DETERMINATION OF AMMONIUM IN KJELDAHL DIGESTS BY GAS-DIFFUSION FLOW-INJECTION ANALYSIS WITH A BULK ACOUSTIC WAVE-IMPEDANCE SENSOR

A novel flow-injection analysis (FIA) system has been developed for th e rapid and direct determination of ammonium in Kjeldahl digests. The method is based on diffusion of ammonia across a PTFE gas-permeable me mbrane from an alkaline (NaOH/EDTA) stream into a stream of diluted bo ric acid. The trapped ammonium in the acceptor is determined on line b y a bulk acoustic wave (BAW)-impedance sensor and the signal is propor tional to the ammonium concentration present in the digests. The propo sed system exhibits a favorable frequency response to 5.0 x 10(-6)-4.0 x 10(-3) mol l(-1) ammonium with a detection limit of 1.0 x 10(-6) mo l l(-1), and the precision was better than 1% (RSD) for 0.025-1.0 mM a mmonium at a through-put of 45-50 samples h(-1). Results obtained for nitrogen determination in amino acids and for proteins determination i n blood products are in good agreement with those obtained by the conv entional distillation/titration method, respectively. The effects of c omposition of acceptor stream, cell constant of conductivity electrode , sample volume, flow rates and potential interferents on the FIA sign als were discussed in detail. (C) 1997 Elsevier Science B.V.