Determination of iodide in multivitamin-mineral supplements using ion chromatography

Significance of the topic

The accurate quantification of iodide in multivitamin-mineral supplements is essential for ensuring proper dietary intake of iodine, a critical micronutrient for thyroid hormone synthesis. Deficiency can lead to developmental disorders, while excessive intake may cause thyroid dysfunction. Regulatory bodies require reliable methods to verify label claims and ensure consumer safety.

Objectives and overview of the study

This study aimed to develop and validate a robust ion chromatography method with pulsed amperometric detection (IC-PAD) for the determination of iodide in multivitamin supplements. A Thermo Scientific Dionex IonPac AS20 column with electrolytically generated KOH eluent was employed. A comparison between silver/silver chloride (Ag/AgCl) and palladium hydrogen (PdH) reference electrodes was also conducted to assess performance and stability.

Methodology and instrumentation

Sample preparation involved:

• Grinding and homogenizing >30 tablets per sample.
• Weighing approximately 0.500 g of powder into a centrifuge tube.
• Adding 20 mL of 0.02 % (w/w) NaOH extraction solution.
• Ultrasonic extraction for >2 h, followed by centrifugation and 0.2 µm PES filtration.
• Dilution of the filtrate (1:20) before analysis.

Chromatographic conditions:

• Instrument: Thermo Scientific Dionex ICS-6000 RFIC-EG with dual conductivity and electrochemical detectors.
• Columns: Dionex IonPac AG20 guard (2×50 mm) and AS20 analytical (2×250 mm).
• Eluent: 30 mM KOH generated by EGC 500 cartridge and CR-ATC 600 trap.
• Flow rate: 0.35 mL/min; injection volume: 25 µL; column temperature: 30 °C; tray temperature: 4 °C.
• Detection: PAD using a disposable silver working electrode with either Ag/AgCl or PdH reference electrode.

Main results and discussion

Iodide eluted at approximately 8.5 min and was fully resolved from other supplement anions. Calibration was linear from 0.001 to 2 mg/L (Ag/AgCl, R²=0.9995) and 0.0025 to 2 mg/L (PdH, R²=0.9991). Method detection limits were 0.0005 mg/L (Ag/AgCl) and 0.0018 mg/L (PdH). Precision studies yielded intraday RSD of 1.3 % and interday RSD of 1.6 %. Spiked recovery ranged from 94 % to 101 %, demonstrating high accuracy. Comparative tests showed equivalent performance of Ag/AgCl and PdH electrodes, with the latter offering enhanced stability and reduced maintenance.
Analysis of four commercial multivitamin brands revealed varying agreement with label values: one matched within 1 %, one slightly exceeded by 11 %, one unlabelled product contained 13.3 mg/kg, and one exceeded its claim by 47 %, underscoring the need for routine quality control.

Benefits and practical applications

This IC-PAD method is rapid (15 min run time), sensitive, accurate, and precise. The reagent-free KOH eluent generator reduces manual preparation and waste. It is well suited for regulatory compliance testing, routine QA/QC in dietary supplement manufacturing, and nutritional research.

Future trends and potential applications

Emerging directions include:

• Extension to other halide and oxyanion determinations in complex matrices.
• Automation of sample extraction and on-line coupling with IC systems for high throughput.
• Miniaturized and portable IC-ED platforms for field testing.
• Integration of solid-state reference electrodes across broader PAD assays.
• Advanced data analytics and AI-driven method optimization.

Conclusion

A robust, validated IC-PAD method for iodide determination in multivitamin supplements has been established, offering high sensitivity, accuracy, and regulatory compliance. Both Ag/AgCl and PdH reference electrodes are viable, with PdH electrodes providing additional operational benefits. This approach supports reliable quality monitoring of iodine in dietary supplements.

Reference

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