Enzymic determination of ammonia in food by flow injection

Ammonia in food samples was determined by its reaction in an immobilised enzyme reactor containing glutamate dehydrogenase (GIDH) in a flow injection system, by measuring the decrease in the absorbance of ultraviolet radiation by reduced nicotinamide adenine dinucleotide (NADH). There was a linear relationship (r = 0.9995) between peak height and ammonia concentration over the range 0.05-0.6 mM. The detection limit was 0.005 mM for an injection volume of 19 microliters. Sampling frequency was 60 h-1 and the precision was better than 1.09% for 11 successive assays. The interference effect of urea and ascorbic acid at concentrations greater than 100 mg per 100 g of product should be taken into account. The interference caused by glycine, creatinine and amino acids is negligible. Only a 20% loss in the activity of the GIDH column was observed after 500 determinations during a 3-month period.