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 spectrophotometric determination of monofluorophosphate (MFP) ions in toothpastes. MFP ions are hydrolyzed on-line by alkalinephosphatase (APase) immobilized on a cellulose nitrate membrane, prior to injection in the FI system. The yielded orthophosphate ions are determined spectrophotometrically (lambda(max) = 690 nm) using the molybdenum blue approach. The chemical and FI variables that affected the enzymatic reaction were studied and optimized. A study of interferences 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 3sigma detection limit of 4.0 x 10(-6) mol l(-1). The application of the proposed FI method to toothpaste samples yielded accurate results (e(r) < 2.0%) compared with a potentiometric reference procedure.