Determination of fluoride in tea using a combustion ion chromatography system

Significance of the Topic

Tea is one of the world’s most widely consumed beverages and has a natural tendency to accumulate fluoride. Excessive fluoride intake can cause dental fluorosis, bone fractures, and skeletal fluorosis. Traditional potentiometric methods for fluoride determination in tea rely on hazardous acids and lengthy sample preparation, creating a need for a rapid, safe, and automated alternative.

Objectives and Overview of the Study

The primary aim was to develop and validate a combustion ion chromatography (CIC) method that streamlines sample preparation and improves analytical performance for fluoride determination in tea. Key objectives included:

• Eliminating corrosive reagents and reducing hands-on time.
• Removing matrix interferences through high-temperature oxidation.
• Demonstrating method sensitivity, accuracy, and compliance with Chinese tea standards.

Methodology and Instrumentation

Tea leaves were finely ground and dried at 100 °C overnight. Approximately 20 mg of sample was placed in a ceramic boat and combusted at 900–1000 °C under a controlled Ar/O₂ atmosphere. Gaseous byproducts, including hydrogen fluoride, were absorbed in deionized water and directly injected into an ion chromatography system.

• Combustion Unit: Mitsubishi AQF-2100H with Automatic Boat Controller, Horizontal Furnace, Gas Absorption Unit, and Liquid Sample Changer.
• IC System: Thermo Scientific Dionex Integrion HPIC with high-pressure RFIC eluent generator, IonPac AG18/AS18 guard and analytical columns (4 × 150 mm), AERS 500 suppressor, and conductivity detector.
• Eluent: 20 mM KOH generated in situ, flow rate 1.0 mL/min, column temperature 30 °C, injection volume 100 µL, run time 10 min.

Main Results and Discussion

• Calibration was linear from 0.002 to 2.00 mg/L (r² > 0.999), with a quadratic fit used for improved accuracy across dilutions up to 10 000×.
• IC detection limit (MDL) was 0.0002 ppm for liquid standards; the CIC method MDL for tea samples was 1 ppm.
• CIC chromatograms showed clean fluoride peaks, whereas direct injection of brewed tea exhibited coeluting organic interferences.
• Measured fluoride levels: 279 ± 3 ppm (Tea 1), 55 ± 5 ppm (Tea 2), and 129 ± 3 ppm (Tea 3), all below the 200 ppm regulatory limit.
• Spike recoveries at 50–100 ppm ranged from 95% to 116%, confirming method accuracy in complex matrices.

Benefits and Practical Applications of the Method

• Automated workflow minimizes exposure to hazardous reagents and reduces manual steps.
• Analysis throughput of ~11 min per sample by overlapping combustion and chromatography.
• Enhanced selectivity and sensitivity for fluoride in the presence of other halides and organic acids.
• Suitable for routine quality control in tea production, regulatory compliance testing, and environmental monitoring.

Future Trends and Opportunities

• Applying CIC for speciation and binding studies of fluoride in different tea constituents.
• Expanding to multi-element combustion IC workflows for simultaneous halogen analysis.
• Developing portable CIC units for on-site screening in manufacturing facilities.
• Coupling with mass spectrometric detection for isotope-specific fluoride tracing.

Conclusion

The presented combustion IC method delivers a rapid, safe, and reliable means to quantify total fluoride in tea leaves. Its automation, elimination of harsh reagents, and robust analytical performance support high-throughput testing and regulatory compliance in the tea industry.

References

1. U.S. EPA. Questions and Answers on Fluoride. EPA, 2011.
2. Ministry of Agriculture of the People’s Republic of China. NY 659-2003: Residue limits for fluoride in tea. 2003.
3. Ministry of Agriculture of the People’s Republic of China. NY/T 838-2004: Test method of fluoride in tea. 2004.
4. Maleki A, et al. Effect of Brewing Time and Water Hardness on Fluoride Release from Different Iranian Teas. Fluoride 49(3):263–273, 2016.
5. Thermo Fisher Scientific. Application Note 1145: Determination of Halogens in Coal Using CIC. 2016.
6. Mitsubishi Chemical Analytech. Operation Manual for NSX-2100 Series Combustion Unit AQF-2100H. 2016.
7. Thermo Fisher Scientific. Technical Note 72211: CIC with a Dionex Integrion HPIC System. 2017.
8. Thermo Fisher Scientific. Technical Note 175: Configuring the Dionex Integrion HPIC System for RFIC. 2016.
9. USP. General Chapter <1225> Validation of Compendial Methods. USP 39. 2016.