Chromatography separability factor

The most common method for screening potential extractive solvents is to use gas—hquid chromatography (qv) to determine the infinite-dilution selectivity of the components to be separated in the presence of the various solvent candidates (71,72). The selectivity or separation factor is the relative volatihty of the components to be separated (see eq. 3) in the presence of a solvent divided by the relative volatihty of the same components at the same composition without the solvent present. A potential solvent can be examined in as htfle as 1—2 hours using this method. The tested solvents are then ranked in order of infinite-dilution selectivities, the larger values signify the better solvents. Eavorable solvents selected by this method may in fact form azeotropes that render the desired separation infeasible. 

Fig. 6-10. Influence of the number of basic interaction sites of the template versus the separation factor measured in chromatography for the corresponding racemate. The templates were imprinted using MAA as functional monomer by thermochemical initiation at 60/90/120 °C (24 h at each temperature) and using acetonitrile as porogen. (From Sellergren et al. [15].)...

An interesting consequence of selective sorption is that conditions for partition chromatography arise which may enhance the normal ion exchange separation factors. This aspect has been utilised by Korkisch34 for separation of inorganic ions by the so-called combined ion exchange-solvent extraction method (CISE). 

Enantiomers of the 8,9-dichloro-2,3,4,4 ,5,6-hexahydro-177-pyrazino[l,2-tf]quinoxalin-5-one (structure 249 Rz = R3 = Cl R1 = R4 = H) could be separated by normal-phase, chiral high-performance liquid chromatography (HPLC) with increased retention and separation factors if ethoxynonafluorobutane was used as solvent, instead of -hexane <2001JCH(918)293>. 

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. 

Displacement chromatography is suitable for the separation of multicomponent bulk mixtures. For dilute multicomponent mixtures it allows a simultaneous separation and concentration. Thus, it permits the separation of compounds with extremely low separation factors without the excessive dilution that would be obtained in elution techniques. 

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). 

As further shown by the authors, for e.g., a fifty-plate column, a separation factor of about 1.5 is needed to achieve a 99% purity. This value of the separation factor is in the range of many practical protein separations. Therefore, the use of a fifty-plate column can achieve high purity indicating that the tens of thousands of plates found in many conventional chromatography columns are not necessary... 

The thermodynamic connection between IE s on gas solubility, infinite dilution Henry s law constants, and transfer free energy IE s, implies that gas-liquid chromatography should be a convenient way to study solvent effect IE s. That in fact is the case, and many authors have reported on chromatographic isotope separations and on the interpretation of the separation factors in terms of the transfer free energy IE s (Section 8.5). 

To illustrate consider gas chromatography. Figure 8.16 shows an idealized plot of detector response vs. time. Here to is the time lapse between injection and elution of inert material, and ti and t2 are retention times for the isotopomers of interest. For difficult separations ti t2 to. The resolution, R, is related to the band widths, R = (t2 — ti)/(2w), and the number of plates (assumed to be the same for both isotopomers) is, n = 4(t/w)2. The separation factor is a = t2/ti, and the... 

The maximum production rate, however, often results in nnacceptable recovery yields. Low recovery yield requires further processing by recycling the mixed fractions. The recovery yield at the maximum production rate strongly depends on the separation factor. In the cases of difficnlt separations, when the separation factor under linear conditions is aronnd or lower than a= 1.1, the recovery yield is not higher than 40%-60%. Even in the case of a=1.8, the recovery yield at the maximum production rate is only about 70%-80%. The situation is still less favorable in displacement chromatography, particularly if the component to be purified is more retained than the limiting impurity. In this case, from one side the impurity, whereas from the other side the displacer, contaminates the product. 

A highly versatile method for enantiomer analysis is based on the direct separation of enantiomeric mixtures on nonraceinic chiral stationary phases by gas chromatography (GC)6 123-12s. When a linearly responding achiral detection system is employed, comparison of the relative peak areas provides a precise measurement of the enantiomeric ratio from which the enantiomeric purity ee can be calculated. The enantiomeric ratio measured is independent of the enantiomeric purity of the chiral stationary phase. A low enantiomeric purity of the resolving agent, however, results in small separation factors a, while a racemic auxiliary will obviously not be able to distinguish enantiomers. 

Equation 23-30 also tells us that resolution increases as the separation factor y increases. The separation factor is the relative velocity of the two components through the column. The way to change relative velocity is to change the stationary phase in gas chromatography or either the stationary or the mobile phase in liquid chromatography. Important equations from chromatography are summarized in Table 23-2. 

As in chromatography, resolution between closely spaced peaks A and B in an electio-pherogram is related to plate count, N, and separation factor, y, by Equation 23-30 resolution = (VA/4)(y — 1). The separation factor (y = anet /unetB) is the quotient of migration times tB/tA. Increasing y increases separation of peaks, and increasing N decreases their width. 

Before proceeding, the author wishes to point out that the separation factor, a, as defined above is the term used by most people working in chromatography and the term recommended by the IUPAC. Readers will encounter statements to the contrary in some references (5,7). 

The application of cellulosic anion exchanger in the separation of trace amounts of rare earths has also been investigated. Diethylaminoe-thyl cellulose paper and 0.026M citric acid were found to be the most satisfactory. A separation factor of 2.6 between Eu and Ce was obtained [123]. It has been found [124] that a mixture of HC1 and various aliphatic alcohols can be successfully used as eluant for the separation of rare earths by paper chromatography (Whatman No. 1). 

Enantioselective gas chromatography can provide three quite different kinds of information (1) the amount of each enantiomer present in a food, determined as the enantiomeric purity or the enantiomer excess, and the separation factor a for each pair of enantiomers (2) enantiospecific sensory evaluation using gas chromatography-olfactometry (GC-O) and (3) data used as part of an authenticity determination. 

Anon. 1993a. Collection of enantiomer separation factors obtained by capillary gas chromatography on chiral stationary phases. J. HighResolut. Chromatogr. 16 338-352. 

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