Vapour generation inductively coupled plasma optical emission spectrometry in determination of total iodine in milk

Abstract

Vapour generation inductively coupled plasma optical emission spectrometry (VG-ICP-OES) was used for the determination of the iodine content in milk samples. Modified alkaline ashing of the milk sample was employed for the total digestion of organic matrix constituents. A mixture of potassium hydroxide (2 mL, 2 mol L⁻¹ in ethanol) and calcium nitrate (2 mL, 0.4 mol L⁻¹ in ethanol) was used in the digestion step as an ashing aid for 2 ml of milk sample (or 200 mg of milk powder). Sample ash was then treated with hydrochloric acid (1.5 mL, 5 mol L⁻¹) and sodium sulfite (1.25 mL, 1 mol L⁻¹) for the elimination of carbonates, which would otherwise cause spectral interference. The vapour of elemental iodine was generated by means of oxidation of iodide by hydrogen peroxide (1 mol L⁻¹) in sulfuric acid (5 mol L⁻¹). Using a gas phase separator, matrix elements were completely eliminated; thus the spectral interference of phosphorus (178.222 nm) was removed and the most intense analytical line of iodine (178.218 nm) could be used for determination. The accuracy of the method was verified by analysis of CRM Non-Fat Milk Powder 1549 from NIST, as well as by the comparative study of VG-ICP-OES and ICP-MS results after alkaline solubilization (TMAH 2.5 mL, 10%). The limit of detection for the VG-ICP-OES method is 20 μg L⁻¹ for original milk samples (based on 3σ of blank value). The concentration of iodine in real milk samples was found in the range 0.2–0.8 mg L⁻¹ with a precision of determination from 0.5 to 3.5% (3 replicates).