Mobile phase, interactions

Figure 7.3 The positions occupied by LC and GC in a generic Type I phase diagram representing the mobile phase. Note that the GC mobile phase is shown as being composed of 100% component a, but this makes no difference chemically because there are no solute-mobile-phase interactions in GC. Reproduced by permission of the American Chemical Society.

The separation of synthetic red pigments has been optimized for HPTLC separation. The structures of the pigments are listed in Table 3.1. Separations were carried out on silica HPTLC plates in presaturated chambers. Three initial mobile-phase systems were applied for the optimization A = n-butanol-formic acid (100+1) B = ethyl acetate C = THF-water (9+1). The optimal ratios of mobile phases were 5.0 A, 5.0 B and 9.0 for the prisma model and 5.0 A, 7.2 B and 10.3 C for the simplex model. The parameters of equations describing the linear and nonlinear dependence of the retention on the composition of the mobile phase are compiled in Table 3.2. It was concluded from the results that both the prisma model and the simplex method are suitable for the optimization of the separation of these red pigments. Multivariate regression analysis indicated that the components of the mobile phase interact with each other [79],... 

It should be noted here that in specifying the rules for the first probe (phenols), it became clear that rules for choosing the column and mobile phase interact significantly with detector rules. 0.1% acetic acid works well as a competing acid additive in terms of chromatography of the phenols. However, carboxylate ions are known to quench the fluorescence of phenols. Thus, if one were to use a fluorescence detector for trace phenol detection, an alternative competing acid, such as 0.1% phosphoric acid should be substituted. It was decided that mobile phase/detector interaction rules would be the first detector rules to be added to the knowledge base. 

Compounds most efficiently separated by LSC are non-ionic and relatively soluble in organic solvents (4). Because the solvent (mobile phase) interacts with the surface of the stationary phase, the separation process is Influenced by... 

Figure 3 shows the effect of temperature on the capacity factor of p-nltroanillne, from 0°C to 77°C. A mobile phase consisting of 10 methanol/water was employed. The retention at 0°C was 23.62 min. while at 77°C was 2.28, a ten fold decrease. This decrease in retention may be attributed to many factors such as increased solubility of the p-nitroaniline with increase in temperature, which results in less solute-stationary phase, and an increase in solute-mobile phase Interactions increase in mass transfer, and decrease in the pressure. Also the binding constant of any solute with cyclodextrin goes to zero at 80°C (11). 

One of the reviewers has pointed out that mobile-phase terms will have the same absolute significance, regardless of the magnitude of adsorbed-phase interactions. This is correct, in terms of their relative effects on separation. However, the observable effects of mobile-phase interactions on experimental A values will be more apparent when these are no longer masked by large adsorbed-phase interactions (which are only approximately describ-able by simple theories such as the one given in this chapter). 

The vertical lines through each point of Fig. 13 correspond to the experimental uncertainty in for an error of 0.005 unit in the associated e° value (a reasonable estimate of the precision of experimental ° values). In the case of the less polar C-solvents of Fig. 13a, all data points appear to fit the dashed curve [Eq. (40)] with acceptable precision. The fit for the more polar C-soIvents of Fig. 13b is poorer, and may reflect mobile-phase interactions as discussed in Section II,C. However, the overall trend in cb versus 0b is clearly that predicted by restricted-access delocalization. 

Normal-phase HPLC explores the differences in the strength of the polar interactions of the analytes in the mixture with the stationary phase. The stronger the analyte-stationary phase interaction, the longer the analyte retention. As with any liquid chromatography technique, NP HPLC separation is a competitive process. Analyte molecules compete with the mobile-phase molecules for the adsorption sites on the surface of the stationary phase. The stronger the mobile-phase interactions with the stationary phase, the lower the difference between the stationary-phase interactions and the analyte interactions, and thus the lower the analyte retention. 

Separation of solutes injected into the system arises from differential retention of the solutes by the stationary phase. The net retention of a particular solute depends upon all the solute-solute, solute-mobile phase, solute-stationary phase and stationary phase-mobile phase interactions that contribute to retention. The t3q3es of solute-stationary phase interactions involved in chromatographic retention include hydrogen bonding, van der Waal s forces, electrostatic forces or hydrophobic forces. 

Knowing that the solvent and the solute are in competition for active sites of the adsorbent, it is easily understood that the more the mobile phase interacts with the adsorbent the less a solute molecule is retained. Therefore, the major factor deter-... 

The chemical description of this interaction is still to be determined. It appears that there exists some threshold solvent power (defined either by the pure carbon dioxide density or the modifier identity and concentration in a modifier/carbon dioxide mixture) at which the solvent can begin to compete successfully with a particular stationary phase for a particular solute. Whether this involves a deactivation of active sites amenable to specific solute adsorption on the silica surface or a secondary solvent effect (43) where the mobile phase interacts with the solute as well as with the adsorption surface is unknown. 

The mobile phase is the solvent that moves the solute (analyte) through the column. In HPLC, the mobile phase interacts with both the solute and the stationary phase and has a powerful influence on solute retention and separation.13,7... 

Selectivity effects are determined by mobile phase interactions between the stationary phase and the solute. Those mobile phases containing proton donors will interact with basic solutes. Conversely, mobile phases containing proton donor acceptors will interact strongly with acidic solvents. Where excessive interactions occur between the solute and the mobile phase, peak tailing can result. However, the inclusion of a basic modifier (triethylamine) can overcome such strong interactions and thereby improve peak shape. 

Each enantiomer carried by the mobile phase interacts in a different manner with the stationary phase which contains an enantiomerically pure chiral element. There again, it is diastereomeric interactions that are translated into different elution rates through the column. In principle, whatever the detector (ultraviolet-visible spectrometer, refractometer, etc.) the response factor is identical for the two enantiomers being analysed. As a result, integration of the peak areas corresponding to each enantiomer leads to a measure of the ee by the simple relationship ee = (Si — S2)/(Si + S2) (Figure 2.66). 

Because liquid-liquid chromatography can be considered as an extension of gas-liquid clu-omatography, it warrants mention in this context. The two techniques differ only in the density of the respective eluants and in the degree of solute-mobile phase interactions. Unfortunately only activity-coefficient ratios can be determined directly, i.e. the ratio of activity coefficient of the solute at infinite dilution in the mobile phase to the activity coefficient of the solute in the stationary phase. Therefore, to obtain an activity coefficient at infinite dilution one of the coefficients must be determined by another method. In many cases, g.l.c. could supply this, thus making the two techniques complementary. The U.c. method is rapid and easy to operate and could make valuable contributions to this field. Locke has pioneered this work which he reviewed in 1969. ... 

Khaledi et al. [6, 7] were concerned with the similarities and differences in retention behavior between the mode of RPLC which employs micellar eluents and that with aqueous-organic solvents. These techniques share the basic components of an RPLC system, that is, a nonpolar stationary phase and a polar aqueous mobile phase. The hydrophobicity of solutes should play an important role in governing the retention in both systems, which is easily controlled by adjusting solute-mobile phase interactions. However, the differences in interaction mechanism can cause significant differences in retention behavior. 

In general, non-linear van t Hoff behavior may be indicative of a change in the mechanism of retention. Basically, any reversible process which alters the enthalpy and entropy of adsorption can, in principle, give rise to non-linear van t Hoff plots. Dissociative processes, such as ionisation, change in conformation, or changes in the extent to which the mobile phase interacts with either the analyte or stationary phase are examples of such reversible processes. In addition, the presence of multiple types of retention mechanisms or multiple types of binding sites may also lead to non-linear van t Hoff plots. 

MOBILE PHASE INTERACTION WITH STATIONARY PHASE... 

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