AMPEROMETRIC AMMONIUM ION AND UREA DETERMINATION WITH ENZYME-BASED PROBES

Amperometric enzyme probes for ammonium and urea have been assembled a nd evaluated using immobilized glutamate dehydrogenase and urease enzy mes coupled with platinum electrodes. Analytical parameters such as pH , buffer, temperature, probe life-time, enzyme immobilization, cofacto r concentration and response time have been optimized. Ammonium was de tected in the range 10(-5)-3 x 10(-4) mol l(-1). Better reproducibilit y 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 th e range 10(-5)-3 x 10(-4) mol l(-1) using the enzyme urease first in s olution and then immobilized on nylon net. The analysis was based on a n amperometric measurement which gives a linear relationship between c urrent and analyte concentration. This considerably improved the sensi tivity of the analysis when compared with the potentiometric-based pro cedures. Moreover, this method does not suffer from the potassium ion interference which affects the potentiometric nonactin-based NH4+ elec trodes. Analysis of ammonium and urea were carried out in standard sol utions and in saliva samples. Results compared with a spectrophotometr ic reference procedure correlated well.