Fluoride determination by its inhibitory effect on immobilised liver esterase

The inhibition of liver esterase (EC 3.1.1.1.) immobilised on controlled pore glass by fluoride ions was studied in a flow injection system. Ethyl butyrate was used as substrate, ethanol and butyric acid being produced as hydrolysis products. To monitor the reaction a second enzymatic reaction was employed, in which alcohol dehydrogenase, immobilised on controlled pore glass, catalysed the oxidation of ethanol and reduction of the coenzyme, NAD+, to NADH, the latter being determined spectrophotometrically at 340 nm. Fluoride inhibition of the immobilised liver esterase was fast and reversible, and there was a linear relationship between %inhibition and fluoride concentration over the range 8 × 10−7−8 × 10−6 M (15–150 μg 1−1) fluoride. The relative standard deviation at mid-calibration range is below 4% (n = 5). The more common inorganic compounds present in water samples do not interfere in the fluoride determination.