Characterization of the composition of bovine urine and its effect on the electrochemical analysis of the model mediator, p-aminophenol

The reliable identification of compounds such as illegal growth promoters in cattle is generally based on expensive gas chromatography-mass spectrophotometric analysis in urine, a method that does not allow on a large-scale screening. The use of simple, semi-quantitative electrochemical biosensors may provide a means of screening for the presence of compounds such as illegal growth promoters. Before such sensors can be utilised, it is necessary to understand which factors influence the response of an electrochemical sensor in bovine urine. The concentration range of protein (0.01-0.04%), uric acid (0.5-0.65 mM), xanthine (0.02-0.12 mM) and ascorbic acid (0.1-0.95 mM) in 26 individual urine samples were determined. Using p-aminophenol (p-AP) as a model system, the electrochemical response increased by 5% in the presence of 6.0 mM uric acid, by 10% on the addition of 0.2 mM xanthine and by 22% in the presence of 1.0 mM ascorbic acid. Exposing urine to air and light for 75 min eliminated interference from ascorbic acid. Addition of Cu²⁺ (10 μM) reduced the time required to 34 min. Binding of species such as growth promoters to proteins may be disrupted by the addition of 8-anilino-1-naphthalene sulphonic acid (ANS) to the urine samples. Addition of 10 μM ANS did not affect the limit of detection of p-AP. The pH of fresh bovine urine samples was monitored over the period 7 to 192 h after collection and ranged from 8.00 to 8.77. The pH of lyophilised urine samples ranged from 8.24 to 9.60. Amperometry was the most sensitive method among a range of electrochemical techniques in the detection of p-AP with a limit of detection (LOD) in urine of 1.0 μg mL^-1 (10 μM) on a glassy carbon electrode.