Amperometric determination of xanthine in tea, coffee, and fish meat with graphite rod bound xanthine oxidase

A method is described for construction of an amperometric xanthine biosensor based on graphite rod modified through adsorption of xanthine oxidase. Enzymatically produced H2O2 from xanthine was split into 2H(+) + O-2 + 2(e-) at 0.6 V and the current was measured, which was directly proportional to xanthine concentration ranging from 1 A degrees 10(-7) to 6 A degrees 10(-7) M with a detection limit of 1 A degrees 10(-7) M. The biosensor exhibited optimum response within 35 sec at pH 7.0 and 35A degrees C. It was employed for determination of xanthine in tea leaves (0.9 A degrees 10(-5)-2.5 A degrees 10(-5) mmol/g), coffee powder (3.2 mu mol/g) and fish meat (90 mmol/g). The content of xanthine in fish meat increased 6.5 times with its storage at room temperature during 15 days. The enzyme electrode could be reused 200 times during the span of 30 days, when stored in reaction buffer at 4A degrees C.