Mechanism of retention

Chapter 2 Separations in High-Performance Liquid Chromatography 

The amount of carbon introduced into the stationary phase by the functional group is referred to as the carbon load, and it is measured as a weight percentage of the bulk silica packing. The carbon load is altered by 

A few polymeric reversed-phase stationary phases are available which provide the advantage that they can be operated over a wider pH range than the silica-based columns. Polymeric columns, however, tend to be less efficient than silica-based ones and are often less retentive. 

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Certain neutral technetium complexes can be used to image cerebral perfusion (Fig. 4). Those in Figure 4a and 4b have been approved for clinical use. Two other complexes (Fig. 4c and 4d) were tested in early clinical trials, but were not developed further. An effective cerebral perfusion agent must first cross the blood brain barrier and then be retained for the period necessary for image acquisition. Tc-bicisate is retained owing to a stereospecific hydrolysis in brain tissue of one of the ester groups to form the anionic complex TcO(ECD) , which does not cross the barrier. This mechanism of retention is termed metaboHc trapping. 

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... 

In the introduction to this chapter, MD-PC was defined as a procedure in which substances to be separated were subjected to at least two separation steps with different mechanisms of retention (5). Discussion of the basic potential modes of operation showed that because of the versatility which resulted from being able to combine mobile phases of different composition, more than two development steps can easily be realized by the use of "D techniques. 

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). 

Dorsey, J.G and DiU, K.A., The molecular mechanism of retention in reversed-phase liquid-chromatography, Chem. Rev., 89, 331, 1989. 

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. 

Mechanisms of retention of uranium during weathering include precipitation of new... 

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. 

Cheng, H.H. 1990. Organic residues in soils Mechanisms of retention and extractability. Intern. J. Environ. Anal. Chem. 39(2) 165-171. 

Besides pesticides, toxic organic substances are of great concern as we attempt to preserve the quality of our environment. Many of these substances have been deposited into aquatic and soil environments. In addition to understanding the equilibrium aspects of these pollutants in soils and sediments, it is imperative that there be an understanding of the rates and mechanisms of retention and mobility. Unfortunately, few of these studies have appeared in the scientific literature. This is most certainly an area of research in the soil and environmental sciences that needs extensive investigation. 

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. 

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