Determination of avoparcin in animal formulations by capillary electrophoresis.

The separation of two closely related glycopeptides, alpha- and beta-avoparcin, which differ by the presence or absence of a Cl atom, is achieved by capillary zone electrophoresis (CZE) using 20 mM borate (pH 9.2) in bare, fused-silica capillaries. The pKa values of these two glycopeptides are determined from spectrophotometric titrations of the mixture. Absorption spectra recorded between pH 3 and 12 produced a maximum pH transition at 258 nm. Titration data fitted to two sigmoidal functions reveal pKas of 7.99 for beta-avoparcin and 10.3 for alpha-avoparcin. The differing pH dependence of the effective electrophoretic mobilities of alpha- and beta-avoparcin resulted in an optimum pH of 9.2 (roughly the average of the pKa values) for their separation. Borate is shown to be necessary for the optimal CZE separation. A linear dynamic range of 0.5-50 ppm is achieved with regression coefficients of 0.9999. The limit of detection ranges from 0.2 to 0.5 ppm using a pressure injection of 15 sec in a 50-cm (effective length) fused-silica capillary with UV detection. Finally, the effects of sodium dodecyl sulfate (SDS) concentration in the migration buffer on apparent mobility, efficiency, migration times, and resolution are discussed. The addition of 75 mM SDS to the running buffer allows baseline resolution of alpha- and beta-avoparcin, including its minor components. Both the CZE and micellar electrokinetic chromatography (MEKC) separations of alpha- and beta-avoparcin are highly reproducible and can readily be applied to the analysis of avoparcin bulk drug and formulated products.

Enantiomeric separations of basic pharmaceutical drugs by micellar electrokinetic chromatography using a chiral surfactant, N-dodecoxycarbonylvaline.

A novel surfactant with a chiral head group, (R)- or (S)-N-dodecoxycarbonylvaline (DDCV), was used to achieve enantiomeric separations of twenty basic pharmaceutical compounds by micellar electrokinetic chromatography (MEKC). Most of these compounds were beta-agonists (anti-asthmatic, bronchodilators) or beta-antagonists (anti-hypertension, anti-angina). DDCV can separate polar as well as more hydrophobic chiral analytes in the same buffer system. The selectivities for these enantiomeric pairs range from 1.03 to 1.23 with good efficiencies. Separations utilizing DDCV are easy to optimize and allow for exact enantiomeric migration order reversal by switching the enantiomeric form of the surfactant. Buffer systems were assessed to minimize Joule heating and to optimize the repeatability of parameters such as migration time, relative migration time, selectivity, peak areas and area ratios. An electrolyte system consisting of 25 mM DDCV, 100 mM zwitterionic CHES (2-[N-cyclohexylamino]ethanesulfonic acid) and 10 mM triethylamine (TEA) was most effective for these runs. The precision for migration times, relative migration times and selectivities was better than 1%, 0.1% and 1% R.S.D., respectively, while the precision for the area ratios ranged from 1% to 4%. The possible effect of analyte structure on selectivity, efficiency and precision of peak area was studied.

Anionic-zwitterionic mixed micelles in micellar electrokinetic chromatography: sodium dodecyl sulfate-N-dodecyl-N,N-dimethylammonium-3-propane-1-sulfonic acid.

A zwitterionic surfactant, N-dodecyl-N,N-dimethylammonium-3-propane-1- sulfonic acid (SB-12), was used in combination with an anionic surfactant, sodium dodecyl sulfate (SDS), to form a novel pseudostationary phase for use in micellar electrokinetic chromatography. This mixed micellar system was characterized in terms of analyte retention, selectivity, efficiency, elution range, and resolution; and compared to results obtained using only SDS. A typically used SDS concentration, 20 mM, was chosen as a reference to which comparisons could be drawn. With 20 mM SDS, the optimum concentration range of 10-20 mM SB-12 provided efficiencies that were 2-4 times greater than with SDS alone, with minimal (< 15%) changes in the elution range and electroosmotic flow. The addition of 40 and 60 mM SB-12 also resulted in efficiencies on average of 600,000-800,000 theoretical plates/m, but at a significant reduction in the elution range and peak capacity. Retention factors (k') for the various neutral analytes increased by 20% with addition of 10 mM SB-12 and by approximately 60% with addition of 40 and 60 mM SB-12, while operating currents remained constant as SB-12 was added. Geometrical isomers p-nitrotoluene and m-nitrotoluene, that co-eluted with 20 mM SDS, were baseline resolved with the addition of 10 mM SB-12; in addition, methylene selectivity was greatest at this composition. No capillary wall interactions or coating effects were observed with the SDS-SB-12 mixed micellar system, in contrast to previously studied anionic-non-ionic mixed micellar system, SDS-Brij 35. Consequently, migration times were very repeatable (< or = 1.2% R.S.D.).

A retention index for micellar electrokinetic chromatography.

A retention index system has been developed for micellar electrokinetic chromatography (MEKC). Three homologous series: alkyl aryl ketones (phenones), 1-nitroalkanes, and alkylbenzenes were studied for use as retention index standards. Micellar systems consisting of sodium dodecyl sulfate (SDS), SDS/Brij 35 (polyoxyethylene lauryl ether), and SDS/SB-12 (N-dodecyl-N, N-dimethylammonium-3-propane-1-sulfonic acid) were used as pseudostationary phases. In addition, three organic modifiers: acetonitrile, methanol, and 1-propanol were used with SDS to evaluate their effect on the retention indices calculated for a set of neutral compounds. Retention indices for the neutral compounds did not vary significantly over the range of surfactant concentrations employed for each of the micellar systems (RSD < 2.0% for non-extrapolated retention indices). However, in the systems where an organic modifier was employed, the calculated retention indices showed some variation (RSD < 3.0%) at different SDS concentrations. The 1-nitroalkanes were found to be the most suitable for use as retention index standards. Alkyl aryl ketones were found to be effective retention index standards for more hydrophobic solutes, but they were not effective for very hydrophilic solutes even with a large amount of organic modifier added to the operating buffer. The alkylbenzenes were too hydrophobic (highly retained) than the alkyl aryl ketones and, therefore, cannot be recommended for use as retention index standards in MEKC.