Micellar mobile phase

The model was tested by the micellar liquid chromatography separ ation of the five rarbornicin derivatives and four ethers of hydroxybenzoic acid. Micellar mobile phases were made with the sodium dodecylsulfate and 1-pentanol or isopentanol as modifier. In all cases the negative signs of the coefficients x and y indicate that at transition of the sorbat from the mobile on the stationar y phase the number of surfactant monomers as well as the number of modifier molecules increases in its microenvironment. 

The pur pose of work is to develop the technique of separ ation of purine bases (caffeine, theophylline, theobromine) and the technique of detection of purine bases in biological fluid by TLC using micellar mobile phases containing of different surfactants. 

Micellar HPLC with micellar mobile phases containing sodium dodecyl sulfate, with and without different alcohols, has been used to determine diuretics in pharmaceuticals [185]. 

High-performance liquid chromatography (HPLC) with a micellar mobile phase or with a selective pre-column or reaction detection system has also been used to determine alkylenebis(dithiocarbamaes). ° Zineb and mancozeb residues in feed were determined by ion-pair HPLC with ultraviolet (UV) detection at 272 nm. These compounds were converted to water-soluble sodium salts with ethylenediaminetetra-acetic acid (EDTA) and sodium hydroxide. The extracts were ion-pair methylated with tetrabuthylammonium hydrogensulfate (ion-pair reagent) in a chloroform-hexane solvent mixture at pH 6.5-8.S. The use of an electrochemical detector has also been reported. ... 

The future development of micellar RPC will probably depend on the development of applications for which micellar mobile phases offer a significant advantage over the conventional use of aqueous-organic solvent mixtures and an increase in the efficiency... 

Immaculada Rapado-Martinez, M., Garcia-Alvarez-Coque, C., and Villanue-va-Camanas, R.M., Performance of micellar mobile phase in reversed-phase chromatography for the analysis of pharmaceuticals containing [i-blockers and other antihypertensive drugs, Analyst, 121,1677, 1996. 

The basis for separation employing micellar mobile phases stems from their ability to differentially solubilize and bind structurally similar solutes. Skeptics view MLC as a fascinating example of the incorporation of secondary equilibria for control or adjustment of retention (101). However, it is the ultimate of secondary equilibria since the types of interactions possible with micellar aggregates cannot be duplicated by any single other equilibrium system, or for that matter, any one or mixture of traditional normal or reversed phase mobile phase systems. This is due to the fact that solutes can interact with the surfactant aggregates via hydrophobic, electrostatic, hydrogen bonding, and/or a combination of these factors. 

A micellar mobile phase can be viewed as being composed of both the surfactant micellar aggregates (pseudophase) and the rest of the... 

Figure 4. Artistic representation of the species and equilibria present when employing surfactant micellar mobile phases in LC.

The micellar mobile phase in all experiments consisted of aqueous 0.285M NaLS, flow rate l.OOmL/min, 10-cm column. 

Lastly, the use of micellar mobile phases allows a convenient means of studying micelle - solute interactions (i.e. determination of binding constants) (1,10 4,105) as well as determination of surfactant CMC values (from breaks in the log k gQ vs. log C, plots)... 

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. 

Gel Filtration. Micellar solutions have also been utilized in gel permeation (filtration) chromatography ( ] ). In fact, the first example of a separation which used a micellar mobile phase was in this area of exclusion liquid chromatography (ELC) ( 86). The last six entries in Table XI summarize some of the separations/work reported concerning micellar mobile phases in ELC. In most of these applications, the work was conducted with stationary phases of relatively small pore size. With these type phases, the relatively large micellar aggregates are confined to the excluded volume of the column and elute rapidly whereas smaller solute molecules in a mixture... 

A cursory review of the literature reveals that the ELC technique with micellar mobile phases has proven to be very beneficial in the characterization of micellar systems (184-186,190-192,227,228). For example, microcolumn exclusion LC has been applied to the determination of the CMC value of surfactants (or micellar-forming proteins), determination of the kinetic rate and equilibrium association constants for surfactant (or protein) micellization (184,192), determination of the size or size distribution of micelles (especially those formed from block copolymers or milk casein) (185,186,191,192,225) as well as for estimation of the time required for formation of micelles (or micelle-forming macromolecules) (186) among others. The size and stability of reversed micelles has also been evaluated using ELC (195). 

The use of ELC to characterize micellar and related aggregates thus appears to be popular and useful. In fact, its use in this manner overshadows the analytical applications of micellar mobile phases to aid ELC separations. However, several recent reports do point out the advantages of micellar mobile phases in ELC (187 189) for the isolation and purification of bacterial and viral proteins. 

This effect has been successfully employed to improve the LC detection of metal ions as their metal complexes (496.497.499). Recently, it has also been demonstrated that metal ions can be detected by direct-current argon plasma emission spectroscopy after LC separation with micellar mobile phases (490). 

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 ... 

In many forms of secondary equilibria separations, the concentration of the equilibrant, or the mobile phase component which participates in the secondary equilibria, controls, at least partially, the strength and selectivity of the mobile phase. In micellar chromatography the concentration of micelles plays this role, which means that for all separations carried out with micellar mobile phases, the strength of the mobile phase can be changed while maintaining an unchanging bulk solvent composition. This unique aspect of micellar mobile phases does indeed allow the solution to "problems that cannot be solved by other means . 

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. 

We have found that the use of 3% n-propanol in the micellar mobile phase and column temperatures of 40° C appear to offer a broadly applicable solution to the low efficiency previously reported for micellar mobile phases. These conditions have resulted in reduced plate heights of 3-4 for SDS, cetyltrimethylammonium bromide (CTAB), and Brij-35 (15). This efficiency optimization scheme then appears to be a broadly-based solution for micellar mobile phases of any surfactant. This means that the surfactant type can be varied to affect separational selectivity with no loss in column efficiency. 

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. 

High-Performance Liquid Chromatography of Organic and Inorganic Anions Use of Micellar Mobile Phase... 

A micellar mobile phase differs from a conventional ion-pairing mobile phase in two important aspects. Firstly, micellar solutions... 

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