Flow injection spectrophotometric determination of fosfestrol, following on-line thermal induced digestion and using an orthophosphate calibration graph

A novel flow injection (FI) system for the spectrophotometric determination of diethyl stilbestrol diphosphate (fosfestrol) in pharmaceutical formulations is described. On-line thermal induced digestion of the analyte by peroxodisulfate ion was performed and the orthophosphate ion generated was determined spectrophotometrically (λ_max=690 nm) using a molybdenum blue based FI approach. As the achieved conversion of the analyte was quantitative, an orthophosphate calibration graph can be used for its determination as well. The chemical and FI variables affecting the digestion were investigated. A linear calibration graph was obtained in the range 5.0×10⁻⁷-1.0×10⁻⁴ mol l⁻¹ fosfestrol. The relative standard deviation was very good (s_r=0.8% at 5.0×10⁻⁵ mol l⁻¹ fosfestrol, n=12) and the 3σ detection limit was 2.5×10⁻⁷ mol l⁻¹. The sampling rate was 60 injections h⁻¹. The average accuracies for the determination of the analyte in a pharmaceutical formulation evaluated by comparison of the results with those obtained by the supplier (Asta Medica) and the method recommended by the US Pharmacopoeia were also very good (e_r of +0.8 and −0.3%, respectively). Average recoveries of known amounts of the analyte ranging between 97.9 and 100.8% were also obtained.