Determination of iron species in food samples with dual direct immersion single-drop microextraction followed by graphite furnace atomic absorption spectrometry

A novel procedure of dual direct immersion single-drop microextraction (DDI-SDME) was developed for the sequential separation and preconcentration of Fe(III) and Fe(II) followed by graphite furnace atomic absorption spectrometry determination. At pH 1.5, Fe(III) can selectively react with N-benzoyl-N-phenylhydroxylamine (BPHA) to form the hydrophobic complexes which can be extracted into one organic drop, while Fe(II) remained in the solution. Then, another organic drop containing BPHA was immersed in the sample solution after the extraction of Fe(III) for the preconcentration of Fe(II) at pH 4.5. This procedure eliminated the time-consuming and labor-intensive step of oxidation of Fe(II) or reduction of Fe(III), which may cause the incomplete conversion of the species and sample contamination. Main conditions influencing the separation and enrichment of Fe species were studied. Under the selected conditions, the detection limits of this procedure were 0.058 ng mL(-1) and 0.074 ng mL(-1) for Fe(III) and Fe(II) with relative standard deviations of 4.8% and 5.6%, respectively. Enrichment factors of 300-fold were obtained for Fe species. The proposed procedure was successfully utilized for detecting Fe(III) and Fe(II) in food samples. To evaluate accuracy of this procedure, a certified reference material of milk powder was analyzed, and the determined value was in good agreement with the certified value.