A novel immobilized enzyme flow-injection system for urea with a bulk acoustic wave impedance sensor as detector

The urea-urease system was chosen as a model system to explore the suitabil ity for the use of a bulk acoustic wave impedance sensor (BAWIS) as a detec tor in an immobilized enzyme FIA system. In the proposed FIA sysem, the ure a in samples was hydrolyzed into ammonium and bicarbonate under the catalys is of the immobilized urease. The ammonium formed was converted into ammoni a in an alkaline stream, and the ammonia diffused across a gas-permeable me mbrane into a recipient solution of dilute hydrochloric acid. The conductan ce change caused by the trapped ammonia was followed by a BAWIS detector. T he system was applied to the analysis of urea in blood and urine samples. I t exhibited a linear frequency response up to 10 mM urea with a detection l imit of 10 mu M. The through-put was 50 h(-1), and the relative standard de viation of the peak heights (n=6) ranged between 0.63-1.97%. The effects of several factors, including those affecting the enzymatic reaction, detecto r performance, and gas-diffusion process, were also investigated. Compared with those conventional enzymatic methods for urea determination, the propo sed immobilized urease FIA/BAWIS system is characterized by its rapid throu ghput, high reproducibility, low-cost apparatus, and easy manipulation.