MEKC

MEKC is a CE mode based on the partitioning of compounds between an aqueous and a micellar phase. This analytical technique combines CE as well as LC features and enables the separation of neutral compounds. The buffer solution consists of an aqueous solution containing micelles as a pseudo-stationary phase. The composition and nature of the pseudo-stationary phase can be adjusted but sodium dodecyl sulfate (SDS) remains the most widely used surfactant. 

Similarly to RPLC, there is a strong Unear correlation between the logarithm of the retention factor measured by MEKC (logfe) for neutral solutes or the neutral form of ionizable compounds and log P j. As shown in the supplementary material (see Sechon 13.5), numerous papers reported good correlations between log Pod and log k. 

Practically, the retention factor k of a neutral solute can be calculated from MEKC measurements  

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Because micelles are negatively charged, they migrate toward the cathode with a velocity less than the electroosmotic flow velocity. Neutral species partition themselves between the micelles and the buffer solution in much the same manner as they do in HPLC. Because there is a partitioning between two phases, the term chromatography is used. Note that in MEKC both phases are mobile. ... 

The elution order for neutral species in MEKC depends on the extent to which they partition into the micelles. Hydrophilic neutrals are insoluble in the micelle s hydrophobic inner environment and elute as a single band as they would in CZE. Neutral solutes that are extremely hydrophobic are completely soluble in the micelle, eluting with the micelles as a single band. Those neutral species that exist in a partition equilibrium between the buffer solution and the micelles elute between the completely hydrophilic and completely hydrophobic neutrals. Those neutral species favoring the buffer solution elute before those favoring the micelles. Micellar electrokinetic chromatography has been used to separate a wide variety of samples, including mixtures of pharmaceutical compounds, vitamins, and explosives. 

Description of Method. The water-soluble vitamins Bi (thiamine hydrochloride), B2 (riboflavin), B3 (niacinamide), and Be (pyridoxine hydrochloride) may be determined by CZE using a pH 9 sodium tetraborate/sodlum dIhydrogen phosphate buffer or by MEKC using the same buffer with the addition of sodium dodecyl-sulfate. Detection Is by UV absorption at 200 nm. An Internal standard of o-ethoxybenzamide Is used to standardize the method. 

Procedure. A vitamin B complex tablet Is crushed and placed In a beaker with 20.00 mL of a 50% v/v methanol solution that Is 20 mM In sodium tetraborate and contains 100.0 ppm of o-ethoxybenzamIde. After mixing for 2 min to ensure that the B vitamins are dissolved, a 5.00-mL portion Is passed through a 0.45- xm filter to remove Insoluble binders. An approximately 4-nL sample Is loaded Into a 50- xm Internal diameter capillary column. For CZE the capillary column contains a 20 mM pH 9 sodium tetraborate/sodlum dIhydrogen phosphate buffer. For MEKC the buffer Is also 150 mM In sodium dodecylsulfate. A 40-kV/m electric field Is used to effect both the CZE and MEKC separations. 

The order of elution when using MEKC is vitamin B3 (5.58 min), vitamin Be (6.59 min), vitamin 82 (8.81 min), and vitamin Bi (11.21 min). What conclusions can you make about the solubility of the B vitamins in the sodium dodecylsulfate micelles ... 

In this experiment the enantiomers of cyclobarbital and thiopental, and phenobarbital are separated using MEKC with cyclodextran as a chiral selector. By adjusting the pH of the buffer solution and the concentration and type of cyclodextran, students are able to find conditions in which the enantiomers of cyclobarbital and thiopental are resolved. 

This experiment describes a quantitative analysis for caffeine, theobromine, and theophylline in tea, pain killers, and cocoa. Separations are accomplished by MEKC using a pH 8.25 borate-phosphate buffer with added SDS. A UV detector set to 214 nm is used to record the electropherogram. An internal standard of phenobarbital is included for quantitative work. 

Weber, P. L. Buck, D. R. Capillary Electrophoresis A Past and Simple Method for the Determination of the Amino Acid Composition of Proteins, /. Chem. Educ. 1994, 71, 609-612. This experiment describes a method for determining the amino acid composition of cyctochrome c and lysozyme. The proteins are hydrolyzed in acid, and an internal standard of a-aminoadipic acid is added. Derivatization with naphthalene-2,3-dicarboxaldehyde gives derivatives that absorb at 420 nm. Separation is by MEKC using a buffer solution of 50 mM SDS in 20 mM sodium borate. 

ON-LINE CONCENTRATION TECHNIQUES IN MICELLE ELECTROKINETIC CHROMATOGRAPHY (MEKC)... 

One of the important problems in the diagnosis of different disease in their early stages is the determination of bio-active substances in biological fluids. We are currently interested in applying capillary electrophoresis (CE) as technique for the rapid and highly efficient separation of corticosteroids in semm and urine. Steroids can analyze by MEKC. 

We have developed the method of the determination of steroids in biological fluids (semm and urine) by MEKC with on-line concentration (sweeping) with detection limit for about 3 ng/ml (S/N=3). 

Figure 6.13 SEE-MEKC electropherogram of the pesticide carbaryl in a tomato sample the peak assigned number (4) coiresponds to the migration time of carbaryl. (from ref. 58).

CE was recently used for anthocyanin analysis because of its excellent resolution. This technique has different modes capillary zone electrophoresis (CZE), capillary gel electrophoresis (CGE), micellar electrokinetic chromatography (MEKC), capillary electrochromatography (CEC), capillary isoelectric focusing (CIEE), and capillary isotachophoresis (CITP)."° CZE is the most popular method for anthocyanin... 

Desiderio, C., Marra, C., and Fanali, S., Quantitative analysis of synthetic dyes in lipstick by MEKC, Electrophoresis, 19, 1478, 1998. 

Another relatively new lipophilicity scale proposed for use in ADME studies is based on MEKC [106]. A further variant is called BMC and uses mobile phases of Brij35 [polyoxyethylene(23)lauryl ether] [129]. Similarly, the retention factors of 16 P-blockers obtained with micellar chromatography with sodium dodecyl sulfate as micelle-forming agent correlates well with permeability coefficients in Caco-2 monolayers and apparent permeability coefficients in rat intestinal segments [130]. 

Similarly to MEKC, strong linear correlations between the logfe measured by MEEKC and the log Pod were reported by several authors as shown in the supplementary material (see Section 13.5). 

The logk obtained by MEKC, MEEKC and LEKC were also compared with membrane permeability reference data by Ornskov et al. [92]. An improved correlation was obtained in the order MEKC>MEEKC>LEKC. Thus, LEKC appears to provide experimental conditions that mimic more closely physiological membranes [93], However, liposomes and vesicles remain unstable and difficult to prepare reproducibly. Their use is then devoted to some parhcular applicahons. 

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