Chromatography multicomponent mixture

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 fac tors without the excessive dilutiou that would be obtained in elution techniques. 

Compared to the SMB system the annular chromatography allows the continuous separation of a multicomponent mixture as it is most often the case in biopharmaceutical separations. 

The mobile phase plays an important role in the separation of components. Often multicomponent mixtures are required to achieve the desired separations. Much work has been done on the TLC separation of, for example, amino acids. Numerous solvent systems have been developed for such purposes, and more than one solvent system is usually necessary before separation of all of the components is achieved. Two-dimensional chromatography is often required such a separation is shown in Fig.2.2. 

Notes. (1) Warming the suspended solid in the solvent may be necessary by removing the porridge to a suitable flask and heating under reflux. Care must be taken if such is the case to supervise this operation carefully as there may be considerable tendency towards bumping . It should also be borne in mind that this batch-extraction process uses open-type vessels and usually large volumes of solvents precautions must therefore be taken in relation to the possible fire and toxic hazards involved in the use of a particular solvent. (2) As a first step, this procedure would involve solvent extraction procedures to divide the multicomponent mixture into acidic, basic and neutral fractions (see above). Subsequently chromatography, fractional crystallisation, etc., would be employed as appropriate. 

Lor a particular analytical separation, each biosolute will have an optimal k] value for maximum resolution with a designated column, flow rate, and mobile phase composition. Similar criteria apply in preparative (overload) chromatography with multicomponent mixtures, where resolution is similarly enhanced following optimization of chromatographic selectivity and zone bandwidth. The conventional approach to process purification with low molecular weight solutes has frequently been based on linear scale-up of the performance of an analytical column system. In these cases, high-resolution separations can be achieved often without the burden of conformational or... 

The isolation and concentration of petroleum products can be performed in several ways. The most efficient method is passive adsorption. In this method, the sample along with a tube filled with Tenax TA adsorbent is placed in a thermostated (60-70 °C) tightly closed container, such as a glass jar, for over 10 h. Under these conditions, a balance between compounds present in the headspace of the sample and the sample adsorbed on the polymer adsorbent is established. Adsorbed compounds are subjected to thermodesorbtion then, the desorbed compounds together with the carrier gas are injected onto a GC column, where they are separated and then identified. This approach has enabled easy detection and identification of trace amounts of petroleum products. Headspace analysis with passive adsorption on Tenax TA is normally used for separation and concentration of analytes. Gas chromatography coupled with an autothermal desorber and a mass spectrometer (ATD-GC-MS) is the best technique for separation of multicomponent mixtures... 

The synthesis of a polymer by a free-radicjd or polycondensation mechanism usually leads to a product with a rather wide distribution of molecular weights. These polydisperse samples must be considered as multicomponent mixtures rather than as chemically pure substances. Using analytical methods like gel permeation chromatography (GPC) it is possible to analyze such mixtures. 

The identification of the chemical forms of an element has become an important and challenging research area in environmental and biomedical studies. Two complementary techniques are necessary for trace element speciation. One provides an efficient and reliable separation procedure, and the other provides adequate detection and quantitation [4]. In its various analytical manifestations, chromatography is a powerful tool for the separation of a vast variety of chemical species. Some popular chromatographic detectors, such flame ionization (FID) and thermal conductivity (TCD) detectors are bulk-property detectors, responding to changes produced by eluates in a characteristic mobile-phase physical property [5]. These detectors are effectively universal, but they provide little specific information about the nature of the separated chemical species. Atomic spectroscopy offers the possibility of selectively detecting a wide rang of metals and nonmetals. The use of detectors responsive only to selected elements in a multicomponent mixture drastically reduces the constraints placed on the separation step, as only those components in the mixture which contain the element of interest will be detected... 

Chapters 10 to 13 review the solutions of the equilibrium-dispersive model for a single component (Chapter 10), and multicomponent mixtures in elution (Chapter 11) and in displacement (Chapter 12) chromatography and discuss the problems of system peaks (Chapter 13). These solutions are of great practical importance because they provide realistic models of band profiles in practically all the applications of preparative chromatography. Mass transfer across the packing materials currently available (which are made of very fine particles) is fast. The contribution of mass transfer resistance to band broadening and smoothing is small compared to the effect of thermodynamics and can be properly accounted for by the use of an apparent dispersion coefficient independent of concentration (Chapter 10). 

All cases of practical importance in liquid chromatography deal with the separation of multicomponent feed mixtures. As shown in Chapter 2, the combination of the mass balance equations for the components of the feed, their isotherm equations, and a chromatography model that accounts for the kinetics of mass transfer between the two phases of the system permits the calculation of the individual band profiles of these compounds. To address this problem, we need first to understand, measure, and model the equilibrium isotherms of multicomponent mixtures. These equilibria are more complex than single-component ones, due to the competition between the different components for interaction with the stationary phase, a phenomenon that is imderstood but not yet predictable. We observe that the adsorption isotherms of the different compounds that are simultaneously present in a solution are almost always neither linear nor independent. In a finite-concentration solution, the amount of a component adsorbed at equilib-... 

Concentration pulse chromatography (also called elution on a plateau, step and pulse method, system peak method, or perturbation chromatography) is experimentally much simpler [100,103,104]. The same experimental procedure is used as for the determination of smgle-component isotherms. First, the column is equilibrated with a solution of the multicomponent mixture of interest in the mobile phase. When the eluent has reached the composition of the feed to the column, a small pulse of the pure mobile phase (vacancy) or of a solution having a composition different from that of the plateau concentration (see end of this section) is... 

Like Helfferich and Klein [9], Rhee et al. [10] studied the separation of multicomponent mixtures by displacement chromatography using the restrictive assumption of the validity of the Langmuir isotherm model and the ideal model. They used a different approach, based on the method of characteristics, and studied the interactions between concentration shocks and centered simple waves [15]. This approach is more directly suited to adsorption chromatography than the... 

In the case of a step input, the numerical solution of the system of Eqs. 16.30 and 16.31 has been discussed in the literature for multicomponent mixtures [16]. The numerical solution of Eqs. 16.30 and 16.31 without an axial dispersion term i.e., with Di = 0) has been described by Wang and Tien [17] and by Moon and Lee [18], in the case of a step input. These authors used a finite difference method. A solution of Eq. 16.31 with D, = 0, combined with a liquid film linear driving force model, has also been described for a step input [19,20]. The numerical solution of the same kinetic model (Eqs. 16.30 and 16.31) has been discussed by Phillips et al. [21] in the case of displacement chromatography, using a finite difference method, and by Golshan-Sliirazi et al. [22,23] in the case of overloaded elution and displacement, also using finite difference methods. 

Water is one of the most widely used solvents because of its availability, low cost, nontoxicity, and safety as well as its ability to dissolve a wide variety of substances. Sometimes when the solubility or any other property of water does not allow it to be used, a polar or a nonpolar organic solvent can be used. In certain applications, neat organic solvents fall short of the mark as far as their dissolving power or other properties are concerned. It is then necessary to use solvent mixtures. These mixtures can be binary (two components), tertiary (three components), or a multicomponent mixture. Many times, one of the components may be a solid. One very common example of this can be found in liquid chromatography where an electrolyte solution (buffer + salt) is used in many applications as a mobile phase. 

The separation of multicomponent mixtures into more than two products usually requires more than one separation device. Chromatography is one of the few separation techniques that can separate a multicomponent mixture into nearly pure components in a single device, generally a column packed with a suitable sorbent. The degree of separation depends upon the column length and the differences in component affinities for the sorbent used. 

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