Determination of citrate in tablets and of oxytetracycline in serum using europium (III) luminescence

A direct method for the determination of citrate and oxytetracycline in samples containing complex matrices like tablets or serum has been developed using the luminescence of the ternary complex formed with Eu(III) ions. The triplet-state energy level of oxytetracycline (OxTc), the excitation maximum (412 nm) and the luminescence lifetime of Eu–OxTc (58 μs) were determined. A 17-fold luminescence enhancement at 615 nm occurs upon addition of citrate within a short 5-min incubation time at neutral pH. This is accompanied by a threefold increase of the luminescence decay time. The optimal conditions for determination of OxTc are equal concentrations of Eu (III) and citrate (C = 1 · 10− 4 mol L− 1) and pH 7.2. For determination of citrate, the optimal concentrations of Eu(III) and OxTc are 1 : 0.5 (CEu = 1 · 10− 4 mol L− 1, COxTc = 5 · 10− 5 mol L− 1) at pH 7.2. The linear range for determination of OxTc in serum is 0.25–250 μg mL− 1, and for citrate in tablets from 0.5 to 10.0 μg mL− 1 (2.3 · 10− 6– 4 · 10− 5 mol L− 1). The detection limit was 0.1 μg mL− 1 for OxTc and 0.2 μg mL− 1 (1 · 10–6 mol L− 1) for citrate, respectively. A comparison of the new method with other methods for determination of citrate is given.