Flumequine (R&D use only) was obtained from Sigma,
ciprofloxacin and enrofloxacin were kindly provided by
the Burgos Health Lab. Tris(hydroxymethyl)aminomethane
(quality for analysis) was obtained from Scharlau laborato-ries, HCl (for analysis) and methanol (HPLC grade) were
supplied by Merck. Deionised water was obtained with a
Millipore Milli-Q gradient A10 system.