Preconcentration and determination of vanadium and molybdenum in milk, vegetables and foodstuffs by ultrasonic-thermostatic-assisted cloud point extraction coupled to flame atomic absorption spectrometry

A new ultrasonic-thermostatic-assisted cloud point extraction procedure (UTA-CPE) was developed for preconcentration at the trace levels of vanadium (V) and molybdenum (Mo) in milk, vegetables and foodstuffs prior to determination via flame atomic absorption spectrometry (FAAS). The method is based on the ion-association of stable anionic oxalate complexes of V(V) and Mo(VI) with [9-(diethylamino) benzo[a]phenoxazin-5-ylidene]azanium; sulfate (Nile blue A) at pH 4.5, and then extraction of the formed ion-association complexes into micellar phase of polyoxyethylene(7.5)nonylphenyl ether (PONPE 7.5). The UTA-CPE is greatly simplified and accelerated compared to traditional cloud point extraction (CPE). The analytical parameters optimized are solution pH, the concentrations of complexing reagents (oxalate and Nile blue A), the PONPE 7.5 concentration, electrolyte concentration, sample volume, temperature and ultrasonic power. Under the optimum conditions, the calibration curves for Mo(VI) and V(V) are obtained in the concentration range of 3-340 mu g L-1 and 5-250 mu g L-1 with high sensitivity enhancement factors (EFs) of 145 and 115, respectively. The limits of detection (LODs) for Mo(VI) and V (V) are 0.86 and 1.55 mu g L-1, respectively. The proposed method demonstrated good performances such as relative standard deviations (as RSD %) (<= 3.5%) and spiked recoveries (95.7-102.3%). The accuracy of the method was assessed by analysis of two standard reference materials (SRMs) and recoveries of spiked solutions. The method was successfully applied into the determination of trace amounts of Mo(VI) and V (V) in milk, vegetables and foodstuffs with satisfactory results. (C) 2016 Elsevier B.V. All rights reserved.