Amperometric ammonium ion and urea determination with enzyme-based probes

Amperometric enzyme probes for ammonium and urea have been assembled and evaluated using immobilized glutamate dehydrogenase and urease enzymes coupled with platinum electrodes. Analytical parameters such as pH, buffer, temperature, probe life-time, enzyme immobilization, cofactor concentration and response time have been optimized. Ammonium was detected in the range 10(-5)-3 x 10(-4) mol l(-1). Better reproducibility and stability were achieved using the enzyme GLDH type III and NADH at a concentration of 10(-3) mol l(-1). Urea has been determined in the range 10(-5)-3 x 10(-4) mol l(-1) using the enzyme urease first in solution and then immobilized on nylon net. The analysis was based on an amperometric measurement which gives a linear relationship between current and analyte concentration. This considerably improved the sensitivity of the analysis when compared with the potentiometric-based procedures. Moreover, this method does not suffer from the potassium ion interference which affects the potentiometric nonactin-based NH4+ electrodes. Analysis of ammonium and urea were carried out in standard solutions and in saliva samples. Results compared with a spectrophotometric reference procedure correlated well.