Dairy products are amongst the top three natural sources of iodine—the other two being seafood and eggs [1,2]. Iodine is an essential mineral for human health, where it is necessary for e.g., the production of thyroid hormones [1–3]. These hormones are especially important for brain and neural development in infancy. However, excessive intake of this trace element may also cause health issues [1–3]. Therefore monitoring iodine intake for humans as well as its content in natural sources are of major interest.
The presented method describes the determination of free iodide in milk samples using Metrohm Low Volume Inline Dialysis for automated sample preparation prior to injection into an ion chromatograph (IC) and subsequent amperometric detection in direct current (DC) mode. An automatic cleaning cycle using a dedicated flexiPAD method was applied to guarantee continuous and reproducible results when using DC mode.
Inline Dialysis reduces the time required for manual sample preparation which helps increase sample throughput, labor efficiency, and repeatability via automation.
Three different commercially available milk samples were analyzed for their iodide content. The milk samples were manually diluted with ultrapure water with a dilution factor of 20 prior to analysis.
Metrohm Low Volume Inline Dialysis was used as an automated sample preparation technique. The analyte of interest (i.e., iodide, I-) can pass through the dialysis membrane (0.2 μm, cellulose acetate), whereas larger molecules (e.g., proteins and enzymes which are present in milk) cannot pass through and are transferred to the waste.
The Metrohm IC Amperometric Detector in DC mode was used for electrochemical detection of iodide. A silver working electrode was used in a thin layer cell together with an Ag/AgCl reference electrode.
Historically, the detection of iodide with IC using DC mode has resulted in low reproducibility during longer sample series due to signal reduction caused by passivation of the working electrode over time. Thus, an additional flexiPAD method was developed for this situation and applied to automatically clean the working electrode after each determination to avoid electrode fouling. The reproducibility of the results is guaranteed, even for longer sample series.
Three different milk samples were analyzed for their iodide content (Tables 1–3). The natural iodide concentration in the samples ranged from below the detection limit of the method up to 141 μg/L. A study with three different spiking concentrations was performed for all three samples, where the recoveries were in the range of 94–107%.
Recovery values were calculated using the following formula:
𝑅 recovery [%]
𝑐f concentration of fortified sample [μg/L]
𝑐u concentration of unfortified sample [μg/L]
𝑐a concentration of analyte added to the sample [μg/L]
Table 1. Results of the spiking study of organic whole milk. The sample was spiked with 50, 100, and 200 μg/L iodide.
| Sample 1 | I- concentration (μg/L) | Recovery (%) |
|---|---|---|
| Natural [I-] | 141 | – |
| Sample spiked with 50 μg/L I- | 189 | 99 |
| Sample spiked with 100 μg/L I- | 251 | 104 |
| Sample spiked with 200 μg/L I- | 363 | 106 |
Table 2. Results of the spiking study of regular whole milk. The sample was spiked with 50, 100, and 200 μg/L iodide.
| Sample 2 | I- concentration (μg/L) | Recovery (%) |
|---|---|---|
| Natural [I-] | 105 | – |
| Sample spiked with 50 μg/L I- | 157 | 101 |
| Sample spiked with 100 μg/L I- | 200 | 98 |
| Sample spiked with 200 μg/L I- | 304 | 100 |
Table 3. Results of the spiking study of another brand of organic whole milk. The sample was spiked with 50, 100, and 200 μg/L iodide.
| Sample 3 | I- concentration (μg/L) | Recovery (%) |
|---|---|---|
| Natural [I-] | < LOD | – |
| Sample spiked with 50 μg/L I- | 78.4 | 107 |
| Sample spiked with 100 μg/L I- | 124 | 100 |
| Sample spiked with 200 μg/L I- | 210 | 94 |
The limit of detection (LOD) for this method was determined according to the signal-to-noise ratio and also in accordance with DIN 32645. LOD was calculated as 36 μg/L (S/N) and 27 μg/L (DIN 32465), respectively.
The following formula was used for the calculation of the LOD according to the signal-to-noise ratio:
𝐿𝑂𝐷 Limit of detection [μg/L]
𝐶𝑂𝑁𝐶 Analyte concentration [μg/L]
𝐻𝐺𝑇 Height of the analyte [nA]
𝑁𝑜𝑖𝑠𝑒 Noise of the determination [nA]
This IC method offers a straightforward, fast, and sensitive solution for reproducible analysis of the iodide concentration in milk. The utilization of an automated cleaning method for the working electrode reduces electrode fouling and increases sample throughput without any additional manual work. Low Volume Inline Dialysis enables automatic sample preparation, increasing the overall method efficiency and costs.
- Sakai, N.; Esho, O. Y.; Mukai, M. Iodine Concentrations in Conventional and Organic Milk in the Northeastern U.S. Dairy 2022, 3 (2), 211–219.
- van der Reijden, O. L.; Zimmermann, M. B.; Galetti, V. Iodine in Dairy Milk: Sources, Concentrations and Importance to Human Health. Best Pract Res Clin Endocrinol Metab 2017, 31 (4), 385–395.
- Gunnarsdottir, I.; Dahl, L. Iodine Intake in Human Nutrition: A Systematic Literature Review. Food & Nutrition Research 2012.
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Internal reference: AW IC CH6-1428-102020