Surface specific retention factors

Below we compare the retention in the form of traditional retention factors (k ) and surface-specific retention factors k of six different analytes on four C18-type columns from four different vendors (Figure 3-31). 

TABLE 3-4. Relative Standard Deviation of Conventional and Surface-Specific Retention Factors on Different Columns... 

The specific retention of sample molecules to be separated in a straight phase or adsorption chromatographic system is mainly determined by two factors their interactions with polar surface centers of the solid stationary phase, and by the different sample solubility in the rather nonpolar mobile phase. The most important interactions... 

The carbon content of a stationary phase is measured by an elemental analyser, as a weight balance before and after heating at 800 °C. Particle size, pore size, and surface area are measured by specific instruments, such as a particle size analyser, nitrogen adsorption porosimeter, and mercury depression analyser, respectively. The precision of the measurement of carbon content is high however, that of the other measurements is relatively poor. Therefore, it is difficult to relate the surface area of different silica gels to analyte retention factors. 

In normal-phase or adsorption chromatography, the chromatographic properties are functions of the specific area. Retention factors increase with the specific surface area. The parameter of the specific surface area of the packing could be of great importance when selectivity and efficiency have to be improved. One very short column packed with a silica of high specific surface area will yield the same results as a long column packed with a silica of low specific surface area (18). 

Mobile phase parameters that have to be optimized include the salt type, concentration, gradient shape, pH, temperature, and possibly the addition of a surfactant or organic solvent [368-372]. The change in free energy on protein binding to the stationary phase is determined mainly by the contact surface area between the protein and stationary phase and by the salt type determined by its ability to increase the surface tension of aqueous solutions. Solvophobic theory predicts that in the absence of specific salt-protein interactions and at sufficient ionic strength the logarithm of the retention factor is linearly dependent on the surface tension of the mobile phase, which in turn, is a linear function of the salt concentration Eq. (4.13)... 

Here, k is the retention factor in a binary mobile phase containing solvents A and B, which can be calculated from the retention volume of the analyte, Vr, and the column hold-up volume, Vm (determined as the elution volume of a nonretained compound, such as trichloroethylene), k = Vr/Vm — 1, fea is k in pure weak solvent. A, o is the activity of the adsorbent in the column. As is the specific surface of the adsorbent, and Sa and Sab are the solvent strength (polarity) parameters of the weak (nonpolar) solvent and of the mixed binary mobile phase, respectively. 

This has been repeatedly confirmed in several studies see, for example. Figure 4.4. Here, the retention (bars) and separation factors (lines) of tricyclic antidepressants in acidic acetonitrile/phosphate buffer are shown. In the case of quite different retention factors depending on the hydrophobic character of the phases (differently strong interactions), apart from a few exceptions very similar separation factors result. Although the analytes are retained for different lengths of time on the stationary phases, they show a similar selectivity behavior. This is observed especially with neutral or - by means of the pH-neutralized molecules. In this case, mainly hydrophobic interactions between the analytes and the surface of the material dominate. These are not especially specific the individual differences of the phases with respect to selectivity do not come into effect. 

The specific retention values are calculated, based in each case on the total mass of reservoir rock, the pore volume and the specific rock surface (Table 2). In this context it must be emphasized that the most substantial of the three correction factors is "polymer loss in immobile fluids". In our calculation a loss factor of 33 per cent of the total injected amount of polymer was assumed. The corrected values of 8 and 14 g/g for both blocks are lower than most literature data for polyacrylamide adsorption, which are mostly based on laboratory tests though (10, 11). Even uncorrected values are relatively low. 

Specific management practices influence triazine runoff and leaching, including fertilizer type, tillage crop residues, and previous crop history, as well as triazine application, formulation, and placement (Baker and Mickelson, 1994). Tillage systems affect various soil properties, such as soil moisture, temperature, pH, organic matter, water flow, and microbial populations, especially at and near the soil surface. These factors can affect transformation, retention, and transport of herbicides in soil. Interactions of and compensations between these processes can influence our prediction of triazine transport in soil. Therefore, triazine movement is usually studied under one management practice at a time. 

Conformational factors play a role with large macromolecules. Changes in the conformation of macromolecules may affect the local environment and, thus, the retention energy of specific coordinating groups. Retention of metals at the surface or inside particles may be influenced by similar effects. 

In the literature, one can hnd theoretical and practical studies relating to heat transfer conditions in scraped-surface heat exchangers (19, 54), which cover factors such as specific weight, specific heat, latent heat of crystallization, dry matter content, retention time, and overall heat transfer conditions. 

Engelhardt and Mliller reported on the differences in the physical properties, such as specific surface area, specific pore volume and average pore diameter - and on the different amounts of stationary and mobile phase per unit column volume for various commercially available silica gels. If the retention for various solutes were normalized for these factors, distinct selectivities were still noticed. This could be explained by differences in the surface pH of the silicas. Irregular ones were usually neutral or weakly acidic, whereas the spherical ones were either acidic (pH ca.4) or basic (pH ca.9) (see Table 1.4).To obtain the required and optimum selectivity, the pH of silica gel can easily be adjusted. For basic compounds more symmetrical peak shapes were obtained on silica with a basic character. 

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