Liquid stationary phase

From the pioneering studies of Ito et al. [117], CCC has been mainly used for the separation and purification of natural products, where it has found a large number of applications [114, 116, 118, 119]. Moreover, the potential of this technique for preparative purposes can be also applied to chiral separations. The resolution of enantiomers can be simply envisaged by addition of a chiral selector to the stationary liquid phase. The mixture of enantiomers would come into contact with this liquid CSP, and enantiodiscrimination might be achieved. However, as yet few examples have been described in the literature. 

Other modes of LC operation include liquid-liquid partition chromatography (LLC) and bonded phase chromatography. In the former, a stationary liquid phase which is immiscible with the mobile phase is coated on a porous support, with separation based on partition equilibrium differences of components between the two liquid phases. This mode offers an alternative to ion exchange in the fractionation of polar, water soluble substances. While quite useful, the danger exists in LLC that the stationary phase can be stripped from the column, if proper precautions are not taken. Hence, it is typical to pre-equil-ibrate carefully the mobile and stationary phases and to use a forecolimn, heavily loaded with stationary phase 9). 

The selection of the support is critical for several reasons [285-287]. The surface of the solid support must be wetted by the stationary liquid phase better than by the mobile phase, otherwise a stable stationary liquid phase film will not be formed. Although sufficiently strong adsorptive properties are required to obtain wetting, some compromise is required, since the support should have negligible adsorptive properties for the components of the sample. In the absence of interfacial adsorption, retention in LLC is very simply defined by equation (4.14)... 

The problems of stationary phase erosion can be largely overcome by solvent-generated LLC, where the stationary liquid phase is generated dynamically by the mobile phase, in this approach, one of the phases of an equilibrated liquid-liquid system is applied as a mobile phase to a solid support which is better wetted by the other phase of the liquid-liquid system. The support is usually silica when the stationary phase is aqueous or a polar solvent and a reversed-phase chemically bonded support when the stationary phase is a nonpolar solvent. Under these Conditions a multimolecular layer is formed on the surface of the solid support which has the properties of the liquid phase in... 

The analysis providing interaction second virial coefficients from chromatography rests upon three principal assumptions 1) vapor-liquid equilibrium exists in the column 2) the solute (component 1) is soluble in both the carrier gas (component 2) and the stationary liquid phase (component 3) 3) the carrier gas and stationary liquid are insoluble. Under assumption 1, we can write... 

The liquid liquid partition chromatography (LLPQ method involves a stationary liquid phase that is more or less immobilized on a solid support, and a mobile liquid phase. The analyte is therefore distributed between the two liquid phases. In conventional LLPC systems, the stationary liquid phase is usually a polar solvent and the mobile liquid phase is an essentially water-immiscible organic solvent. On the other hand, in reversed-phase chromatography (RPQ, the stationary liquid is usually a hydrophobic... 

The pore diameter of the silica support should lie between 100 and 500 A. With pore diameters below 100 A the values-of retarded peaks are much higher than for pore diameters of 100 A or larger. At pore diameters above 500 A, even at moderate eluent velocities, mechanical erosion of the stationary liquid phase dimi.nishes column lifetime. 

Such in situ coated columns with an equilibrium distribution of liquid phase have long operational lifetimes because the amount of liquid stationary phase remains constant in the column as long as its concentration in the mobile phase and the temperature remain constant. The column can be coated successively with different liquid phases after removing the stationary liquid phase with a suitable solvent and reactivating by treatment with dry eluent before recoating. 

The solute molecules can enter and leave the particles only by diffusion. The particles are porous, like sponges, and most of the absorbent surface area is given by the surface of the inner pores. In bLC most of the stationary liquid phase is also inside the particles. Ofily porous layer beads which have been used in the past and may find sonie applications in the future represent an exception. The diffusion through the particles takes some time. The average time necessary for a molecule to diffuse across a distance dp is... 

Pattern recognition can be applied for the determination of structural features of unknown (monofunctional) compounds (Huber and Reich ). The information about the chemical structure is contained in a multidimensional gas-liquid retention data/stationary liquid phases set. The linear learning machine method is applied in a two step classification procedure. After the determination of a correction term, the skeleton number, a classification step for the determination of the functional group is executed. It is remarkable that 10 stationary phases are sufficient for the classification. 

The first type consist of relatively short tubes, with an inner diameter of several millimetres, packed with an inert support on which the stationary liquid phase is held. The preferred sorbents are variously calcined, acid- or base-washed and silanized forms of diatomaceous earth (Chromosorb, Gas Chrom Q, Supelcoport)165,168,178,180-184,191,184,195,197,200,... 

Comparison of Release Data with Proposed Model. A summary of the cesium release data obtained with helium as carrier gas is shown in Figure 5, where the percentage of cesium remaining is plotted vs, heating time on semilogarithmic coordinates. In all of the experiments the flow rate was maintained at 300 cc./min. and the temperature at 730° = = 5°K. The geometric parameters are indicated on each curve. Since only about 5% of the sodium was vaporized in each experiment, the assumption of a stationary liquid-phase boundary is justified. 

The wall-coated column in Figure 24-2c features a 0.1- to 5-p.m-thick film of stationary liquid phase on the inner wall of the column. A support-coated column has solid particles coated with stationary liquid phase and attached to the inner wall. In the porous-layer column... 

Solid support coated with stationary liquid phase... 

Fused silica fiber with stationary liquid-phase coating... 

Weight of stationary liquid phase. WL. Weight of liquid phase in the column. 

The value for KD is high when most of the component is retained in the stationary (liquid) phase. The distribution constant is temperature dependent, so the column temperature must be stated when reporting KD data. Generally, the distribution constant is halved for each 30°C increase in temperature. 

Bj2 - vi )/RT, v = the partial molar volume of the sample at infinite dilution in the stationary (liquid) phase, (1) refers to the sample, (2) to the carrier gas, (3) to the stationary liquid, and is a function of the column inlet and outlet pressures, and pQ. 

Because of the similarities in the theory and practice of these two procedures, they will be considered together. Both are examples of partition chromatography. In paper chromatography, the cellulose support is extensively hydrated, so distribution of the solutes occurs between the immobilized water (stationary phase) and the mobile developing solvent. The initial stationary liquid phase in thin-layer chromatography (TLC) is the solvent used to prepare the thin layer of adsorbent. However, as developing solvent molecules move through the stationary phase, polar solvent molecules may bind to the immobilized support and become the stationary phase. 

Two types of columns are used. A packed column is one filled with inert, solid particles coated with a liquid stationary phase. Standard tubing is about 0.5 cm in diameter, with lengths ranging from 1 m to 20 m however, columns for large-scale preparative work may be up to 5 cm in diameter and several meters long. Commonly used solid supports are diatomaceous earth, Teflon powder, and glass beads. The stationary liquid must be chosen on the basis of the compounds to be analyzed. A more recently developed and more widely used type of column is the open-tubular or capillary column. This is prepared by coating the inner wall of the column with the stationary liquid phase. The inside diameter of a typical capillary tube is 0.25 mm, and... 

I Liquid-liquid partition chromatography, where the sample components are partitioned between a moving liquid phase and a stationary liquid phase deposited on an inert solid. The two solvent phases must be immiscible. The stationary phase may he a large molecule chemically bonded lo the surface of a solid (bonded liquid phase) lo prevent loss by solubility in the moving phase. This method can also be subdivided into normal-phase systems, in which Ihe moving phase is less polar than the stationary phase, and reverse-phase systems, in which it is more polar. 

An alternative and very direct approach is to separate the enantiomers on a column in which the stationary liquid phase is a chiral compound. The diastereomeric interaction is between A+ or A and the chiral liquid phase, and may be sufficiently different to permit separation of A+ from A. The ratio of the amounts of A+ and A corresponds to the enantiomeric purity. 

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