Fluoroquinolones as sensitizers of lanthanide fluorescence: application to the liquid chromatographic determination of ciprofloxacin using terbium

A time-resolved luminescence detection procedure is proposed for the in vivo liquid chromatographic determination of the important fluoroquinolone antibiotic ciprofloxacin. In a first step, the sensitized luminescence of lanthanides (samarium, terbium or europium) by three main fluoroquinolones, i.e. ciprofloxacin, pefloxacin and ofloxacin, was investigated. For this purpose, a preliminary study was made in order to determine the photophysical characteristics of the compounds of interest. Then, in the presence of lanthanide ions, the possibility of chelation, suggested by the α-keto acid skeleton of the fluoquinolone nucleus, was verified. Among the three lanthanides studied, Tb³⁺ was the most efficiently sensitized. Among the fluoroquinolones tested, ciprofloxacin was the best sensitizer. In a second step, the formation of a terbium chelate with ciprofloxacin was demonstrated in a classical reversed-phase eluent [methanol-water (50 + 50) containing trichloroacetic acid (pH 2.5)] using a postcolumn device. In this way, the ciprofloxacin-terbium chelate was selectivity excited at 320 nm and the terbium emission detected at 545 nm (⁵D₄ → ⁷F₅ main terbium transition). Postcolumn sensitization was performed using an alkaline methanolic reagent (1 × 10^-3 M Tb³⁺ -2 × 10^-2M triethylamine-1 × 10^-4 trioctylphosphine oxide). The chemical compatibility between the eluent and the postcolumn reagent was studied. The analytical characteristics were established. With the proposed liquid chromatographic method, the time-resolved luminescence detection mode allowed the detection of 0.25 pmol injected of ciprofloxacin, which easily meets the sensitivity required for the determination of fluoroquinolones in biological samples.