The Capillary Electrophoretic Method

Although each capillary electrophoretic method has its own unique considerations, the following description of the determination of a vitamin B complex provides an instructive example of a typical procedure. 

A number of developments have increased the importance of capillary electrophoretic methods relative to pumped column methods in analysis. Interactions of analytes with the capillary wall are better understood, inspiring the development of means to minimize wall effects. Capillary electrophoresis (CE) has been standardized to the point of being useful as a routine technique. Incremental improvements in column coating techniques, buffer preparation, and injection techniques, combined with substantive advances in miniaturization and detection have potentiated rugged operation and high capacity massive parallelism in analysis. 

Lin et al. [95] used capillary electrophoresis with dual cyclodextrin systems for the enantiomer separation of miconazole. A cyclodextrin-modified micellar capillary electrophoretic method was developed using mixture of /i-cyclodextrins and mono-3-0-phenylcarbamoyl-/j-cyclodextrin as chiral additives for the chiral separation of miconazole with the dual cyclodextrins systems. The enantiomers were resolved using a running buffer of 50 mmol/L borate pH 9.5 containing 15 mmol/L jS-cyclodextrin and 15 mmol/L mono-3-<9-phcnylcarbamoyl-/j-cyclodextrin containing 50 mmol/L sodium dodecyl sulfate and 1 mol/L urea. A study of the respective influence of the /i-cyclodcxtrin and the mono-3-(9-phenylcarbamoyl-/i-cyclodextrin concentration was performed to determine the optical conditions with respect to the resolution. Good repeatability of the method was obtained. 

Nishi et al. [110] used dextran and dextrin as chiral selectors in capillary-zone electrophoresis. Polysaccharides such as dextrins, which are mixtures of linear a-(l,4)-linked D-glucose polymers, and dextrans, which are polymers of D-glucose units linked predominantly by a-(l,6) bonds, have been employed as chiral selectors in the capillary electrophoretic separation of enantiomers. Because these polymers are electrically neutral, the method is applicable to ionic compounds. The enantiomers of basic or cationic drugs such as primaquine were successfully separated under acidic conditions. The effects of molecular mass and polysaccharide concentration on enantioselectivity were investigated. 

The zone capillary electrophoretic methods are applied the most and are therefore discussed in more detail. The others are discussed briefly. 

Brown et al. [47] have described a cyclodextran modified capillary electrophoretic method for the determination of polyaromatic hydrocarbons... 

During the last few years, a number of specific monographs for different pharmaceutical products have appeared in pharmacopoeias in which CE is prescribed as one of the analytical procedures. Several comparative studies have been reported in which established analytical procedures described in pharmacopoeial monographs were compared with capillary electrophoretic methods or in which CE was evaluated as a valuable additional technique.Based on the results, some analytical procedures have been replaced by CE-based alternative methods in a number of monographs. For other products, CE has been included in the monograph from the initial version onwards. In addition, CE has been applied... 

Yesilada, A., Tozkoparan, B., Gbkhan, N., Oner, L., and Ertan, M. (1998). Development and validation of a capillary electrophoretic method for the determination of degradation product in naphazoline HCl bulk drug substance. /. Liq. Chromatogr. Related Technol. 21(17), 2575-2588. 

TABLE I Potential Factors in the Robustness Testing of Capillary Electrophoretic Methods... 

Leonard, S., Van Schepdael, A., Ivanyi, T., Lazar, L, Rosier, J., Vanstockem, M., Vermeersch, H., and Hoogmartens, J. (2005). Development of a capillary electrophoretic method for the separation of diastereoisomers of a new human immunodeficiency virus protease inhibitor. Electrophoresis 26, 627-632. 

Fernandes, J. B., Griffiths, D. W., Bain, H., Fernandes, F. A. N. (1996). The development and evaluation of capillary electrophoretic methods for the determination of the major phenolic constituents of potato Solanum tuberosum) tubers. Phytochem. Analysis, 7, 253-258. 

Electromigration methods compose a family of analytical separation methods based on differences in the mobilities of charged analytes in the electric field. In this chapter, we discuss mainly such electromigration methods that are performed in thin capillaries with inner diameter (i.d.) <0.1 mm. These methods are commonly known as capillary electrophoretic methods where the most important modes are capillary zone electrophoresis (CZE), micellar electrokinetic capillary chromatography (MEKC), capillary gel electrophoresis (CGE), and capillary electrochromatography (CEC). 

Altria et al. [135] used a validated capillary electrophoresis method for the analysis of omeprazole among other acidic drugs and excipients. The results of validation experiments for the capillary electrophoretic... 

Of course, conventional chromatographic and capillary electrophoretic methods are popular for the analyses of drugs, pharmaceuticals, and pollutants in biological and environmental matrices. In the last few decades, new developments in nanoanalyses have emerged in the literature due to their high separation efficacy, speed, inexpensive running cost, and small volume requirement, which are major... 

Although affinity capillary electrophoresis (ACE) in its classical mode (one of the reagent is dissolved in a BGE, another is injected) is the most widely used technique in the literature, other capillary electrophoretic methods exist which are even more favorable concerning the information about binding parameters obtainable the Hummel-Dreyer (HD) method, frontal analysis (FA), the vacancy peak (VP) method, and vacancy affinity capillary electrophoresis (VACE) (see, e.g., Refs. 49-57). All the methods need as a precondition that the equilibrium between the reactants (say, protein P, drug D, and complex formed PD) is established rapidly compared to the dislocation of the electropho-retically migrating zones. The experimental setup of the HD and the ACE methods is identical, and so is the setup for the VP and the VACE methods. FA differs from all the other techniques. 

While capillary electrophoretic methods have been employed and do find some application in casework [9, 10], the method of choice is currently high performance liquid chromatography (HPLC). The exact method to be employed will depend upon the amphetamine in question, but the method described below [11] can be applied to amphetamine, methylamphetamine, MDA and MDMA without any difficulties. 

Hardy, S., Jones, P. Development of a capillary electrophoretic method for the separation and determination of trace inorganic and organomercury species utilizing the formation of highly absorbing water soluble dithizone sulphonate complexes. J. Chromatogr. A 791, 333-352 (1997)... 

Iwaki M, Murakami E, Kakehi K. Chromatographic and capillary electrophoretic methods for the analysis of nicotinic acid and its metabolites. J Chromatogr B Biomed Sci Appl 2000 747 229-40. 

In the past, the most common method of analysis of small anions has been ion-exchange chromatography. For cations, the preferred techniques have been atomic absorption spectroscopy and inductively coupled plasma emission spectroscopy. Recently, however, capillary electrophoretic methods have begun to compete with these traditional methods for small ion analysis. Several major reasons for adoption of electrophoretic methods have been recognized lower equipment costs, smaller sample size requirements, much greater speed, and better resolution. 

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