Analysis of erythromycin and benzoylperoxide in topical gels by liquid chromatography.

Gels containing a combination of erythromycin and benzoylperoxide are frequently used in the treatment of acne vulgaris. A method was developed to determine the content of both erythromycin and benzoylperoxide in these gels. Erythromycin was extracted from the gel in conditions where the oxidative power of benzoylperoxide was neutralised by addition of ascorbic acid and this extract was analysed on an Xterra RP(18) column, with a mobile phase containing acetonitrile-0.2 M K2HPO4-water (35:5:60, v/v/v). The detection wavelength was 215 nm. A second extraction procedure was developed for the analysis of benzoylperoxide. The extraction solution was analysed on a Hypersil C(18) BDS column and a mobile phase containing acetonitrile-water (58:42, v/v). Detection was performed at 254 nm. The flow rate was 1.0 ml/min in both methods. The selectivity, repeatability, linearity and recovery of both methods were examined. Special attention was given to determination of the recovery and the uncertainty on the recovery. This allowed evaluation of the bias of the extraction method. The method developed was used to examine the stability of a gel for topical use.

Interlaboratory study of a liquid chromatography method for erythromycin: determination of uncertainty.

Erythromycin is a mixture of macrolide antibiotics produced by Saccharopolyspora erythreas during fermentation. A new method for the analysis of erythromycin by liquid chromatography has previously been developed. It makes use of an Astec C18 polymeric column. After validation in one laboratory, the method was now validated in an interlaboratory study. Validation studies are commonly used to test the fitness of the analytical method prior to its use for routine quality testing. The data derived in the interlaboratory study can be used to make an uncertainty statement as well. The relationship between validation and uncertainty statement is not clear for many analysts and there is a need to show how the existing data, derived during validation, can be used in practice. Eight laboratories participated in this interlaboratory study. The set-up allowed the determination of the repeatability variance, s(2)r and the between-laboratory variance, s(2)L. Combination of s(2)r and s(2)L results in the reproducibility variance s(2)R. It has been shown how these data can be used in future by a single laboratory that wants to make an uncertainty statement concerning the same analysis.

Analysis of clindamycin by micellar electrokinetic chromatography with a mixed micellar system.

This study details the development and validation of an optimized method with micellar electrokinetic chromatography for the analysis of clindamycin. The method uses a mixed micellar phase containing anionic sodium dodecylsulfate (SDS) and non ionic Brij 35 on an untreated fused-silica capillary. The influences of buffer concentration, pH, SDS, Brij 35 and organic modifier were investigated. Special attention was given to the role of the non ionic Brij 35 in the mixed micellar system. Optimization with a central composite design resulted in optimal separation conditions: background electrolyte containing 25 mM sodium tetraborate pH 7.75, 90 mM SDS, 14 mM Brij 35 and 21% acetonitrile. The applied voltage was 15 kV and the capillary temperature 15 degrees C. The method was robust and gave good linearity and repeatability. The limits of detection and quantitation were 0.05 and 0.15%, respectively, relative to a 2.5 mg/ml clindamycin solution. Two commercial bulk products were analysed with this system.

Analysis of metacycline by capillary electrophoresis.

The development and validation of an optimized capillary electrophoresis method for the determination of metacycline in the presence of its related substances by capillary electrophoresis is shown. The influence of methanol as organic modifier, buffer pH, buffer concentration, capillary length, column temperature, Triton X-100 and methyl-beta-cyclodextrin was investigated. A central composite design was performed in order to optimize the method. The optimal separation conditions were: uncoated fused-silica capillary (39 cm total length, 31 cm effective length, 50 microm ID); as background electrolyte a solution of 160 mM sodium carbonate and 1 mM EDTA (pH 10.35)/methanol (89:13 v/v); temperature, 15 degrees C; voltage, 12 kV. The method showed good selectivity, repeatability, linearity, and sensitivity. The limits of detection and quantitation are 0.024% and 0.06%, respectively, relative to a 2.5 mg/mL solution. Six commercial samples were analyzed quantitatively.