Micellar mobile phase chromatographic

The major contributions which result in the reduced chromatographic efficiency have been ascribed to slow mass transfer principally due to poor wetting of the surfactant modified stationary phase (109), poor mass transfer between the micelle and stationary phase (113), and poor mass transfer in the stationary phase (100,106). In some cases, the use of small amounts of alcohol additives (MeOH, n-PrOH) and operation at elevated temperature (MO0 C) result in chromatographic efficiencies comparable to that seen in traditional LC using hydro-organic mobile phases (109,113,154,206). In our own work, we have found n-pentanol to be superior to n-propanol in this regard (refer to Table IX) (112). Further work is clearly needed in this efficiency area in order to clarify the exact reason(s) for the reduction in efficiency. It appears that a combination of factors can contribute to this effect with the dominant efficiency reduction mode dependent upon the nature of the solute, micellar mobile phase, and stationary phase packing material employed (100,112,135). 

Micellar mobile phases have been utilized in numerous recent paper, thin-layer, and high-performance liquid chromatographic separations. Table XI summarizes the separations performed to date. 

It was then recognized early in the development of the technique that there were possibilities for dramatic differences in the chromatographic performance of hydroorganic and micellar mobile phases. Since that review appeared there have in fact been several examples of micellar mobile phases providing solutions to inherent limitations of hydroorganic mobile phases allowing chromatographic capabilities that are not possible with traditional mobile phases. Yet in spite of these advances it was said in 1986 ( 3 ... 

A major drawback in the early reports of micellar chromatography was a serious loss of efficiency when compared to traditional hydroorganic mobile phases. If micellar mobile phases are ever to be widely accepted as a viable chromatographic technique, the efficiency achieved must at least approach that of conventional reversed-phase LC. 

Micellar mobile phases will never replace traditional hydroorganic mobile phases. They do, however, deserve serious consideration by practicing chromatographers as they can provide the solution to certain fundamental limitations of hydroorganic mobile phases. Hopefully the advantages will overcome the skepticism and resistance to change shown by many chromatographers and micellar mobile phases will soon assume a role of importance. 

Rosado-Maria, A., Gasco-Lopez, A.I., Santos-Montes, A., Izquierdo-Homillos, R. High-performance liquid chromatographic separation of a complex mixture of diuretics using a micellar mobile phase of sodium dodecyl sulphate application to human urine samples. J. Chromatogr. B 748, 415 24 (2000)... 

The most serious limitations of pure micellar solutions are their weak elution strength and poor efficiencies. As early as 1983, the addition of a small percentage of 1-propanol was found to enhance the efficiencies and decrease the asymmetries of chromatographic peaks. Later, the term hybrid micellar mobile phases was given to the ternary eluents of water/organic solvent/ micelles. Although 1-propanol is still the most frequently used additive, other alcohols (methanol, ethanol, 1-butanol, and 1-pentanol) and organic solvents common... 

Chromatographic efficiency seems to be linked to the additive-to-surfactant concentration ratio in the micellar mobile phase. The plate numbers increase with this ratio but reach a maximum level (e.g., at pentanol/SDS = 6 and acetonitrile/CTAC= 12). - The organic solvent/surfactant ratio affects the exchange rates of the solute between micelle/stationary and aqueous phases. It also controls the extent of the surfactant coverage and the fluidity of the organic layer on the stationary phase. 

Rapado-Martinez, I., Garcia-Alvarez-Coque, M. C. and Villanueva-Camanas, R. M. Liquid chromatographic procedure for the evaluation of /1-blockers in pharmaceuticals using hybrid micellar mobile phases. J. Chromatogr. A 765(2) 221—231, 1997. 

Brando, C., Hoffman, T. and Bonvini, E. (1 990) High-performance liquid chromatographic separation of inositol phosphate isomers employing a reversed-phase column and a micellar mobile phase. Journal of Chromatography B 529, 65-80. 

Micellar mobile phases have posed, however, some serious problems, that have slowed the development and widespread use of MLC. First, the chromatographic efficiency is often much lower in MLC than that... 

The organic additives of the micellar mobile phase affect the surfactant-adsorbed layer. This changes the chromatographic selectivity and efficiency obtained for a set of analytes with the same column. Selectivity and efficiency are studied in other parts of this book. 

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]. 

Limitations such as those described for naphthalene (with the C18 column) and hydroquinone (with the cyano column) would be more serious if one were limited to a single stationary phase. Fortunately, several different polarity bonded stationary phases are available. As a result, a very wide variety of compounds can be chromatographed using micellar mobile phases... 

The effect of temperature on the retention of compounds provides thermodynamic data that describe the chromatographic process. Dorsey et al. [13] proposed the use of high temperatures in MLC and applied the Van t Hoff nation to micellar mobile phases ... 

Figure 7.6 shows the separation of the mixture of vanillins on a C18 column with the following mobile phases methanol-water 20 80 (v/v), 0.02 M SDS and 0.02 M CTAB. Several things are apparent from an examination of the chromatograms. First, the test mixture is completely separated by the SDS micellar mobile phase, but not by the aqueous-organic or CTAB mobile phases. Second, the efficiency of the chromatographic process is poor for all three mobile phases, and this implies that the variability in the resolution of the test mixture, among the three mobile... 

Most reported procedures for the determination of compounds in Micellar Liquid Chromatography (MLC) make use of micellar mobile phases containing an organic modifier, usually a short-chain alcohol or acetonitrile. These modifiers increase the elution strength, which is particularly important for the most hydrophobic solutes, and often improve the shape of the chromatographic peaks. The most hydrophilic alcohols do not penetrate the micelles, but butanol and pentanol can be inserted into the... 

These results show that the type of general relationship depends not only upon the set of compounds but also on the characteristics of mobile and stationary phases. This was confirmed in a study of the chromatographic behavior of a group of 11 benzene derivatives and 12 PAHs, in SDS and CTAB micellar mobile phases modified with methanol, 1-propanol or 1-butanol at different percentages [19]. The authors demonstrated effectively that the hydrophobicity range of compounds is an important fector in the k-log Pow or log k-log Pow correlations. For the whole set of 23 aromatic compoimds studied (log Pqw from 0.64 to 5.03), k always correlated better with log Pow than log k, irrespective of the nature of the surfactant present in the mobile phase, and the nature and percentage of the alcohol used as a modifier. However, for a group of 15 benzene and naphthalene derivatives... 

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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