title: Determination of ammonium in Kjeldahl digests by gas-diffusion flow-injection analysis with a bulk acoustic wave-impedance sensor.
abstract: A novel flow-injection analysis (FIA) system has been developed for the rapid and direct determination of ammonium in Kjeldahl digests. The method is based on diffusion of ammonia across a PTFE gas-permeable membrane from an alkaline (NaOH/EDTA) stream into a stream of diluted boric acid. The trapped ammonium in the acceptor is determined on line by a bulk acoustic wave (BAW)-impedance sensor and the signal is proportional to the ammonium concentration present in the digests. The proposed 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) mol l(-1), and the precision was better than 1% (RSD) for 0.025-1.0 mM ammonium at a through-put of 45-50 samples h(-1). Results obtained for nitrogen determination in amino acids and for proteins determination in blood products are in good agreement with those obtained by the conventional distillation/titration method, respectively. The effects of composition of acceptor stream, cell constant of conductivity electrode, sample volume, flow rates and potential interferents on the FIA signals were discussed in detail.
authors.affiliation: College of Chemistry and Chemical Engineering, Hunan University, Changsha 410 082, People's Republic of China.
authors.forename: X L
authors.initials: XL
authors.lastname: Su
authors.forename: L H
authors.initials: LH
authors.lastname: Nie
authors.forename: S Z
authors.initials: SZ
authors.lastname: Yao