Frontal separation

Figure 3.2 Three major methods in chromatography. The commonest form of chromatography involves the introduction of a small volume of sample onto a column and is known as zonal chromatography. Movement down the column is effected by the mobile phase, which may be simply a solvent (A) allowing partition of the test molecules between the stationary and mobile phases. Alternatively, the mobile phase may be a solvent containing solute molecules (B), which actively displace test molecules from the stationary phase. A less frequently used method known as frontal separation (C) does not involve a separate mobile phase but a large volume of the sample is allowed to pass through the column and as the various components separate, concentration fronts develop and their movement can be monitored.

It is to be noted that with complete flow reversal the separation process in column I is the analogue of the frontal separation process or the frontal analysis in a flxed bed of the sorbent the separation process in column II is the analogue of the reverse frontal analysis and the process in both columns, with conditions, phase composition, and temperature unchanged in passing from one column to the other is the analogue of displacement chromatography. The analogy is related only to the... 

Figure 6 Distribution of component concentrations as a function of column height during separation of a three-component mixture (a) frontal separation (b) displacement separation.

In case impurities are of no value and the only task is to recover the intermediately sorbable B component, the ion-exchange resin on which frontal separation is carried out must not contain impurities that sorb better than B and the solution of E displacer must not contain impurities that sorb less effectively than B. On this basis, A and C ions can be applied as auxiliary ones. This is sometimes done when separating mixtures of rare earth elements. 

It is suitable to perform frontal separation on the ion-exchange resin loaded with the more weakly sorbed C mixture component and to carry out the recovery of the desired B component from the ion-exchange resin obtained upon frontal separation either with the same C ion or with A and C mixtures at such a ratio of the phase flows which provides complete displacement of B from the ion exchanger. 

Solution can be collected from the zone containing B and A while the BX can be obtained from it upon frontal separation (Fig. 9). The ion-exchange resin leaving column II returns to the zone of column I from which solution is collected. Concentrated A and C impurities are removed from time to time at the ends of column I near flow reversal zones. It is also possible to use an auxiliary section in column II to displace separated ions from ion exchanger by the better sorbed E ions. 

Development of the Chromatogram. The term development describes the process of performing a chromatographic separation. There are several ways in which separation may be made to occur, eg, frontal, displacement, and elution chromatography. Frontal chromatography uses a large quantity of sample and is usually unsuited to analytical procedures. In displacement and elution chromatography, much smaller amounts of material are used. 

A chromatographic separation can be developed in three ways, by displacement development, by frontal analysis, and by elution development, the last being almost universally used in all analytical chromatography. Nevertheless, for the sake of completeness, and because in preparative chromatography (under certain conditions of mass overload) displacement effects occur to varying extents, all three development processes will be described. 

This type of chromatographic development will only be briefly described as it is rarely used and probably is of academic interest only. This method of development can only be effectively employed in a column distribution system. The sample is fed continuously onto the column, usually as a dilute solution in the mobile phase. This is in contrast to displacement development and elution development, where discrete samples are placed on the system and the separation is subsequently processed. Frontal analysis only separates part of the first compound in a relatively pure state, each subsequent component being mixed with those previously eluted. Consider a three component mixture, containing solutes (A), (B) and (C) as a dilute solution in the mobile phase that is fed continuously onto a column. The first component to elute, (A), will be that solute held least strongly in the stationary phase. Then the... 

At this stage the benzene is still just separated from the naphthalene. A 16 ml sample volume demonstrates the complete frontal analysis chromatogram of the mixture. The first and second steps containing pure benzene and benzene + naphthalene, respectively, the center peak containing all three solutes and the last two steps containing naphthalene plus anthracene and pure anthracene, respectively. 

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

A relatively large volume of sample can be applied to the wet layer from the edge of the layer from the eluent distributor, forming a partly separated starting band by the frontal chromatography stage. 

Ferric ion was immobilized on a Chelating Sepharose Fast Flow column preparatory to the separation of seven enkephalin-related phosphopep-tides.17 Non-phosphorylated peptides flowed through the column, and the bound fraction contained the product. The capacity of the column was found to be 23 pmol/mL by frontal elution analysis. Cupric ion was immobilized on Chelating Superose for the isolation of bovine serum albumin.18 Cupric ion was immobilized on a Pharmacia HiTrap column for the separation of Protein C from prothrombin, a separation that was used to model the subsequent apparently successful separation of Factor IX from prothrombin Factor IX activity of the eluate was, however, not checked.19 Imidazole was used as the displacement agent to recover p-galactosidase from unclarified homogenates injected onto a nickel-loaded IMAC column.20 Pretreatment with nucleases and cleaning in place between injections were required procedures. A sixfold purification factor was observed. 

FIGURE 1-1 Coronal section of the human brain at the thalamic level stained by the Heidenhain technique for myelin. Gray matter stains faintly, all myelinated regions are black. The thalamus ( ) lies beneath the lateral ventricles and is separated at this level by the beginning of the third ventricle. The roof of the lateral ventricles is formed by the corpus callosum (small arrows). Ammon s horns are shown by the large arrows. Note the outline of gyri and sulci at the surface of the cerebral hemispheres, sectioned here near the junction of the frontal and parietal cortices. 

Two other means of separating and removing components from a column are frontal analysis and displacement development, but these are of secondary importance. In frontal analysis sample is continuously applied to the top of the column. Eventually, as the stationary phase becomes... 

The breakthrough curves measured for the monolithic columns with different proteins are very sharp and confirm again the fast mass transport kinetics of the monoliths [133, 134]. The frontal analysis used for the determination of the breakthrough profile can also be used for calculation of the dynamic capacity of the column. For example, the capacity for the 60x16mm i.d. monolith at 1% breakthrough is 324 mg of ovalbumin and represents the specific capacity of 40.0 mg/g of separation medium or 21.6 mg/ml of column volume. 

Although separations may be caused by elution, frontal and displacement analyses, yet the elution technique is the most common. This method makes use of a stream of carrier-gas flowing through the column. Precisely, a sample is injected into the carrier-gas as a plug of vapour that is swept into the head of the packed chromatographic column. Separation of components that comprise the sample results from a difference in the multiple forces by which the column materials tend to retain each of the components. 

Frontal analysis does not involve the use of separate solvent systems but separation takes place within the liquid sample usually as a result of one compo-... 

The precise measurement of competitive adsorption isotherms not only of theoretical importance but may help the optimization of chromatographic processes in both analytical and preparative separation modes. The methods applied for the experimental determination of such isotherms have been recently reviewed [90], Frontal analysis using various flow rates can be successfully applied for the determination of competitive adsorption isotherms [91]. 

Chin The anatomical frontal portion of the mandible, also known as the mentum, that contains the line of fusion of the two separate halves of the mandible (symphysis menti). This line of fusion divides inferiorly to enclose a triangular area called the mental protuberance. On each side, inferior to the second premolar tooth, is the mental foramen for... 

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