Determination of l-asparagine using flow-injection systems with spectrophotometric and potentiometric detection

Two different flow-injection analysis (FIA) approaches are described for the determination of asparagine in food samples. For the first method an asparaginase reactor was integrated in a gas-diffusion flow-injection system. This method is based on the measurement of the pH change in an acceptor solution caused by diffused ammonia generated by asparagine hydrolysis catalzyed by immobilized asparaginase. The detection was carried out spectrophotometrically using an acid-base indicator solution as acceptor, and potentiometrically using a pH electrode and bidistilled water as acceptor. The linear range was observed from 0.2−2.3 mmol l−1 asparagine for spectrophotometric detection with a relative standard deviation (r.s.d.) of 1.7% at 0.75 mmol l−1 asparagine (n = 5) and 0.1−4 mmol l−1 for potentiometric detection with a r.s.d. of 2.5% at 1 mmol l−1 asparagine (n = 5). A sample frequency of 35 h−1 was achieved. The results of sample assays using spectrophotometric and potentiometric detection were in good agreement, a recovery of 94.3−98.2% for spectrophotometric detection and 99.1–102.3% for potentiometric detection was observed. This method is highly selective and the sample preparation is very simple. The immobilized enzyme is stable for more than two months or 500 assays. For the second system an asparaginase biosensor is constructed by fixing an enzyme membrane on a pH electrode. This biosensor was used in a FIA system for the determination of asparagine in food samples. A sample frequency of 30 h−1 was achieved. The calibration is linear from 0.1 − 2.0 mmol l−1 for asparagine. The sensor can be used repeatedly at room temperature for at least two weeks with little decline in response.