Simultaneous flow-injection determination of fluoride, monofluorophosphate and orthophosphate ions using alkaline phosphatase immobilized on a cellulose nitrate membrane and an open-circulation approach Original Research Article

A new flow-injection (FI) method is reported for the simultaneous determination of fluoride, monofluorophosphate (MFP) and orthophosphate ions in toothpaste. Fluoride and orthophosphate are determined in a first run using potentiometric (fluoride ion-selective electrode (ISE)) and spectrophotometric (molybdenum blue) detection, respectively, via a double-injection valve. The MFP ion is determined in a second run after quantitative hydrolysis to orthosphophate by alkaline phosphatase (APase) immobilized on a cellulose nitrate membrane, incorporated in an open-circulation FI system. The developed method enables the determination of both orthophosphate and MFP ions in the range of 1.0×10−6 to 2.0×10−4 mol l−1, using a single calibration graph and the determination of fluoride ion in the range of 4.0×10−6 to 1.0×10−2 mol l−1, while the relative standard deviations (sr) were satisfactory in all cases (sr=0.5% at 1.0×10−4 mol l−1 PO43−, 1.7% at 1.0×10−4 mol l−1 MFP and 1.6% at 1.0×10−3 mol l−1 F−; n=12). The developed method was applied successfully to the analysis of a commercial toothpaste sample, with the mean recoveries varying between 97.9 and 99.4% for all analytes.