Determination of Iodide and Iodate in Soy- and Milk-Based Infant Formulas

Significance of the Topic

Iodine is an essential micronutrient required for thyroid hormone synthesis and normal growth and development. Accurate quantification of iodine species in infant formula is critical because both deficiency and excess intake can cause health disorders. Regulatory authorities set compositional limits and mandate monitoring of iodine in fortified foods to ensure nutritional safety and efficacy.

Objectives and Study Overview

This study presents a robust ion chromatography–pulsed amperometric detection method for determining iodide and iodate in milk- and soy-based infant formulas. The aim was to optimize sample preparation, including acetic acid digestion and optional microwave or heat-assisted extraction, and to evaluate method performance for free and bound iodine species.

Methodology and Instrumentation

• Sample Preparation: Formulas underwent acetic acid hydrolysis at room temperature or 70 °C, followed by centrifugation, filtration through 0.2 μm membranes, and cleanup with OnGuard II RP cartridges. Microwave digestion with acetic acid was also tested.
• Conversion of Iodate to Iodide: Reduction with 5 g/L ascorbic acid under acidic conditions converted iodate to iodide prior to analysis.
• Chromatography and Detection: Separation on Dionex IonPac AG11/AS11 columns (4 × 250 mm and 4 × 50 mm guard) with 50 mM nitric acid eluent at 1.5 mL/min and 30 °C. Detection used a silver working electrode in PAD with Ag/AgCl reference.
• Instrumentation: Thermo Scientific Dionex ICS-5000 IC system with DC detector, ED electrochemical detector, AS autosampler, and Chromeleon CDS.

Main Results and Discussion

• Linearity and Sensitivity: Iodide response was linear from 0.005 to 10 mg/L (R² = 0.9998), with a quantitation limit of 0.005 mg/L (S/N ∼10) and baseline noise of 3–6 pC.
• Precision: Retention time and peak area RSDs were <0.6% and <6%, respectively, over 800 injections.
• Free vs. Bound Iodide: Room-temperature digest recovered free iodide consistent with label claims for most formulas, whereas 70 °C digestion and microwave extraction released additional bound iodine, improving recovery for one underperforming sample.
• Iodate Analysis: Complete conversion of iodate to iodide using ascorbic acid yielded recoveries of 94–103% after heat-assisted digestion and 80–128% at room temperature.
• Accuracy: Spiked recoveries in infant formulas ranged from 82–115% across three days, demonstrating minimal matrix effects and reliable quantification.

Benefits and Practical Applications

This IC-PAD protocol offers high selectivity, sensitivity, and throughput for routine quality control of iodine species in fortified foods. It avoids hazardous perchloric acid digestion, accommodates complex protein and fat matrices, and can be adapted for total iodine determination, supporting regulatory compliance and nutritional monitoring.

Future Trends and Applications

Advances may include online sample preparation, higher resolution columns, and coupling to mass spectrometry for lower detection limits. Expansion to speciation of other halogens and automation for large-scale survey studies can further enhance food safety and nutritional analytics.

Conclusion

The described IC-PAD method delivers accurate, precise, and efficient quantification of iodide and iodate in milk- and soy-based infant formulas. Optimized extraction conditions enable assessment of free and bound iodine, while ascorbic acid reduction facilitates total iodine measurement. The approach meets regulatory needs and supports comprehensive nutritional analysis.

Reference

1. Haldimann M, Alt A, Blanc K, Blondeau K. Iodine content of food groups. J Food Comp Anal. 2005;18:461–471.
2. FAO/WHO. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. 2001.
3. Azizi F, Hedayati M, Rahmani M, et al. Reappraisal of the risk of iodine-induced hyperthyroidism. J Endocrinol Invest. 2005;28:23–29.
4. Anke M, Groppel B, Müller M, Scholz E, Kramer K. The iodine supply of humans depending on site, food offer and water supply. Fresenius J Anal Chem. 1995;352:97–101.
5. WHO. Human Vitamin and Mineral Requirements. 2002.
6. Fisher DA. Upper limit of iodine in infant formulas. J Nutr. 1989;119:1865–1868.
7. Benofti J, Benotti N. Protein-bound iodine, total iodine. Clin Chem. 1963;9(4):408–416.
8. Mitsuhashi T, Kaneda Y. Gas chromatographic determination of total iodine in foods. J AOAC Int. 1990;73(5):790–792.
9. Hammer D, Andrey D. Comparison of ion-selective electrode and ICP-MS to determine iodine in nutritional products. J AOAC Int. 2008;91(6):1397–1401.
10. Crecelius EA. Determination of total iodine in milk by X-ray fluorescence spectrometry and iodide electrode. Anal Chem. 1975;47:2034–2035.
11. Naozuka J, Mesquita Silva da Veiga AM, Oliveira PV, de Oliveira E. Determination of chlorine, bromine, and iodine in milk samples by ICP-OES. J Anal At Spectrom. 2003;18:917–921.
12. Pacquette LH, Levenson AM, Thompson JJ. Determination of total iodine in infant formula by ICP-MS: Single-laboratory validation. J AOAC Int. 2012;95(1):169–176.
13. Osterc A, Stoš K, Stibilj V. Investigation of iodine in infant formula. Food Control. 2006;17:522–526.
14. Niemann RA, Anderson DL. Determination of iodide and thiocyanate in powdered milk. J Chromatogr A. 2008;1200:193–197.
15. Chadha RK, Lawrence JF. Determination of iodide in dairy products. J Chromatogr. 1990;518:268–272.
16. Liang L, Cai Y, Mou S, Cheng J. Disposable vs. conventional silver electrode for iodide determination. J Chromatogr A. 2005;1085:37–41.
17. Thermo Fisher Scientific. Application Update 122: The Determination of Iodide in Brine. 2012.
18. Yang SX, Fu SJ, Wang ML. Determination of trace iodine by cathodic stripping voltammetry. Anal Chem. 1991;63:2970–2973.
19. Yebra MC, Bollaín MH. Indirect method to determine total iodine in milk products by FAAS. Talanta. 2010;82(2):828–833.
20. AOAC Int. Official Method 992.24. Iodine in ready-to-feed milk-based infant formula. 2007.
21. Federal Ministry for Consumer Protection. LMBG 49.00-6. 1998.
22. CEN. EN15111:2007. 2007.
23. ISO. ISO 14378:2009. 2009.
24. Knapp G, Maichin B, Fecher P, Hasse S. Iodine determination in biological materials. Fresenius J Anal Chem. 1998;362:508–513.
25. AOAC Int. SMPR 2012. Determination of iodine in nutritional formula. 2012.
26. Wang LM, Lin YK, Wang BT, Yan Z, Li Y. Determination of iodine in infant formula cereals by ion chromatography. J Instrumental Analysis. 2011;30(1):99–102.
27. Sanchez LF, Szpunar J. Speciation analysis for iodine in milk by SEC-ICP-MS. J Anal At Spectrom. 1999;14:1697–1702.