Electrokinetic Chromatographic Methods

There are obviously many methods to determine partition coefficients, directly and indirectly. Many of the newer high throughput procedures provide a rapid, easy to use, reproducible cost-effective way to assay the hydrophobicity of large numbers of diverse compounds however, there are a few caveats. In some cases, the range of partition coefficients that has been validated is limited. In HT screens with 1-octanol, a 10,000 to 1 distribution ratio would be difficult to access and assess thus the reliability of log Poet values at the ends of the spectrum ( 3 or 5) would be much lower than shake-flask values when dealing with minute volumes such as those present in a 96-well plate. 

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Micellar electrokinetic capillary chromatography with photodiode array detection was used for the determination of polyaromatic hydrocarbons in soil [65]. A detection limit of lOpg and linear calibration over five orders were observed. Compared to a standard gas chromatographic analysis method, the miscellar electrokinetic chromatographic method is faster, has a higher mass sensitivity and requires smaller sample sizes. 

A micellar electrokinetic chromatographic method allows the separation of optically isomeric diltiazem hydrochloride using bile salts as chiral surfactants. The chiral separation of diltiazem hydrochloride from several analogs is achieved at ambient temperature using 0.05M sodium taurodeoxycholate in a 0.02M aqueous phosphate-borate buffer solution of pH 7.0. Separation is performed using a fused-silica capillary tube (650 mm x 50 mm I.D.) and a voltage up to +25 kV. Detection is achieved on-column using UV adsorption at 210 nm (31). 

Separation of kavalactones was also achieved by supercritical fluid chromatography using methanol-modified carbon dioxide as the mobile phase. An optimal separation was achieved on either an amino or protein C4 column at 125 atm and 80 °C. Semipreparative separation of kavalactones was also obtained with two columns connected in series (Ashraf-Khorassani etal., 1999). A micellar electrokinetic chromatographic method with diode-array detection has also been developed for the identification and quantitative determination of the major kavalactones from extracts of P. methysticum... 

Log P values can be measured experimentally or predicted from molecular structure. Experimental approaches run the gamut from the highly reliable and traditional shake-flask method to newer methodologies such as electrokinetic chromatographic methods that are rapid, accurate and precise, and can be automated. However, for validation purposes, partition coefficient data of large, structurally diverse datasets obtained from such systems should be compared with that from the standard octanol/water system. 

Hillaert S., De Beer T.R., De Beer J.O., Van den Bossche W., Optimisation and validation of a micellar electrokinetic chromatographic method for the analysis of several angiotensin-II-receptor antagonists. J. Chromatogr. A, 984, 135-146 (2003). 

Lee et al. [30] described a micellar electrokinetic capillary chromatographic method for the determination of some antiepileptics including valproic acid. They used a fused silica capillary column (72 cm x 50 pm) and SDS as the micellar phase and multiwavelength UV detection. Reaction conditions, such as pH and concentration of running buffer were optimized. Solutes were identified by characterizing the sample peak in terms of retention time and absorption spectra. Recoveries were 93-105%. 

Furlanetto, S., Orlandini, S., Marras, A. M., Mura, P., and Pinzauti, S. (2006). Mixture design in the optimization of a microemulsion system for the electrokinetic chromatographic determination of ketorolac and its impurities method development and validation. Electrophoresis 27, 805-818. 

GLC determination of reducing end sugar residues in oligo- and poly-saccharides following reduction, hydrolysis and acetylation has been recommended as an alternative to colorimetric and H NMR methods. The iridoid glycosides catalpol and aucubin have been quantified by use of a fast micellar electrokinetic capillary chromatographic method. 

Bishop, S. C., et al. "Micellar Electrokinetic Chromatographic Screening Method for Common Sexual Assault Drugs Administered in Beverages." Forensic Science International, 141 2004, 7-15. 

The stereochemistry of drugs and precursors can be vital in determining synthetic routes. Horvever, since enantiomers are nearly chemically identical, standard chromatographic methods cannot separate them. To effect a chromatographic separation of enantiomers, stereospecific interactions have to be incorporated. As described in Chapter 5, the use of chiral stationary phases is one method of discriminating enantiomers while capillary electrokinetic chromatography (MEKC) using chiral cy-clodextrins is another. Both of these protocols have been applied to methamphetamine and related compounds and precursors, substances that will be discussed in detail later in the chapter. 

Recently a new method was developed for the complete liquid chromatographic separation and diode array detection of standard mixtures of the 14 most frequently used synthetic colorants. Protocols for RP-HPLC - " and IP-HPLC techniques have been extensively described and the techniques were compared with micellar electrokinetic capillary chromatography, - which has been shown to be suitable for the analysis of synthetic colorants. 

Chromatographic and related electrophoretic methods for the separation of transition metal complexes or their ligands were reviewed . Micellar electrokinetic chromatography (MEKC) presents a new development in the field of capillary zone electrophoresis (CZE). The use of micellar solutions expands the application of CZE to electronically neutral solutes, as well as charged ones. Thus, electrically neutral / -diketonates Cr(dik)3, Co(dik)3, Rd(dik)3, Pt(dik)2 and Pd(dik)2 were separated by CZE in micellar solutions of sds. A linear log-log relationship was found between the distribution coefficient and the partition coefficient of the complex between dodecane and water, which was used for prediction of both the distribution coefficients and the migration times of different metal complexes . 

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