Automated analytical biosystem for urea monitoring

The present work describes the construction and automation of a flow injection (FI) system for on-line monitoring of urea. The FI system comprises a nylon column on the wall of which the enzyme urease is covalently immobilized. Urea is hydrolyzed to ammonium and hydrogen carbonate ions in the presence of urease. The final measurement is done by an ammonium ion sensitive electrode. This tubular shaped electrode is a potentiometric device that features a solid internal contact made of a graphite-epoxy composite. Interferents are separated from the analyte by means of a gas diffusion cell. The FI system is automated using electronic units that sample, electrically isolate, amplify and process the electrode signal. This hardware also controls the movements of the injection valve. The entire system is controlled by a computer program that governs the analytical process from sampling to signal processing. The urea analyzer was optimized following two approaches: firstly, signal and sensitivity maximization keeping the dispersion small, and secondly optimization when working with haemodialysis effluents. The first approach produced sensitivities ranging from 54 to 56 mV/decade and linear ranges from 3 × 10−5 to 3 × 10−3 M. The second approach produced 50–52mV/decade sensitivities and linear ranges between 1 × 10−3 and 2 × 10−2 M. The lifetime of the continuously working reactor was more than 30 days. The analyzer was applied to urea monitoring in haemodialysis processes both discretely and continuously, without any significant differences at 95% confidence levels when compared with classical measuring methods.