FeatureArticle VoltametricandFlowInjectionDeterminationofOxytetracycline ResiduesinFoodSamplesUsingCarbonFiberMicroelectrodes L. Ag¸Ì, A. Guzman, M. Pedrero, P. Ya¬nƒez-Sedenƒo, J. M. Pingarro¬n* Department of Analytical Chemistry, Faculty of Chemistry, University Complutense of Madrid, 28040-Madrid, Spain *e-mail: pingarro@quim.ucm.es Received: May 16, 2002 Final version: August 7, 2002 Abstract A voltammetric method for the determination of the antibiotic oxytetracycline (OTC) in food samples is reported. Carbon fiber microelectrodes (CFMEs), which allow voltammetric measurements to be performed in a small volume (1 mL) of the analyte extract from the samples, are employed. Repeatable electroanalytical responses were obtained with no need of applying cleaning treatments to the CFME. Under the optimized square-wave conditions, a linear calibration plot for OTC was obtained in the 1.0  10 6 ±1.0  10 4 molL 1 range, with a detection limit of 2.9  10 7 molL 1 (150ngmL 1 ) OTC. The determination of OTC by a flow-injection method with amperometric detection using a homemade flow cell specially designed to work with CFMEs, was also evaluated using pure acetonitrile as the carrier. The SW voltammetric method was applied to the determination of OTC in spiked milk and eggs samples, at 100ngmL 1 and 200 ng g 1 levels, respectively. The procedure involved the extraction of the analyte in ethyl acetate, evaporation of the solvent and reconstitution of the residue in acetonitrile  5.0  10 4 molL 1 tetrabutylammonium perchlorate medium. Recoveries of 96  8 and 91  8% were obtained for milk and eggs, respectively, by applying the standard additions method. Keywords: Oxytetracycline, Carbon fiber microelectrodes, Milk, Eggs, Flow injection 1. Introduction Administrationofbroad-spectrumantibioticstocattle,pigs, sheep and poultry is a routine practice in veterinary medicine. Among them, tetracyclines (TCs) have been widelyusedforthepreventionandtreatmentofdiseasesina variety of food-producing animal species. However, the widespreaduseofthesecompoundsincreasesthelikelihood that residues of the drugs may remain in tissue intended for human consumption or in milk. Consequently, the Euro- peanUnionhassetthemaximumresiduelimits(MRLs)for tetracyclines in various food products. So, for example, MRL in bovine milk is of 100 ngg 1 [1]. The determination of TCs in biological samples implies some important difficulties as a consequence of their polar amphoteric nature, which gives rise to unfavorable proper- ties for chromatography and extraction from samples [2]. Moreover, they are unstable in acidic or alkaline media and photodegradable. In recent years, several HPLC methods for the determination of various TCs in different samples have been described [2±8]. Reversed-phase chromatogra- phy using C 18 columns and acidic oxalate buffer±acetoni- trile gradients [3±5], ion-pairing chromatography using alkyl sulfonates [6, 7], and metal chelate affinity chroma- tography [8] have been employed for the analysis of animal tissues and foods. LC-MS and tandem MS methods, which havebeenrecommendedbyEUforconfirmatoryassays[9], havebeenalsoreported[5,10,11].Capillaryelectrophoresis (CE) [12±15] and micellar electrokinetic chromatography (MEKC) [16, 17] have been also used in the separation of TCsandimpuritiesincommercialproducts,aswellasforthe determination of residues in biological samples. Further- more, a variety of chromatographic methods for the determination of individual TCs, such as minocycline [17], chlortetracycline [18, 19], doxycycline [20] and oxytetracy- cline [21±25] have been reported. The use of chromatographic methods for the determina- tionofTCsinfoodstuffs,tissuesorbiologicalfluidsrequires acarefulsamplepreparation.Eitherliquid-liquidextraction or solid phase extraction (SPE) have been usually em- ployed.RegardingSPEprocedures,amajordrawbackisthe irreproducibility found when TCs are analyzed. Thus, for example, collaborative studies [4] have demonstrated an important variability in the recovery of the analytes when different lots of solid-phase extraction cartridges were employed. On the other hand, the electrochemical properties of some tetracycline antibiotics at mercury and glassy carbon electrodeshavebeenstudied[26].Polarography[27,28]and adsorptive stripping voltammetry [29±31] at mercury electrodes have been used to develop electroanalytical methods for TCs. Amperometric detection at glassy carbon electrodes after liquid chromatography has been also used to determine tetracycline antibiotics in pharmaceutical formulations[32,33].Furthermore,amethodusingcapillary electrophoresis with voltammetric detection has been also described [34]. Most of these methods allow one to achieve detection limits at the level of the maximum residue limits 601 Electroanalysis 2003, 15, No.7 ¹ 2003 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim 1040-0397/03/0704-0601 $ 17.50+.50/0