Determination and speciation of trace iron in fruit-flavored milk samples by extractive spectrophotometry

This study aims to develop a novel method for the determination of trace iron (as Fe(II) and total Fe) in fruit-flavored and unflavored milk by extractive spectrophotometry. The method is based on the ion-pair formation among charge transfer sensitive reagents and Fe(II) at pH 3.0. Extraction of the Fe from the sample matrix was greatly accelerated and facilitated by ultrasonication. After extraction, the micelle-rich phase was diluted with acetone, and monitored at 340 nm by spectrophotometer. The variables affecting the sensitivity for quantitative extraction were evaluated and optimized in detail. The performance of the method was determined under optimal conditions. A good linear relationship was obtained in the range of 1-62.5 µg L -1 with limits of detection and quantification of 0.31/1.03 and 0.27/0.89 µg L -1 for free Fe(II) and total Fe after pre-reduction of Fe(III) to Fe(II) with ascorbic acid-KI mixture, respectively. The recovery rate of the method was in range of 91.6-96.1% with a relative standard deviation of less than 6.1%. A significant matrix effect, except for signal enhancement of 6.5 and 6.7% from comparison of slopes of calibration curves, was not observed. After validation by analysis of two certified samples, the method was successfully applied to the analysis of the selected milk samples. • UA-CPE was combined with spectrophotometry, showing good feasibility. • Triton X-114 was effectively used to improve the extraction efficiency of UA-CPE. • Limitations associated with the sensitivity of spectrophotometry were greatly reduced. • Its analytical performance with sensitivity enhancement of 46/42-fold was improved. • Low LODs of 0.31/0.18 and 0.27/0.29 µg L -1 were obtained with a RSD lower than 6.1%.