ASSAY AND PURITY CONTROL OF MINOCYCLINE BY THIN-LAYER CHROMATOGRAPHY USING UV AND FLUORESCENCE DENSITOMETRY - A COMPARISON WITH LIQUID-CHROMATOGRAPHY

A thin-layer chromatography (TLC) method using UV and fluoresecence de nsitometry is described for the assay and purity control of minocyclin e (MC). With a mobile phase dichloromethane-methanol-water (57:35:8, v /v/v) and a silica gel thin-layer, previously sprayed with 10% m/v sod ium edetate adjusted to pH 9.0, 4-epiminocycline and 7-didemethylminoc ycline were well separated from MC and from each other, 7-monodemethyl minocycline and 6-deoxy-6-demethyltetracycline (6-DODMTC) were not sep arated from each other and were only partially separated from minocycl ine. 6-DODMTC was selectively determined by fluorescence densitometry, while quantification of other impurities and the assay of MC were per formed by UV densitometry. Results obtained with qualitative TLC were compared with those obtained by a liquid chromatography (LC) method us ing a poly(styrene-divinylbenzene) copolymer stationary phase. The cor relation coefficient for TLC and LC results was >0.999. For TLC the re lative standard deviation for the assay of MC at 1.25 mg ml(-1) was <3 .0% (n = 4), while for LC it was < 1.0% (n = 4).