An open-tubular enzyme reactor and an ammonium ion selective electrode using nonactin were prepared and connected sequentially to determine serum urea by flow injection method. Urease was immobilized on the inner surface of aminosilanized silicone rubber tubing. The effects of sample size, pH value and flow rate of the carrier solution on the enzyme reactor were investigated. The following practical standard conditions for the measurement were decided; sample size: 20 μl, pH value: 6.5, and flow rate of the carrier solution: 1.0ml min–1. The calibration range for urea concentration was from 2.0 up to 300 mM. The relative standard deviations for 10 replicate measurements of a urea standardand a control serum were 0.65% and 1.19%, respectively. Long-term stability of the urease reactor was improved by coating thin film of cellulose acetate over the whole immobilized enzyme layer. The measurement system was applied to serum samples from 20 healthy women. Good correlation between the present method and colorimetry was observed.
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