Surfactant-containing mobile phases

Examination of equations 5, 6, and 7 reveals that retention can be controlled by variation of the surfactant micelle concentration, variation of pH (for ionizable species), and by manipulation of the solute-micelle binding constant (K. ) which, in turn can be influenced by additives (salt, alcohol referto data on DDT, Table VI) or the type (charge and hydrophobicity) of micelle-forming surfactant employed (refer to data in Table VII for 1-pentanol). Table VIII summarizes some of the factors that influence retention for surfactant-containing mobile phases and compares the effect of changes in these factors upon the retention behavior observed in both micellar liquid and ion-pair chromatography (81). 

The main disadvantages of micellar chromatography are the observed diminished chromatographic efficiency, higher column back pressure, and in preparative work, the need to separate the final resolved analyte from the surfactant (95) (a later section of this review will discuss this latter problem and its resolution in further detail). The higher column back pressure and part of the decreased efficiency stem from the fact that surfactant-containing mobile phases are more viscous compared to the usual hydro-organic mobile phases employed in conventional RP-HPLC (refer to viscosity data in Table X)... 

Summary of Some Selected Separations Reported which have Utilized Surfactant-Containing Mobile Phases ... 

J.F. Clos and J.G. Dorsey, Enhanced Stability of Electrochemical Detection with Surfactant Containing Mobile Phases in LC and Flow-Injection Analysis, Anal. Lett., 23 2327 (1990). 

Mass-action model of surfactant micelle formation was used for development of the conceptual retention model in micellar liquid chromatography. The retention model is based upon the analysis of changing of the sorbat microenvironment in going from mobile phase (micellar surfactant solution, containing organic solvent-modifier) to stationary phase (the surfactant covered surface of the alkyl bonded silica gel) according to equation ... 

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. 

ANALYSIS OF MEDICINES BY TEC IN MOBILE PHASES CONTAINING SURFACTANTS AND CYCLODEXTRINES... 

The recent use of HPLC for the analysis of sulfophenyl carboxylates (SPCs) has been one of the most interesting applications of this technique for the study of the environmental behaviour of anionic surfactants. SPCs are separated by reversed-phase ion-paired chromatography, in which a hydrophobic stationary phase is used and the mobile phase is eluted with aqueous buffers containing a low concentration of the counter-ion [19]. 

One of the main problems to be solved in the analysis of cationic surfactants is the strong adsorption of the surfactant to glassware, tubing and apparatus. To avoid losses, the solvent system used should contain a substantial percentage of organic solvent. Additionally, mobile phases containing more than 20-25% methanol will help to inhibit micelle formation [46]. 

In MEKC, the supporting electrolyte medium contains a surfactant at a concentration above its critical micelle concentration (CMC). The surfactant self-aggregates in the aqueous medium and forms micelles whose hydrophilic head groups and hydrophobic tail groups form a nonpolar core into which the solutes can partition. The micelles are anionic on their surface, and they migrate in the opposite direction to the electroosmotic flow under the applied current. The differential partitioning of neutral molecules between the buffered aqueous mobile phase and the micellar pseudostationary phase is the sole basis for separation as the buffer and micelles form a two-phase system, and the analyte partitions between them (Smyth and McClean 1998). 

Polysorbate 80 is widely used as a nonionic surfactant in liquid pharmaceutical products such as inhalation, suspension, and nasal suspension products, due to its properties of solubilization, reduction of surface and interfacial tension, and wetting. Direct analysis of Polysorbate 80 is quite time consuming. Size-exclusion chromatography (SEC) has been reported [5] in which a mobile phase contained the surfactant at concentrations above the critical micelle concentration. Polysorbate 80 appeared as a very broad peak and coeluted with other peaks, which makes quantification in Nasonex impossible. 

Aqueous micellar solutions were first used as mobile phases in LC in 1980. A micellar solution is one that contains a surfactant at a concentration above the critical micelle concentration, about 10-2 M. The nature of micelles and their use in analytical chemistry, including chromatography, has been recently described.82... 

Early researchers sought to choose appropriate surfactants for mobility control from the hundreds or thousands that might be used, but very little of the technology base that they needed had yet been created. Since then, work on micellar/polymer flooding has established several phase properties that must be met by almost any EOR surfactant, regardless of the application. This list of properties includes a Krafft temperature that is below the reservoir temperature, even if the connate brine contains a high concentration of divalent ions (i.e., hardness tolerance), and a lower consolute solution temperature (cloud point) that is above the reservoir temperature. 

In both MEKC methods reported, the mobile phase consisted of borate buffer containing surfactant and acetonitrile. It was foimd that mobile phase prepared at pH 9.75 gave better resolution compared to other conditions, and increasing pH also increased the migration time of ezetimibe. An analyte concentration of 25 mM was chosen due to its lower current, and the sharp peaks observed [41]. In addition, for the analysis of ezetimibe in combination with another drug such as simvastatin, increasing the borate concentration increased both resolution and migration times. 

Fig. 1.4. (A) Separation of Ihe individual oligomers in a Serdox NNP 4 sample of clhoxylalcd nonylphenol non-ionie surfactants on a Scpan>n SGX Amine. 7 pm, column tl5() x. . .4 mm i.d.) with acclonitrilc-water-dichloromethane 49 I 50 mobile phase at 0.5 ml/min. lifeiection UV. 230 nm. (B) Separation of the individual non-sulphatcd (first group of peaks) and sulphalcd anionic (second group) oligomers in a partially sulphated Serdox NNP 4 sample of clhoxylalcd nonylphenol on a Separon SGX Amine. 7 pm, column (150 X 3.3 mm i.d.) with Ihe mobile phase containing 0.04 M cetyl trimethylammonium bromide (CTAB) in acetonitrile-water-dichloromeihane f>8.6 1.4 . 3() at 0.5 ml/min. Detection UV. 2.30 nm.

Fig. M2. Dependence of retention factors, k. on the number of oxyethylene units, n. in the individual ethoxylaied nonylphenol non-ionic. surfactants oligomers on a Sila,sorb SPH Cm (7.. ) pm) column (300 x 4.0 mm i.d.) in mobile phases containing 60 (/). 50 (2). 45 (.3). 40 (4) and 35 (5) 9r 2-propanoI in water. Points experimental data lines best-fit plots of Eq. (1.19) with the quadratic term equal to 0.

Electrokinetic chromatography is conducted in capillary tubes and mobility is generated by the application of electrical potential but separation arises as a result of interactions with a micellar stationary phase formed by surfactants added to the mobile phase. Therefore electrokinetic chromatography contains elements of both liquid chromatography and capillary electrophoresis. 

---