Retention mechanisms partition coefficients

In a sediment system, the hydrolysis rate constant of an organic contaminant is affected by its retention and release with the sohd phase. Wolfe (1989) proposed the hydrolysis mechanism shown in Fig. 13.4, where P is the organic compound, S is the sediment, P S is the compound in the sorbed phase, k and k" are the sorption and desorption rate constants, respectively, and k and k are the hydrolysis rate constants. In this proposed model, sorption of the compound to the sediment organic carbon is by a hydrophobic mechanism, described by a partition coefficient. The organic matrix can be a reactive or nonreactive sink, as a function of the hydrolytic process. Laboratory studies of kinetics (e.g., Macalady and Wolfe 1983, 1985 Burkhard and Guth 1981), using different organic compounds, show that hydrolysis is retarded in the sohd-associated phase, while alkaline and neutral hydrolysis is unaffected and acid hydrolysis is accelerated. 

For LC a similar relationship should apply if the retention mechanism shows the expected theoretical dependence on carbon number. The situation is more complex since the partition coefficient is a function of many intermolecular forces. Several papers have been published showing a homologous series retention like that described for GC.5 In principle then, the retention index concept should also apply in those cases. However, little interest has been shown in developing an index for LC, probably because the paraffins are not usually run by LC and the modes of analysis by LC are much more variable and complex, so that the data are not as widely usable. 

Mirrlees MS, Moulton SJ, Murphy CT et al. (1976) Direct measurement of octanol-water partition coefficients by high-pressure liquid chromatography. J Med Chem 19 615-619 Pagliara A, Khamis E, Thrinh A et al. (1995) Structural properties governing retention mechanisms on RP-HPLC stationary phases used for lipophilicity measurements. J Liquid Chromatography 18 1721-1745 Slater B, McCormack A, Avdeef A et al. (1994) pH-Metric log P. 4. Comparison of Partition Coefficients Determined by Shake-Flask, HPLC and Potentiometric Methods. J Phar-maceut Sci 83 1280-1283... 

Partitioning is the first and probably the simplest model of the retention mechanism. It assumes the existence of two different phases (mobile and stationary) and instant equilibrium of the analyte partitioning between these phases. Simple phenomenological interpretation of the dynamic partitioning process was also introduced at about the same time. Probably, the most consistent and understandable description of this theory is given by C. Cramers, A. Keulemans, and H. McNair in 1961 in their chapter Techniques of Gas Chromatography [12]. The analyte partition coefficient is defined as... 

The basis of all these theories is the assumption of the energetic additivity of interactions of analyte structural fragments with the mobile phase and the stationary phase, and the assumption of a single-process partitioning-type HPLC retention mechanism. These assumptions allow mathematical representation of the logarithm of retention factor as a linear function of most continuous parameters (see Chapter 2). Unfortunately, these coefficients are mainly empirical, and usually proper description of the analyte retention behavior is acceptable only if the coefficients are obtained for structurally similar components on the same column and employing the same mobile phase. 

Sorption of Cu(tfac)2 on a column depends on the amount of the compound injected, the content of the liquid phase in the bed, the nature of the support and temperature. Substantial sorption of Cu(tfac)2 by glass tubing and glass-wool plugs was observed. It was also shown that sorption of the copper chelate by the bed is partialy reversible . The retention data for Cr(dik)3, Co(dik)3 and Al(dik)3 complexes were measured at various temperatures and various flow rates. The results enable one to select conditions for the GC separation of Cr, Al and Co S-diketonates. Retention of tfac and hfac of various metals on various supports were also studied and were widely used for the determination of the metals. Both adsorption and partition coefficients were found to be functions of the average thickness of the film of the stationary phase . Specific retention volumes, adsorption isotherms, molar heats and entropy of solution were determined from the GC data . The retention of metal chelates on various stationary phases is mainly due to adsorption at the gas-liquid interface. However, the classical equation which describes the retention when mixed mechanisms occur is inappropriate to represent the behavior of such systems. This failure occurs because both adsorption and partition coefficients are functions of the average thickness of the film of the stationary phase. It was pointed out that the main problem is lack of stability under GC conditions. Dissociation of the chelates results in a smaller peak and a build-up of reactive metal ions. An improvement of the method could be achieved by addition of tfaH to the carrier gas of the GC equipped with aTCD" orFID" . ... 

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. 

The retention mechanism is very simple. The only physicochemical parameter responsible for solute retention is the liquid-liquid partition coefficient, P. The retention volume, is simply... 

Supports for SEC of proteins are designed to be neutral and very hydrophilic to avoid disruption of protein structure and interaction of the solutes with the support by ionic or hydrophobic mechanisms. The base matrix can be either silica or polymer efforts are made to totally mask its properties with a carbohydratelike stationary phase. The pore structure is critical to successful SEC. Not only must the total pore volume (F,) be adequate for separation, the pore diameter must be consistent and nearly homogeneous for attainment of maximum resolution between molecules with relatively small differences in molecular size (radius of gyration or molecular weight). A twofold difference in size is usually required for separation by SEC. Pore homogeneity can be assessed from the slope of the calibration curve of the logarithm of the molecular weight versus the retention time or the partition coefficient (Kd) = (F - Fq)/F , where F is... 

It should be pointed out that column selectivity should frequently be understood in a very broad sense. Reliable predictions are frequently difficult. Different temperature dependence of partition coefficients for various solutes may further modify such predictions. Mixed retention mechanisms are also observed due to interactions between the solid supports and chromatographed molecules. 

OV-275. For solutes retained solely by gas-liquid partitioning (e.g. nitromethane, dioxane and ethanol) on Carbowax 20M the plots have a zero slope. The n-alkanes are retained by a mixed retention mechanism on Carbowax 20M as indicated by the slope. The relatively large intercept is an indication that gas-liquid partitioning makes a significant contribution to the retention mechanism. Interfacial adsorption is important for all compounds on OV-275 and is dominant for the n-alkanes, which have a near zero intercept, indicating that gas-liquid partitioning is of minor importance to their retention. The lack of a reliable method to estimate the surface area of the liquid phase prevents Eq. (2.3) from being used to determine the gas-liquid adsorption coefficient. 

There are certain conditions that must be fulfilled if Eqs. (2.2), (2.3) and (2.4) are to be used to calculate partition coefficients. The basic assumption is that the individual retention mechanisms are independent and additive. This will be true for conditions where the infinite dilution and zero surface coverage approximations apply or, alternatively, at a constant concentration with respect to the ratio of sample size to amount of liquid phase. The infinite dilution and zero surface coverage approximations will apply to small samples where the linearity of the various adsorption and partition isotherms is unperturbed and solute-solute interactions are negligible. The constancy of the solute retention volume with variation of the sample size for low sample amounts and the propagation of symmetrical peaks is a reasonable indication that the above conditions have been met. For asymmetric peaks, however, the constant concentration method must be employed if reliable gas-liquid partition coefficients are to be obtained [191]. It is difficult to state absolutely the conditions for which contributions to retention from the structured liquid phase layer can be neglected. This will occur for some minimum phase loading that depends on the support surface area, the liquid phase... 

In terms of the isotherm formalism, the situation with regard to inert as well as interactive solutes with interactive solvents amounts to that with inert and interactive solutes with non-interactive solvents. That is, since equation 22 appears to provide a description of the partition coefficients of systems of the latter to within experimental error, empirical fitting of equation 13 would seem to be superfluous. However, it may not always be the case that solute and/or solvent positive or negative deviations from Raoult s law can be defined separately. This is particularly true of liquid chromatography, wherein the mechanisms of solute retention are at best only poorly defined. Thus, and for the purpose of achieving analytical separations, the isotherm relation may in fact prove to be advantageous insofar as it is a well-defined continuous function and, hence, can be incorporated immediately into the Laub-Purnell window-diagram scheme of optimization. We consider the matter in further detail in the Section that follows. 

The implications on selectivity ofthe direct transfer furnishes a new evidence of the solubility limit theory. The retention mechanism of several hydrophobic compounds (i.e., benzene derivatives, polycyclic aromatic hydrocarbons (P AHs), and dihydropyridines) was studied in SDS and CTAB micellar systems, by comparing experimental selectivity coefficients with those theoretically calculated assuming a direct transfer mechanism [7,8]. A mathematical expression was derived by using the three-partition equilibria theory, which explains the tendency of selectivity coefficients to the ratio of P s coefficients of the solutes, when the concentration of surfactant increases. Expressing the equation that relates the retention with the concentration of micelles as a function of Pms and Pwm-... 

Figure 7.9 shows the variation of the theoretical (eq. 7.5) and experimental selectivity coefficients (calculated from the retention factors), as a function of micelle concentration, in SDS-5% 1-propanol and CTAB-5% 1 -butanol mobile phases, for three pairs of solutes pyrene-acenaphthene which are both very hydrophobic and for which a direct transfer mechanism can be assumed for any surfactant concentration, pyrene-toluene in which only for pyrene can a direct transfer mechanism be assumed for all surfactant concentrations, and pyrene-benzamide in which benzamide does not experience a direct transfer, except at very high surfactant concentration. When both solutes experience direct transfer, the experimental and theoretical selectivity coefficients are veiy similar (Fig. 7.9 a, d). It is possible to predict the selectivity coefficient from P s partition coefficients. In contrast, when one of the two solutes does not experience a direct transfer mechanism, the theoretical and experimental selectivity are different but this difference decreases under the conditions in which the direct transfer is favored (Fig. 7.9 b, c, e, f). 

As already mentioned in Sect. 4.1, the capacity ratio can be obtained from the retention time of a sample i according to Equation (6). Since k[ is proportional to the partition coefficient K[, the thermodynamic description of <[ can also be used for k[ with respect to its dependences on temperature T, pressure p, or density p. A weakness of all these approaches, however, is the incompleteness of information concerning the retention mechanisms in the stationary phase. For the assumption of an adsorption mechanism see Ref. [7]. 

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