NADH ELECTROCHEMICAL SENSOR FOR THE ENZYMATIC DETERMINATION OF L-LACTATE AND D-LACTATE AND 3-HYDROXYBUTYRATE USING A FLOW-INJECTION ANALYSIS

A NADH sensor in the submicromolar range, assembled in a wall-jet cell using a flow injection analysis (FIA) procedure, was studied and coup led with enzyme reactors. Spectroscopic graphite was used as electrode material without any mediator. The measurements were performed at an applied potential of +500 mV (vs. Ag/AgCl). L-, D-lactate. and 3-hydro xybutyrate dehydrogenases were immobilized on aminopropyl-controlled p ore glass beads using glutaraldehyde in a packed-bed enzyme reactor. L -, D-lactate, and 3-hydroxybutyrate were oxidized in the presence of N AD+, and an equivalent amount of NADH was produced and measured. By ca reful choice of pH, buffer, and NAD+ concentration, the analytes could be measured in the range of 1 X 10(-6) to 1 x 10(-4) M in a few secon ds. Large blank effects were evidenced with this procedure if the carr ier and the sample differed in ionic strength of about 1 X 10(-3) M wh en the total ionic strength is 0.8 M. Therefore, the use of such a sys tem for real samples requires further investigation.