Conventional RPLC

The elution strength of the organic solvent in MLC has been described according to Eq. 3. However, a simple logarithmic relationship (similar to that used in conventional RPLC) is customary ... 

Satisfactory results are obtained with compounds of different nature. Of particular interest is the case of basic compounds, such as phenethylamines and 3-blockers, which experience large efficiency enhancements in SDS systems.This makes the use of special columns less necessary. The surfactant layer adsorbed on the column prevents the interaction of basic compounds with free silanol groups, which accounts for the low efficiencies observed with conventional columns in aqueous-organic RPLC. For acidic compounds such as sulfonamides, the efficiencies are comparable with both MLC and conventional RPLC, but for low polar compounds such as steroids, efficiencies are comparably poorer in MLC. However, in this technique, low polar steroids are eluted at sufficiently short retention times via a small amount of... 

In conventional RPLC, a systematic decrease in selectivity usually occurs when the volume fraction of organic solvent is increased.In contrast, in the presence of micelles, the selectivity may increase, decrease, or remain unchanged with the addition of both surfactant and organic solvent. Although the elution strength increases with the concentration of both micelle and organic solvent, their effect on the selectivity can be quite different, even opposite. 

Finally, sample preparation is expedited due to the solubilization capability of micellar media, avoiding laborious steps to separate the matrix, previously performed to sample injection. All these features have allowed the development of multiple applications that are highly competitive against conventional RPLC. 

Ethane sulfonates may be formed from chloroacetanUide herbicides in soil if enzymatic activation proceeds via glutathione. As these compounds are more polar and, thus, more mobile in the soil-water system than the respective parent herbicide, ethane sulfonates are more frequently found in groundwater.The use of ESI in the negative ion mode is self-evident from the acidity of the sulfonate group, but as these ethane sulfonates are less polar than the aromatic sulfonates considered above, ion-pairing is not required. Instead, conventional RPLC with an acidified eluent may be used. ... 

True micellar phases (surfactant + water) can be considered mobile phases with low elution strength. If organic modifiers, mainly alcohols, are added in small proportions to the micellar phases, a significant enhancement of the efficiency as well as elution strength is observed. Chapter 7 is dedicated to this topic. When micellar solutions and/or hybrid micellar phases with alcohol additions are used as mobile phases, it is possible to accurately predict the retention of each compound in a mixture. This capability is more decisive than in conventional RPLC because the MLC peaks are broader. The elaborated equations for modeling and optimization purposes given in Chapter 8 are easily applied with the aid of the MICHROM software included with this book. Appendix I is given as an aid to run the software. 

The complexity of MLC is much greater dmn that of conventional RPLC with aqueous-organic solvents, because of Ihe number of possible interactions with both mobile and stationary phases (Fig. 5.1). Hie solutes in the mobile phase can interact electrostatically with the charged outer-layer of ionic micelles, and hydrophobically with their lipophilic interior. The steric factor can also be important. The modification of the stationary phase by adsorption of surfactant monomers, which creates a "micelle-like" surface, gives rise to similar interactions with the solutes. The combination of these interactions cannot be duplicated by any traditional pure or mixed solvent system. While micellar solutions will never totally replace traditional aqueous-organic eluents, they offer several interesting alternatives to separation work. 

When micelles are not present in the mobile phase, eq. 5.1 is reduced to the partitioning equation of conventional RPLC ... 

The addition of small percentages of 1-propanol to micellar mobile phases was first recommended by Dorsey et al. [13], to enhance the chromatographic efficiency and decrease the asymmetry of chromatographic peaks. Since then, several organic solvents have been studied as modifiers in MLC. Of these, short and medium chain alcohols (z. e., methanol, ethanol, propanol and butanol) have shown to be the most suitable. Less frequent has been the use of pentanol [14, 15], Only a few rqports have appeared on the MLC behavior of solutes in the presence of other organic solvents commonly employed in conventional RPLC, such as acetonitrile [13, 16, 17], and tetrahydrofiiran [18, 19]. Micellar mobile phases allow the use of organic solvents in aqueous solution at molar concentrations well above their normal solubility limit in water alone. For example, the water solubility of pentanol is ca. 0.30 M, whereas in 0.285 M SDS micellar medimn, it increases to ca. 0.94 M [17]. 

As a result of the increasing use of modifiers in MLC, the need for an adequate description of the retention in hybrid micellar mobile phases appeared. Khaledi et al. [7, 8] were the first which intended to model the retention of solutes in these systems. They assumed that the linear relationship between log k and the volume fraction of organic modifier, (p, followed in conventional RPLC over a small range of values of (p, was also valid in hybrid MLC at constant micelle concentration ... 

The linear relationship between k and nc observed with micellar mobile phases may also result from the energetics unique to the pseudo-phase environment. In conventional RPLC with aqueous-organic mobile phases, the linear dependence between log k and nc has been attributed to the direct proportionality between log k and the free energy of retention, which is in turn a linear combination of the free energy increments associated with the constituent parts of the molecule [8]. The predominant contribution to the firee energy derives fi omthe cavity formation within the mobile phase solvent structure that is required to accommodate the solute molecule transferred from the stationary phase ... 

As can be deduced from eq. 9.29, the linearity range of the plots increases when the micellar concentration in the mobile phase decreases. There is therefore an apparent parallelism with the fact that a decrease in organic modifier of aqueous-organic mobile phases, in conventional RPLC, results in better correlations of log k vs. log. ... 

The chromatographic retention of diuretics decreased as the concentration of SDS in mobile phase increased, which made retention times closer to each other. Among the micellar mobile phases, 0.03 M SDS gave the best correlations between retention factors and site of action. It is interesting to note that the correlations hold in spite of the largely diverse chemical structures of diuretics. The order of retention attained with other mobile phases in conventional RPLC systems was checked to be completely different from that of MLC, and a correlation with regard to physiological properties could not be drawn. 

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