Polarographic determination of lisinopril in pharmaceuticals and biological fluids through treatment with nitrous acid

A simple and highly sensitive polarographic method was developed for the determination of lisinopril in dosage forms and biological fluids. The method is based on treatment of the compound with nitrous acid followed by measuring the cathodic current produced by the resulting nitroso derivative. The polarographic behavior was studied adopting direct current (DCt), differential pulse (DP) and alternating current (AC(t)) polarography. A well-defined, diffusion-controlled cathodic wave over the pH range of 1.0-8.0 was obtained in Britton-Robinson buffers (BRb). At pH 3.0, the value of limiting diffusion-current constant (K) was 8.42 +/- 0.23 (n = 7). The limiting diffusion current-concentration relationship was found to be rectilinear over the range of 2-24 mug/mL and 0.1-20 mug/mL using DCt and DP polarographic modes, respectively. The minimum detectability was (S/N = 2) 0.02 mug/mL (4.54 x 10(-8) M). The proposed method was successfully applied to the determination of lisinopril either per se or in dosage forms and the results obtained were in good agreement with those given using a reference method. The proposed method was further applied to the determination of lisinopril in spiked human urine and plasma. The percentage recoveries adopting the DP polarographic mode were 99.71 +/- 1.87 and 97.16 +/- 1.09, respectively.