Selectivity of phase systems

Some steps towards the creation of such an expert system for the selection of phase systems in chromatography have recently been taken. Karnicky et al. have reported on attempts to build such a system for LC [204]. The selection of the most appropriate phase system for a chromatographic separation is a complicated matter. The choice will be largely determined by the characteristics of the sample (see Figure 2.1) and by the analytical... 

Figure 3.7 Illustration of the selection of phase systems for LC according to eqn.(3.30). A solute with a polarity of 12.5 (middle scale) can be eluted from silica (Ss= 16 top scale) with a non-polar mobile phase (Sm=9 bottom scale) or with a polar solvent in a reversed phase system. The shaded areas indicate the latitude with respect to the selection of the mobile phase. Figure taken from ref. [311]. Reprinted with permission.

Fig. 4.10 Selection of phase systems dependent on the separation problem.

The impact of the value of (k 2) is obviously linear, but controls the overall analysis time. It emphasizes the importance of carefully selecting the phase system not only to make the value of (a) as large as possible but also to keep (k 2) to a minimum. 

The mobile phase used was 2% tetrahydrofuran (THF) in a mixture of 30% methanol and 70% water. This is an interesting example of the use of a small quantity of THF to increase the dispersive character of the mobile phase while maintaining the high polarity of the methanol water mixture. To achieve the same increase in dispersive interactions by increasing the methanol content would probably require as much as 40-45% methanol. At this concentration the polarity of the mobile phase would have drastically changed and the selectivity of the system for the more polar materials probably lost. It is also seen that the overall sensitivity of the system is high, components being present at a level of about 100 ng. 

The most common technique for the radiochemical determination of complexing constants utilizes partition methods which are based on reactions between two phases under static or dynamic conditions (chromatography). Partition methods offer the advantages of simplicity and rapidity and are amenable to a broad selection of phase compositions and arrangements. One of the most reliable partition methods, especially useful for measuring 0 , is solvent extraction. It is best applied to systems which exhibit compound formation (11). 

Even with so large a simplification, however, the convergence of Eq. (3.32) for a realistically sized chemical system and a random selection of phase points is too slow to be useful. What is needed is a scheme to select important phase points in a biased fashion. 

This situation is fundamentally different from the one in GC, where interactions with the stationary phase and the vapour pressure of the pure solute were the relevant factors (see section 3.1.1). In LC, both the interactions in the mobile phase and in the stationary phase can be influenced in order to optimize the selectivity of the system, and neither is beyond control in the sense that vapour pressure is in GC. 

The main parameters in LLC are the polarities of the mobile and the stationary phase. Increasing the polarity difference between the phases enhances both the selectivity of the system (figure 3.7) and the stability, due to a reduced mutual solubility of the phases. 

In many respects programmed analysis does not differ from chromatography under constant conditions. Retention is still determined by the distribution of solute molecules over the two chromatographic phases and the selectivity of the system is still determined by differences between the distribution coefficients of the solutes. However, if the operation conditions are changed during the elution, then the distribution coefficients may change with time, thus affecting both retention and selectivity. 

The key to the success of the oxidation examples cited above is the ability of the catalysts used to exert proper kinetic control on the possible side reactions. Without it, thermodynamically favorable but undesired products such as CO2 and H2O are made instead. Controlling oxidation kinetics to stop at the desired oxygenated products is quite difficult, and has yet to be solved for many other systems. For instance, although many attempts have been made to develop a commercial process for the oxidation of propylene to propylene oxide, both the activity and the selectivity of the systems proposed to date, mostly based on silver catalysts, are still too low to be of industrial interest " propylene oxide is presently manufactured by processes based on chlorohydrin or hydrogen peroxide instead. In spite of these difficulties, though, recent advances in selective liquid phase oxidation of fine chemicals on supported metal catalysts have shown some promise, offering high yields (close to 100%) under mild reaction conditions." ... 

Equation (1-7) shows that in an ideal case the selectivity of the system is only dependent on the difference in the analytes interaction with the stationary phase. It is important to note that the energetic term responsible for the eluent interactions was canceled out, and this means that the eluent type and the eluent composition in an ideal case does not have any influence on the separation selectivity. In a real situation, eluent type and composition may influence the analyte ionization, solvation, and other secondary equilibria effects that will have effect on the selectivity, but this is only secondary effect. 

The reactivity of oxide supported metals has received considerable attention because of the importance of such systems in heterogeneous catalysis. The morphology (structure and size) of the supported particle and its stability, the interaction of the particle with the support, and the crossover of adsorbed reactants, products and intermediates between the metal and oxide phases are all important in determining the overall activity and selectivity of the system. Because of the relative insensitivity of an optical technique such as IR to pressure above the catalysts, and the flexibility of transmission and diffuse reflection measurement techniques, vibrational spectroscopy has provides a considerable amount of information on high area (powder) oxide supported metal surfaces. Particularly remarkable was the pioneering work of Eichens and Pliskin [84] in which adsorbed CO was characterised by IR spectroscopy on... 

As the solvents used will affect both the capacity factors and the selectivity of the system, the mobile phase is selected on its ability to elute the solutes in an acceptably short time with a reasonable resolution between each one. The choice of mobile phase components may also be influenced by the desire to suppress the ionisation of one or more of the analytes or to reduce peak tailing. These considerations, whilst very important are generally viewed separately from the factors which affect the k and a values. 

Third, the stationary phase should permit separate elution of the substances into the mobile phase and the selectivity toward samples of interest has to be sufficient to lead to separations with good resolution. The selectivity of solvent systems can be estimated by determination of the partition coefficients for each substance. The batch partition coefficients D are calculated as the ratio of the component concentration in the organic phase to that in the aqueous phase. The dynamic partition coefficients of compounds are determined from an experimental elution curve [7]. Several solvent systems for organic separation were investigated [4-8]. The most efficient evolution usually occurs when the value of the partition coefficient is equal 1. However, in some CCC schemes, the best results are obtained with lower partition coefficient values ofO.3-0.5 [1,4]. 

The choice of the organic phase affects the selectivity of the system. 

M perchloric acid - 0.8 M sodium perchlorate (ca. 35% of the total weight of the silica gel used) (Fig.5.9). The mobile phase, n-butanol - dichloromethane - n-hexane (1 7 2), was saturated with the stationary phase. A precolumn was used to improve the equilibration of the mobile phase. n-Butanol was found to improve the peak symmetry it lowered, however, the selectivity of the separation of related compounds. Dichloromethane determined the selectivity of the system and n-hexane was found to be inert and could be used to decrease the polarity of the mobile phase and - thus - increase the retention times. 

Wittwer investigated the influence of the volatilization of amines in the mobile phase by testing the same solvent system, containing amnonia in various concentrations (without changing the water content of the mobile phase) in combination with a silica gel column. For the compounds tested, common adulterants or impurities of illicit heroin samples, only a few changes in the elution order were observed, particularly for the early eluting compounds, and furthermore an increase of retention time was observed upon decreasing ammonia concentration (Table 7.11). However, the relative retention varied little for most test compounds. The water content of the mobile phase was found to play an important role in the selectivity of the system. Retention times were reduced considerably on increase of the water content of the mobile phase but some compounds were more affected than others. Therefore, the water content of the mobile phase should be controlled... 

Szepesi et al. reported an ion-pair separation of eburnane alkaloids on a chemically bonded cyanopropyl stationary phase. As counter-ion, di-(2-ethyl hexyl)phosphoric acid or (+)-10-camphorsulfonic acid were used in a mobile phase consisting of hexane - chloroform -acetonitrile mixtures (Table 8.8, 8.9). Because of the poor solubility of the latter pairing ion, diethylamine (Table 8.9) was added to the mobile phase. Addition of diethylamine considerably reduced the k1 of the alkaloids, due to suppression of the ionization of the alkaloids. However, due to the strong acidic character of the pairing ion, ion-pairs were still formed under these conditions. The camphorsulfonic acid containing mobile phases were found to be very useful for the separation of optical isomers (Table 8.10, 8.11, Fig.8.8) 6. It was also found that the selectivity of the system could be altered by choosing different medium-polarity solvents (moderator solvents) as dioxane, chloroform or tetrahydrofuran. The polar component of the solvent system affected peak shape. Based on these observations, a method was developed to analyze the optical purity of vincamine and vinpocetine. For the ana-... 

An old alchemist maxim, similia similibus solvuntur" ( like dissolves like"), is the oldest rule for selecting suitable solvents, meaning that the nature of the solute determines the nature of the solvent. Due to the classification of phase systems in Section 4.2.2, organic and aqueous solvents are distinguished. Non-polar to medium-polar substances show best solubility in typical organic solvents while medium-polar to polar substances show best solubility in aqueous solvents. 

Obviously, the variances V and hence the level of CV entanglement depend on all the parameters of the system including the phases. The minimal possible level of V is realized for an appropriate selection of phases 9t, namely for 9 + 2 = — arg (ai a2) — Further, in most cases we assume that this... 

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