting factor for all electrophoretic separations. When heat is dissipated rapidly, as in capillary electrophoresis, rapid, high resolution separations are possible. For electrophoretic separations the higher the separating driving force, ie, the electric field strength, the better the resolution. This means that if a way to separate faster can be found, it should also be a more effective separation. This is the opposite of most other separation techniques. 

Figures 13.25-13.28 show the ultrahigh resolution separations in chloroform of polystyrene standards, polytetramethylene glycol, urethanes and isocyanates, and epoxy resins, respectively. Multiple column sets of anywhere from two to six columns in series have been used for well over a year with no apparent loss of efficiency. The 500- and 10 -A gels can easily tolerate 15,000 psi or more. In fact, the limiting factor in the number of columns that can be used in series is generally the pump or injector in the FIPLC system. A pump capable of 10,000 psi operation should allow the use of a column bank of 10-12 50-cm columns with a total plate count of 500,000 or more.

In the elucidation of retention mechanisms, an advantage of using enantiomers as templates is that nonspecific binding, which affects both enantiomers equally, cancels out. Therefore the separation factor (a) uniquely reflects the contribution to binding from the enantioselectively imprinted sites. As an additional comparison the retention on the imprinted phase is compared with the retention on a nonimprinted reference phase. The efficiency of the separations is routinely characterized by estimating a number of theoretical plates (N), a resolution factor (R ) and a peak asymmetry factor (A ) [19]. These quantities are affected by the quality of the packing and mass transfer limitations, as well as of the amount and distribution of the binding sites. 

Racemate Separation factor a Resolution Note factor R S Refe- rence... 

Resolution The degree to which two peaks are separated. This is a function of the number of theoretical plates, N, in a column and the separation factor between the two components. 

To a first approximation the three terms in equation (1.46) and (1.47) can be treated as independent variables. For a fixed value of n Figure 1.8 Indicates the influence of the separation factor and capacity factor on the observed resolution, when the separation factor equals 1.0 there is no possibility of any separation. The separation factor is a function of the distribution coefficients of the solutes, that is the thermodynamic properties of the system, and without some... 

Figure 1.8 Influence of varying the separation factor and capacity factor on the observed resolution for two closely spaced peaXs.

Figure 1.9 illustrates the relationship between resolution, the separation factor, the average capacity factor and the column efficiency for some real chromatographic peaks [lOS]. The central portion of the figure illustrates how resolution increases with the capacity factor for a fixed separation factor and column efficiency. At first the resolution increases quickly as the... 

Resolution in capillary gel electrophoresis of DNA sequencing was shown to be directly proportional to the product of the number of bases and the relative peak distance, i.e., to the mean separation of peaks.43 Reformulation of the treatment of the capacity factor has been used to simplify and clarify the interpretation of the separation factor in electrophoresis.44 Peak... 

Aboul-Enein and Ali [78] compared the chiral resolution of miconazole and two other azole compounds by high performance liquid chromatography using normal-phase amylose chiral stationary phases. The resolution of the enantiomers of ( )-econazole, ( )-miconazole, and (i)-sulconazole was achieved on different normal-phase chiral amylose columns, Chiralpak AD, AS, and AR. The mobile phase used was hexane-isopropanol-diethylamine (400 99 1). The flow rates of the mobile phase used were 0.50 and 1 mL/min. The separation factor (a) values for the resolved enantiomers of econazole, miconazole, and sulconazole in the chiral phases were in the range 1.63-1.04 the resolution factors Rs values varied from 5.68 to 0.32. 

The chiral recognition ability of a CSP is quantitatively evaluated from the results of chromatographic separation of enantiomers. Figure 3.4 shows a chromatogram of the resolution of benzoin (19) on cellulose tris(3,5-dimethylphenylcarbamate). The (+)-isomer elutes first followed by the (—)-isomer complete baseline separation is achieved. The results of the separation can be expressed by three parameters—capacity factors (k1), separation factor (a), and resolution factor (Rs)—defined as follows ... 

Compared with GC and HPLC, the most important advantage of CE is its high peak efficiency. It can give a baseline resolution of peaks even when the separation factor is low. Volatile chiral samples are best analyzed by GC, whereas HPLC and CE are more suitable for nonvolatile samples. CE is the best choice for a charged compound or for a high-molecular-weight sample. 

Perhaps, the most important goal of all chromatographic method development projects is to obtain the desired resolution (separation) of the analyte peaks in the desired time. On the basis of the above discussions, resolution can be described as resulting from three key factors set up of the instrument, intermolecular interactions in the column and residence time in the column. First, resolution must come from properly setting up the instrument and operating it correctly. In GC, this is not always trivial, as many operations such as installing a... 

The effect on sample resolution with changes in k, N or a values is shown in Fig. 15.10. For example, an increase in separation factor a results in a displacement of one band center relative to the other, and a... 

Total resolution of [269] into its two enantiomers was achieved by liquid-liquid chromatography through complexation to L-valine adsorbed initially on diatomaceous earth (Timko et al., 1978). On the basis of comparative chromatographic studies, the separation factors (a) and the EDC values were correlated (Cram et al., 1975) (Table 59). 

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