Determination of iodine content in Fijian foods using spectrophotometric kinetic method

Abstract

The spectrophotometric kinetic method was validated for the determination of iodine in different food samples. The method is based on the iodide catalysed reduction of Ce4+ to Ce3+ by As3+. The absorbance of the kinetic indicator reaction was measured at 370 nm for exactly 1 min at 37 °C. The change in absorbance per min, as a measure of initial rate, was plotted against the different iodine concentrations to achieve a linear calibration equation with the R2 value of 0.9998 which showed excellent reproducibility. The limit of detection (LOD) was 1.54 ng/mL and the limit of quantification (LOQ) was 4.90 ng/mL. The incineration of the food organic matter was achieved by ashing the food samples at 600 °C using KOH and ZnSO4 in steps for 3 h. Trace levels of iodine (ng) were determined successfully using the validated spectrophotometric kinetic method for the 9 food samples (36 sub-samples). The Fiji seaweeds, lumiwawa (brown seaweed) showed the highest iodine content being 6373.30 ± 0.39 ng/g followed by sea grapes (green seaweed) 1162.81 ± 0.61 ng/g, lettuce 114.81 ± 0.08 ng/g, English cabbage 108.40 ± 0.06 ng/g, Chinese cabbage 104.01 ± 0.06 ng/g, pumpkin 101.24 ± 0.08 ng/g, long bean 97.61 ± 0.10 ng/g, banana 76.18 ± 0.10 ng/g and tomato 40.32 ± 0.04 ng/g. The coefficient of variation for the sample analysis was <5.31% with a mean and standard deviation of 2.55 ± 0.17% for the food samples analysed. The recovery analysis of iodine from standard samples ranged from 99.84 ± 0.91% to 100.24 ± 5.92% with an excellent average recovery of 100.06 ± 3.16%. The analytical coefficient of variation was calculated to be 0.34% for the food samples analysed. This shows exceptional system analytical stability of the method used in this study.