Kinetic determination of ansamicins in pharmaceutical formulations and human urine. Manual and semiautomatic (stopped-¯ow) procedures A. Espinosa-Mansilla * , M.I. Acedo Valenzuela, F. Salinas, F. Can Äada Can Äada Department of Analytical Chemistry, University of Extremadura, 06071 Badajoz, Spain Received 26 January 1998; received in revised form 7 July 1998; accepted 31 July 1998 Abstract Manual and semiautomatic kinetic methods to determine ansamicins (rifamycin SV and rifampicin), based in the catalytic effect of Cu(II) ion on the aerial oxidation of rifamycin, is described. The effect of pH, temperature and presence of foreign species are studied. A selective and fast kinetic-photometric determination of ansamicins, between 0.50 and 10 mg ml 1 , is proposed by the application of a semiautomatic procedure in which the sampling-time is 20 s per sample. The method has been applied to determine rifamycin SV and rifampicin in pharmaceutical formulations and urine. Interferences like isoniazide and pyrazinamide are avoided. Statistical comparison with a chromatographic method is reported. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Ansamicins; Kinetic; Stopped-¯ow; Urine; Pharmaceutical analysis 1. Introduction Rifamycin SV and rifampicin belong to ansamicin group of antibiotics, with excellent therapeutic action. Specially rifampicin, a semisynthetic rifamycin from the naturally rifamycin B, is a potent chemotherapeu- tic agent very used in the pulmonary, urogenital and leprosy therapy. The most analytical methods for the analysis of rifamycin including spectrophotometry [1±4] and HPLC [5±15]. Rifampicin is the rifamycin more stu- died, probably due to its more extended use. In some of this methods, rifampicin is analyzed together others antituberculous antibiotics like isoniazid and pyrazi- namide, in pharmaceuticals and plasma samples [7,9,12]. 25-Deacetylrifampycin is a metabolite of the rifampicin obtained by biotransformation in the liver. Another degradation product is the rifampicin quinone obtained from the oxidation in alkaline med- ium [16]. Walash et al. [1] have analyzed mixtures of this compound and rifampicin in pharmaceuticals by spectrophotometry. Since rapid and precise analytical methods for quantitative analysis of rifamycin are required. Some speci®c advantages in the application of semiauto- matic kinetic method can be expected: 1. elimination of some experimental steps prior to absorbance measurements (e.g. ®ltration, extrac- tion, etc.), hence considerably simpler and faster Analytica Chimica Acta 376 (1998) 365±375 *Corresponding author. Fax: +34-24-289375. 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00537-6