Determination of Total and Free Sulfite in Foods and Beverages

Significance of the Topic

The accurate measurement of total and free sulfite in foods and beverages is critical for regulatory compliance, quality control, and consumer safety. Sulfites serve as preservatives and antioxidants but can trigger allergic and asthmatic reactions. Differentiating reversibly bound (total) from free sulfite is especially important in products like wine, where free sulfite governs microbial stability and sensory properties. A robust, selective, and efficient analytical approach addresses limitations of traditional Monier–Williams procedures, including long turnaround times, false positives, and electrode fouling.

Objectives and Study Overview

This application note presents the development and validation of an improved ion‐exclusion chromatography method with pulsed amperometric detection (PAD) for simultaneous determination of total and free sulfite in complex matrices. Key goals were to:

• Replace conventional platinum electrodes with a disposable Pt working electrode to minimize fouling and maintenance
• Reduce eluent consumption by using a 4 mm Dionex IonPac ICE‐AS1 column set
• Employ methanesulfonic acid (MSA) eluent to improve safety without compromising separation or sensitivity
• Demonstrate method performance across representative samples: red and rosé wines, coconut water, dried apricot, and a fortified nutritional bar

Methodology and Instrumentation

Sample Preparation:

• Free sulfite: extracted at pH 2 with MSA/mannitol buffer to preserve carbonyl adducts
• Total sulfite: extracted at pH 9 with sodium phosphate/mannitol buffer to release bound forms
• Internal sample dilutions optimized to minimize matrix effects and maintain sulfite stability (10–125× dilutions depending on sample acidity and solid content)
• Mannitol added to reduce sulfite oxidation; samples analyzed within 8 h of preparation

Instrumentation:

• Thermo Scientific Dionex ICS-5000 system with DP Dual Pump, DC Detector compartment, and AS-AP autosampler (250 µL syringe, 4 °C tray)
• Dionex IonPac ICE-AS1 Guard (4×50 mm) and Analytical (4×250 mm) columns
• 375 µL PEEK reaction coil to remove residual oxygen and stabilize background
• Electrochemical Detector with disposable platinum working electrode and Ag/AgCl reference
• Eluent: 20 mM methanesulfonic acid at 0.2 mL/min, column compartment at 30 °C, detector at 25 °C
• Pulsed amperometric waveform optimized for sulfite detection to clean and regenerate the electrode surface

Main Results and Discussion

Separation and Selectivity:
The method leverages Donnan exclusion, partitioning, and size-exclusion to retain sulfite while allowing stronger acid anions to elute unretained. PAD confers high selectivity over conductivity detection by monitoring oxidation at specific potentials.

Calibration and Detection Limits:

• Total sulfite: quadratic calibration (0.2–10 mg/L, r² = 0.9960), LOD 0.039 mg/L, LOQ 0.13 mg/L
• Free sulfite: quadratic calibration (0.1–5 mg/L, r² = 0.9994), LOD 0.028 mg/L, LOQ 0.093 mg/L

Recovery and Precision:
Spiked recoveries ranged from 82% to 115% for all matrices except the nutritional bar (incomplete recovery attributed to low-level instability and matrix interactions). Intraday peak height RSDs were <3% for standards and <2.5% for spiked wine samples. Electrode stability over two weeks showed peak height RSDs of 8.6% (total sulfite) and 15.7% (free sulfite), demonstrating disposable electrode robustness.

Sample Results:

• Red wine: 72.4 mg/L total, 43.6 mg/L free sulfite
• Rosé wine: 145 mg/L total, 38.1 mg/L free sulfite
• Coconut water: 3.80 mg/L total, 3.71 mg/L free sulfite
• Dried apricot: 304 mg/L total, 16.7 mg/L free sulfite
• Nutritional bar: 3.97 mg/L total, 2.67 mg/L free sulfite

Benefits and Practical Applications

• Reduced eluent consumption and operating costs via smaller 4 mm column and lower flow rate
• Elimination of frequent electrode polishing through disposable Pt electrodes and PAD waveform
• High selectivity and sensitivity suitable for complex food and beverage matrices
• Compliance with regulatory limits and enhanced quality control in industrial and academic laboratories

Future Trends and Opportunities

Potential developments include further miniaturization of ion‐exclusion systems, integration with automated sample preparation workflows, and adaptation to on‐line monitoring in production environments. Expanding the method to other sulfite‐releasing additives and exploring alternate electrochemical detection modes may broaden applicability.

Conclusion

A robust, user‐friendly ion‐exclusion chromatography method with PAD and disposable platinum electrodes has been demonstrated for reliable determination of total and free sulfite in diverse food and beverage samples. The approach offers high sensitivity, selectivity, and reproducibility while reducing maintenance and consumable costs.

Reference

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