The Mechanism of Retention

Molecular Interactions, the Thermodynamics of Distribution, the Plate Theory and Extensions of the Plate 

Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 

There are two fundamental chromatography theories that deal with solute retention and solute dispersion and these are the Plate Theory and the Rate Theory, respectively. It is essential to be familiar with both these theories in order to understand the chromatographic process, the function of the column, and column design. The first effective theory to be developed was the plate theory, which revealed those factors that controlled chromatographic retention and allowed the 

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The standard free energy can be divided up in two ways to explain the mechanism of retention. First, the portions of free energy can be allotted to specific types of molecular interaction that can occur between the solute molecules and the two phases. This approach will be considered later after the subject of molecular interactions has been discussed. The second requires that the molecule is divided into different parts and each part allotted a portion of the standard free energy. With this approach, the contributions made by different parts of the solvent molecule to retention can often be explained. This concept was suggested by Martin [4] many years ago, and can be used to relate molecular structure to solute retention. Initially, it is necessary to choose a molecular group that would be fairly ubiquitous and that could be used as the first building block to develop the correlation. The methylene group (CH2) is the... 

Enantioresolution in capillary electrophoresis (CE) is typically achieved with the help of chiral additives dissolved in the background electrolyte. A number of low as well as high molecular weight compounds such as proteins, antibiotics, crown ethers, and cyclodextrins have already been tested and optimized. Since the mechanism of retention and resolution remains ambiguous, the selection of an additive best suited for the specific separation relies on the one-at-a-time testing of each individual compound, a tedious process at best. Obviously, the use of a mixed library of chiral additives combined with an efficient deconvolution strategy has the potential to accelerate this selection. 

Naphthalenedisulfonate-acetonitrile as the only mobile phase with a silica column coated with a crosslinked aminofluorocarbon polymer has proven to be an effective combination for the separation of aliphatic anionic surfactants. Indirect conductivity and photometric detection modes are used to monitor these analytes. The retention of these surfactants is found to depend on both the ionic strength and the organic solvent content of the mobile phase. The mechanism of retention is considered to be a combination of both reverse phase and ion exchange processes. Selective separation of both alkanesulfonates and... 

Although SPE can be done in a batch equilibration similar to that used in LLE, it is much more common to use a small tube (minicolumn) or cartridge packed with the solid particles. SPE is often referred to as LSE, bonded phase or sorbent extraction SPE is a refinement of open-column chromatography. The mechanisms of retention include reversed phase, normal phase, and ion exchange. 

Principles and Characteristics Liquid chromatography is the generic name used to describe any chromatographic procedure in which the mobile phase is a liquid. It may be classified according to the mechanism of retention in adsorption, partition, size-exclusion, affinity and ion-exchange (Scheme 4.4). These mechanisms form the basis for the chromatographic modes of... 

The complex 99mTc(V)-DMSA (64) has been used for a long time for the imaging of renal blood flow and morphology of the kidneys. The exact composition and structure of the agent is still unknown, as is the mechanism of retention. A structure of 64 (where R = COOH) has been proposed with three possible conformations, syn-endo, syn-exo, and anti, of the carboxylic acid groups with respect to the Tc = 0 core (280). 

High shear forces are prevelant in the approach flow system to the paper machine (i.e. as the fibre suspension approaches the point of deposition on the wire), and these have a large impact upon the efficiency of retention aids (Figure 7.8). A study of the effect of shear can often be helpful in establishing the mechanism of retention. Bridging flocculation is irreversibly sensitive to shear (i.e. when the shear forces are removed the suspension does not reflocculate) whereas charge neutralisation is reversibly sensitive to shear. 

As in the case of rosin sizing, the first step is to retain the emulsified size particle in the wet web. The mechanism of retention is probably by heterocoagulation of the cationic size particles to the negatively charged fibre surface. The charge characteristics of the stabilising polymer become important as demonstrated by the effect of pH on the retention of AKD emulsion particles stabilised with a tertiary cationic starch (Figure 7.17). 

Wheeler, J.F., Beck, T.L., Klatte, S.J., Cole, L.A., and Dorsey, J.G, Phase-transitions of reversed-phase stationary phases—cause and effects in the mechanism of retention, J. Chromatogr. A, 656, 317, 1993. 

While the mechanism of retention for various compounds can be only partially understood, the application of the technique provides much needed information for characterization of different types of materials. 

A second important conclusion from this analysis is that data obtained in a study on the dependence of retention factor on the hetaeron concentration alone do not suffice to ascertain the mechanism of the particular process that governs retention. It is so because for all the four mechanisms I, II, Ilia, and Illb, which were discussed above, the dependence of retention factor on hetaeron concentration in the eluent. is identical. The problems arising from this and the need for extrachromatographic data in attempting to establish the mechanism of retention are discussed further in Section VI,C. 

In the early years of GC, more consideration was given to partition (GLC) than to adsorption (GSC) systems. For GLC, the mechanism of retention was well understood, all of the mathematics were derived, and the chromatographic peak shapes were symmetrical. At that time, GSC had been utilized only for the separation of permanent gases. In recent years much has been accomplished in the determination of thermodynamic parameters in GSC separations. Part of the reason for the upsurge of interest was due to the desire to predict sample separations at any temperature, since most GSC data was reported at only one temperature. 

Carbon-based material on a silica template has been pioneered by Knox (34). It can be used at any pH. However, the mechanism of retention on this support is quite different from that for the average alkyl-bonded silica (35). Further information on reversed-phase retention can be found in Ref. 36. 

There are two ways to classify liquid chromatographic methods. The first and more common classification is based on the mechanism of retention, and from this the chromatographic modes discussed in Chapter 2 are derived. For example, the normal-phase mode can be performed by taking advantage of either the adsorption mechanism or the partition mechanism. The gel-filtration mode is performed using the mechanism of size exclusion. The second classification discussed below is based on the separation principle and is found mostly in the literature published before the 1990s. 

Liquid chromatographic methods may be classified according to either the mechanism by which analytes are retained on the column or the mechanism by which they are removed from it. The mechanism of retention classification is the most popular scheme, and five major retention mechanisms have been identified ... 

The mechanism of retention on chiral phases that is based on multiple hydrogen bonding formation involves the formation of base pairs and triple hydrogen bonds between the solutes and the chiral stationary phase 95 Fundamental work in this area has been done by Hara and Dobashi,96 97 using amino acid amide and tartaric acid amide phases. In addition, N,N -2,6-pyridinediyl bis(alkanamides) chemically bonded to silica gel have been described for the resolution of barbiturates 95... 

Figure 3.28 reduces to the simple mechanism of figure 3.27 if both Kx and K Y are very small. If Kx is small (i.e. the solute molecule is mainly in the mobile phase) but K Y is large (the pairing ion is mainly absorbed into the stationary phase), then the mechanism of retention in IPC becomes similar to that of IEC. Typical ion-pairing as well as typical ion-exchange mechanisms may play a role in practical IPC systems. 

The capacity factor of the solute varies with the composition of the mobile phase. This aspect is related to the mechanism of retention (chapter 3). 

The shape of the gradient program should be adapted to the mechanism of retention (i.e. to the particular form of LC) in order to achieve an LSS gradient (eqn.5.5). For... 

The empirical physicochemical parameters have a good informative value for determining the mechanism of retention operating in a given chromatographic sy.stem. There are exhaustive compilations of such parameters like >/-octanol-water partition coefficients [45,46] or the LSER-based analyte parameters [47,48]. The problem is. however, that there is a lack of such descriptors for many analytes of interest in actual QSRR studies. 

Turowski et al. [116] using PCA classified typical commercially available and several newly prepared stationary phases with regard to the mechanism of retention of nucleosides and cyclic nucleotides. Structural features of the analytes were identified which most strongly affected retention on individual stationaiy phase materials. 

Study of the mechanism of adsorption in TLC is more difficult than in column hquid chromatography. The nonlinear isotherm model in TLC can be designed in a qualitative way only, after investigation of chromatographic band shape and of the concentration distribution within this band phenomena characteristic of TLC band formation can also have a major effect on the mechanism of retention. 

Broadening of Chromatographic Bands as a Resuit of the Mechanism of Retention... 

The semiempirical model of adsorption chromatography, analogous to that of Martin and Synge, was established only in the late 1960s by Snyder [3] and Soczewinski [4] independently, and it is often referred to as the displacement model of solute retention. The crucial assumption of this model is that the mechanism of retention consists in competition among the solute and solvent molecules for the active sites of the adsorbent and, hence, in a virtually... 

The slope of a plot of the log of the retention time versus the inverse of the temperature is proportional to the enthalpy of retention for the probe molecule on the coal. This is a thermodynamic measure of the strength of the interaction between the probe molecule and the coal. The temperatures at which major changes in slope are observed represent the points where the mechanism of retention has changed, indicating that a significant change in the chemical or physical structure of the coal has occurred. 

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