Microwave-assisted distillation of iodine for the indirect atomic absorption spectrometric determination of iodide in milk samples

Abstract

To complete these microwave distillation studies, a method for iodide determination in milk-type samples is proposed. The iodide contained in the sample is oxidized to iodine, which is distilled by means of microwave energy and reduced back to iodide. This iodide is combined with Hg(II) and 2,2′-dipyridyl to give an ion pair, which is selectively extracted into IBMK. Mercury is determined in the extract by ETAAS in order to determine iodide. The yield of the distillation process (100%) was evaluated using the Sandell–Kolthoff reaction. In addition, for the solvent extraction, no buffer was needed; 3/5 was the best phase ratio (organic/aqueous); 30 s was the shaking time and the extracts were stable for 24 h, at least. The yield of the extraction procedure (studied also using the Sandell–Kolthoff reaction) was 87.5 and 87.1%, for standards and samples, respectively. Regarding the ETAAS measurements, the ashing temperature selected for both samples and standards was 150 °C; 800 and 1000 °C were the atomization temperatures chosen for samples and standards, respectively. Palladium in IBMK was the chemical modifier at a concentration of 3.0 µg ml−1, both for samples and standards. The sensitivity was 3.50 × 10−3 l µg−1 (standard additions); the LOD and LOQ were 0.2 and 0.6 µg ml−1, referred to sample; the mean characteristic mass within the linear range of concentrations (0–17.5 µg l−1) was 27.3 pg; the mean analytical recovery within that range was 97.5% for a liquid whole milk sample and 100.6% for a CRM, which also gave results within certification. The method was applied to liquid whole milk, milk powder and infant formulae samples.