Rapid flow injection spectrophotometric determination of monofluorophosphates in toothpastes after on-line hydrolysis by alkaline phosphatase immobilized on a cellulose nitrate membrane

A new, rapid flow injection (FI) method is reported for the spectrophotomet ric determination of monofluorophosphate (WP) ions in toothpastes. MFP ions are hydrolyzed on-line by alkaline phosphatase (APase) immobilized on a ce llulose nitrate membrane, prior to injection in the FI system. The yielded orthophosphate ions are determined spectrophotometrically (lambda (max) = 6 90 nm) using the molybdenum blue approach. The chemical and FI variables th at affected the enzymatic reaction were studied and optimized. A study of i nterferences was also carried out. The proposed method is very precise (s(r ) = 0.7% at 1.0 x 10(-4) mol l(-1) MFP. n = 12), fast (sampling rate of 72 h(-1)) and allows the determination of MFP ions in the range of 4.0 x 10(-5 ) to 6.0 x 10(-4) mol l(-1) with a satisfactory 3 sigma detection limit of 4.0 x 10(-6) Mol l(-1). The application of the proposed Fl method to toothp aste samples yielded accurate results (e(r) < 2.0%) compared with a potenti ometric reference procedure.