Trace Detection of Paraquat in Tea Leaves

Importance of the topic

Paraquat is a widely used contact herbicide known for its efficient weed control but carries severe toxicity risks. Regulatory bodies in many regions have banned or heavily restricted its use following numerous poisoning incidents and fatalities. Rapid on-site detection of paraquat residues in consumable crops like tea is critical for consumer safety and regulatory compliance.

Objectives and Study Overview

This application demonstrates a simple, field-deployable protocol for trace detection of paraquat on tea leaves. By combining a rapid chloroform extraction with surface-enhanced Raman spectroscopy (SERS) using gold nanoparticles, the study aims to achieve low-level detection without the need for complex laboratory instruments.

Methodology

Tea leaf samples were finely ground and spiked with paraquat at concentrations from 50 µg/g down to 50 ng/g. After drying, each sample was extracted with chloroform, and an aliquot of the extract was mixed with a gold colloid suspension and NaCl solution. The mixture was directly analyzed using the Metrohm Instant SERS Analyzer (MISA) with acquisition parameters set for rapid, sensitive detection. Spectral processing—including baseline correction and background subtraction—was performed using MISA Cal software.

Used Instrumentation

• MISA Advanced portable SERS analyzer with Orbital-Raster-Scan spectrograph
• MISA Vial Attachment for liquid sampling
• ID Kit – Au NP containing gold colloid, vials, spatula, and pipettes
• MISA Cal software for spectral acquisition and library matching

Main Results and Discussion

Overlaying SERS spectra from the spiked tea samples confirmed clear paraquat signatures down to 1 µg/g. Characteristic Raman peaks allowed reliable identification at concentrations well below typical maximum residue limits. The method demonstrated robust performance across the tested range, with automated software ensuring consistent spectral interpretation.

Contributions and Practical Applications

• Enables non-expert users to conduct on-site paraquat screening
• Reduces dependence on centralized labs and lengthy turnaround times
• Provides automated data processing and library matching for traceability

Future Trends and Applications

Advancements in portable SERS detectors will facilitate integration with wireless reporting and cloud-based spectral databases, enabling real-time surveillance in agricultural supply chains. Expanding this approach to detect multiple contaminants across food and environmental samples can further enhance safety monitoring.

Conclusion

The presented protocol delivers rapid, sensitive on-site detection of paraquat residues on tea leaves. Its ease of use, low detection limits, and portability make it a valuable tool for protecting consumer health and ensuring adherence to international safety standards.

Reference

• Metrohm AG. Application Note AN-RS-025: Trace Detection of Paraquat in Tea Leaves.