Fluorometric determination of urea in alcoholic beverages by using an acid urease column-FIA system

An acid urease column was applied to a fluorometric flow-injection analysis (FIA) system as a recognition element for determination of urea in rice wines.

The acid urease has specific properties of showing its catalytic activity in low pH range and tolerance to ethanol in comparison to those of a urease from jack-beans. The enzymes were covalently immobilized onto porous glass beads with controlled pore size and then, packed into a small polymer column. The flow-type of the biosensing system was assembled with a sample injection valve, the immobilized enzyme column, and a flow-through quartz cell attached to a fluorescent spectrophotometer. Citrate buffer (50 mM, pH 5.0) as the carrier solution was continuously pumped through the system. Sample solutions were introduced into the system via a rotary injection valve. A standard urea solution was measured through monitoring variations in fluorescent intensity attributable to fluorescent isoindole derivatives formed by coupling with ammonia molecules released in the enzymatic hydrolysis of urea and orthophthalaldehyde reagents. The fluorescent intensity was measured under the conditions of λ_ex = 415 nm and λ_em = 485 nm. A wide, linear relationship was obtained between the concentration of urea (1.0–100 μM) and the variation in fluorescent intensity. The monitoring did not suffer from ethanol and various amino acids contained in rice wines. Real samples pretreated with ion exchange resins for removal of endogenous ammonia were introduced into the FIA system and urea in the samples was determined. These results were compared with those obtained with use of an F-kit method. The proposed FIA system should present sensitive, selective and convenient analysis of urea in alcoholic beverages.