Non-aqueous capillary electrophoresis of p-quinone anion radicals

Capillary electrophoresis for pharmaceutical analysis

This chapter describes the application of capillary electrophoresis (CE) to pharmaceutical analysis. The areas of pharmaceutical analysis covered are enantiomer separation, analysis of small molecules such as amino acids or drug counter-ions, pharmaceutical assay, related substances determinations, and physiochemical measurements such as log P and pKa of compounds. The different electrophoretic modes available and their advantages for pharmaceutical analysis are described. Recent applications of CE for each subject area are tabulated with electrolyte details. Information on electrolyte choice and method optimization to obtain optimal separations is included.

Semipermanent capillary coatings in mixed organic-water solvents for CE

This report describes the creation of semipermanent capillary coatings that are compatible with organic-water solvent systems in CE. The coatings are created by simply rinsing the fused-silica capillary with long double-chain cationic surfactants, such as dimethyl-ditetradecyl ammonium bromide (2C(14)DAB), dihexadecyldimethyl ammonium bromide (2C(16)DAB), and dimethyldioctadecyl ammonium bromide (2C(18)DAB). These surfactants generate semipermanent bilayer coatings on the capillary surface, which display a high degree of stability in buffers containing up to 60% v/v of organic solvents, such as methanol and ACN. The coating stability increases with increasing hydrophobicity of the surfactant, i.e., with increasing chain length. For instance, the EOF changes by only 1.2% in a 2C(18)DAB-coated capillary after 130 capillary volumes of rinsing with 60% v/v methanol containing buffer. The bilayer coatings allow separations to be performed without the need to regenerate the coating between runs or to maintain the EOF modifier in the run buffer. Rapid separations (<2 min) of anions and basic drugs with migration time reproducibility of less than 0.5% RSD and efficiencies of 0.4-0.6 million plates/m are obtained. In addition, selectivity changes for small anions and cationic drugs are also observed when the organic solvent content is adjusted.

Capillary electrophoresis for the analysis of small-molecule pharmaceuticals

This paper reviews the application of CE to the analysis of small-molecule pharmaceuticals. The areas of pharmaceutical analysis covered are enantiomer separation, the analysis of small molecules such as amino acids or drug counter-ions, pharmaceutical assay, determination of related substances and physicochemical measurements such as log P and pK(a) of compounds. The different electrophoretic modes available and their advantages for pharmaceutical analysis are described. Recent applications of CE for each subject area are tabulated with electrolyte details.

Capillary Zone Electrophoresis of Some Extremely Weak Bases in Acetonitrile

In water capillary zone electrophoresis cannot be used to investigate basic compounds, which are so weak that their pK(a,HB+) values are less than zero. In acetonitrile the basic strength of such compounds is increased by many orders of magnitude. Accordingly, several extremely weak bases are protonated at low pH in acetonitrile, thus, allowing their investigation by CZE. In this work the CZE separation of thioacetamide, acetamide, thiourea, benzamide, and 4-nitrobenzamide as well as that of N-methylformamide, N,N-dimethylformamide, formamide, and dimethyl sulfoxide is demonstrated in acetonitrile using 10 mmol/L perchloric acid as an electrolyte. The effect of BGE additives, like water and acetic acid, on the CZE performance is discussed. The problem of finding a suitable electroosmotic flow marker at low pH in acetonitrile is addressed, and nitromethane is shown to be a proper marker compound under such extreme conditions. This work demonstrates how organic solvents can enlarge the field of application of CZE.

Capillary electrophoresis of methyl-substituted phenols in acetonitrile

The separation of mono- and dimethylphenols by capillary electrophoresis in pure acetonitrile was investigated. In acetonitrile, uncharged phenols interact with background electrolyte anions forming negatively charged complexes, which can be separated from each other by capillary electrophoresis. The background electrolyte anions tested were acetate, bromide and chloride. The calculated formation constants for phenol-anion complexes were highest with acetate and smallest with bromide. Complex formation was found to be sensitive to traces of water in the background electrolyte. The separation of methylphenols was also carried out in acetonitrile at high pH using background electrolytes prepared from diprotic acids and tetrabutylammonium hydroxide. At high pH the phenols were partly dissociated, providing an additional mechanism for the separation. All methylphenols were separated with the use of malonate background electrolyte. However, this approach was prone to interference from methanol resulting from the tetrabutylammonium hydroxide solution.

Analysis of active chemical species generated by electrolysis using non-aqueous capillary electrophoresis. Detection of the anion radical and the divalent anion of tetracyanoquinodimethane

We have investigated detection of the anion radical and the divalent anion of tetracyanoquinodimethane (TCNQ) by acetonitrile-CE under anaerobic conditions. With electrolysis at a potential of 0.0 V (vs. Ag/AgCl), an acetonitrile solution of TCNQ turned green, characteristic of the TCNQ anion radical (TCNQ-). Only one peak of the anionic compound was observed in CE of the electrolysis solution and it should be that of TCNQ-. Then, the electrolysis potential was shifted to -0.8 V expected to be sufficient potential for the further reduction of TCNQ-, and the solution turned almost colourless. In CE analysis of the latter solution, another anionic component possessing a larger electrophoretic mobility than that of TCNQ- was detected, and it was decomposed immediately under aerobic conditions. This product was strongly suggested to be the divalent anion of TCNQ, and the present method would contribute notably to detection of the unstable species.

Novel dynamic polymer coating for capillary electrophoresis in nonaqueous methanolic background electrolytes

Coated capillaries can be advantageous in many capillary electrophoretic applications where nonaqueous background electrolytes are used. In the present work, a new dynamic polymer coating (poly(glycidylmethacrylate-co-N-vinylpyrrolidone)) for methanol-based background electrolytes is introduced. The magnitude and stability of electroosmotic flow was investigated with coated capillaries at pH* values of 3, 7.8, and 10.4 in methanol. At pH* 7.8 and 10.4 the electroosmotic flow was negligible and repeatable. On the other hand, at pH* 3 a weak, unstable electroosmotic flow was observed, due to a change in the conformation of the polymer under acidic conditions. The dynamically coated capillaries were successfully applied to the separations of cationic drugs, phenols, and benzoic acids. The synthesis and characterization of the polymer are described in detail.