Iodine is a key nutrient for regulating important biochemical reactions, but it can only be acquired through the diet, either through food or dietary supplements. So that consumers are aware of how much iodine they ingest, food and supplements must comply with U.S. Food and Drug Administration (FDA) regulations. To meet their iodine labeling requirements, methods for sensitive, precise and accurate iodine determination are needed.
Iodine intake: Finding the right balance
The production of thyroid hormones for critical metabolic processes requires iodine, as it facilitates the thyroid gland’s activity. With too little iodine, children and adults can suffer growth and development abnormalities[i]. However, too much iodine can cause thyroid disorders[ii]. Therefore, an appropriate iodine level is required for regular metabolic functioning.
Because the human body cannot produce iodine, the mineral must be introduced by dietary means. While iodine is abundant in several food sources such as fish and dairy, many people don’t consume these foods, particularly for health or personal reasons. To ensure sufficient dietary uptake, iodine is often artificially added to foods, or supplied in multivitamin-mineral supplements.
Determining iodide levels to comply with regulations
Due to the importance of iodine level regulation in the body, accurate iodine levels must be determined to comply with regulations, such as the FDA’s Nutrition Facts and Supplement Facts labels[iii]. In fact, determination of iodine is so important it has been included in the National Institute of Standards and Technology (NIST) Dietary Supplement Laboratory Quality Assurance Program (DSQAP)[iv].
Typically, dietary iodine comes in the form of potassium iodide or sodium iodide, so iodide determination can be used to quantify iodine levels. Most commonly, ion chromatography (IC) is used, combined with electrochemical detection (ED)[v]. IC and ED have been used to detect iodine and iodate (another iodine source) in infant formulas using acid digestion5a with excellent results.
While the acid digestion method is effective, it can cause interference if the sample needs to be investigated for other nutrients. This means that a new sample of the multivitamin needs to be prepared for different detection methods, such as determination of chloride using a conductivity detector (CD). Using an alkaline digestion approach instead can overcome this challenge by providing a universal sample that can be effectively used for both iodide and chloride determination in multivitamins, saving preparation time and costs.
Sensitive, precise and accurate determination of iodide
We reported the alkaline digestion ED method for determining iodine in multivitamin-mineral supplement samples in a recent application note. In that note, we used a Thermo Scientific™ Dionex™ IonPac™ AS20 IC column in the Thermo Scientific™ Dionex™ ICS-6000 Dual Channel HPIC™ system with RFIC™-EG module, and conductivity and electrochemical detectors (Figure 1). The Dionex IonPac AS20 IC column is a hydroxide-selective, high-capacity anion-exchange column that can determine what are typically strongly retained anions such as iodide. This column’s selectivity for these highly retained anions means lower-strength ionic eluents can be used for determination.
Figure 1: Schematic diagram of an RFIC system for the determination of iodide.
Our results demonstrated:
- Precision — An intraday precision = 1.3%, and an interday precision = 1.6% were shown.
- Accuracy — Spiked samples were analyzed, and recovery ranged from 94% to 101%.
- Sensitivity — The method detection limit was 0.0005 mg/L.
We also found that a palladium hydrogen (PdH) solid-state reference electrode is a promising alternative to silver/silver chloride (Ag/AgCl). In the limited experiments we ran, PdH gave similarly excellent results. Additionally, PdH boasts improved stability, a longer lifetime, and less maintenance[i].
A consistent approach for regulatory compliance
We further demonstrated the IC-ED method is ideal for regulatory monitoring due to its reliability, accuracy, and precision. We determined iodine (as iodide) in four brands of multivitamins, testing each sample at least three times using independently prepared samples on separate days. We obtained consistent results that were consistent with the labeled value and had a relative standard deviation (RSD) range from 0.9 to 2.5% (Table 1).
| Multivitamin | Measured iodine* (mg/kg) (n≥3) | RSD | Label iodine (mg/kg) | Measured/ label (%) |
| MV1 | 106 ± 1.4 | 1.3 | 95.2 | 111% |
| MV2 | 90.1 ± 1.5 | 1.6 | 91.0 | 99% |
| MV3 | 13.3 ± 0.1 | 0.9 | N/A | N/A |
| MV4 | 173 ± 4.3 | 2.5 | 117.4 | 147% |
Table 1: Determination of iodide in multivitamin samples. *Data used Ag/AgCl reference electrode.
We therefore show that IC-ED is a valuable tool for determination of iodine in multivitamin-mineral supplements for exemplary application in a regulatory setting, demonstrating excellent precision, accuracy and sensitivity.
Read more about using IC-ED to determine iodine in multivitamin-mineral supplements in our application note, and don’t miss our upcoming blog post on using high-performance anion-exchange chromatography with pulsed amperometric detection (HPAE-PAD) for determination of sugars in foods.
Read more about using IC-ED to determine iodine in multivitamin-mineral supplements in our application note, and don’t miss our upcoming blog post on using high-performance anion-exchange chromatography with pulsed amperometric detection (HPAE-PAD) for determination of sugars in foods.
[1] a) Iodine, Fact Sheet for Health Professionals, https://ods.od.nih.gov/factsheets/Iodine-
Health Professional/ (Accessed April 11, 2022). b) Iodine, Fact Sheet for Consumers, https://ods.od.nih.gov/factsheets/Iodine-Consumer/ (Accessed April 11, 2022). c) Institute of Medicine, Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc; A Report of the Food and Nutrition Board, National Academic Press, Washington, DC, 2001.
[2] Azizi, F.; Hedayati, M.; Rahmani, M.; Sheikloleslam, R.; Allahverdian, S.; Salarkia, N. Reappraisal of the Risk of Iodine-Induced Hyperthyroidism: An Epidemiological Population Survey. J. Endocrinol. Invest. 2005, 28, 23–29.
[3] Trumbo, P.R. FDA regulations regarding iodine addition to foods and labeling of foods containing added iodine, Am. J. Clin. Nutr. 2016 Sep; 104(Suppl 3), 864–867. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5004497/ (Accessed April 11, 2022).
[4] Dietary Supplement Laboratory Quality Assurance Program, https://www.nist.gov/programs-projects/dietary-supplement-laboratory-quality-assurance-program (Accessed April 11, 2022).
[5] a) Thermo Scientific Application Note 37: Determination of Iodide and Iodate in Soy- and Milk-Based Infant Formulas, Sunnyvale, CA, USA, 2016, https://appslab.thermofisher.com/App/1431/determination-iodide-infant-formula (Accessed April 11, 2022). b) Liang, L.; Cai, Y.; Mou, S.; Cheng, J. Comparisons of Disposable and Conventional Silver Working Electrode for the Determination of Iodide Using High-Performance Anion-Exchange Chromatography with Pulsed Amperometric Detection, J. Chromatogr. A 2005, 1085, 37–41. c) Chadha, R.K.; Lawrence, J.F. Determination of Iodide in Dairy Products and Table Salt by Ion Chromatography with Electrochemical Detection. J. Chromatogr. 1990, 518, 268–272.
[6] Thermo Scientific Technical Note 73348: Carbohydrate determinations by HPAE-PAD using a PdH reference electrode, Sunnyvale, CA, USA, 2020, https://appslab.thermofisher.com/App/4378/a-pdh-reference-electrode-for-hpaepad (Accessed April 11, 2022).