Trace Detection of Thiram on Apples

Significance of the Topic

Thiram is a widely used fungicide and animal repellent applied to fruit crops and ornamental plants. Although effective in preventing fungal diseases, residues of thiram can pose risks to human health and aquatic ecosystems. Regulatory bodies in the US impose maximum residue limits on thiram, while the EU has restricted its use due to potential inhalation toxicity. Rapid, on-site detection of thiram residues supports food safety monitoring and regulatory compliance.

Objectives and Study Overview

This study demonstrates a field-adapted workflow for detecting trace levels of thiram on apple skins using the portable Metrohm Instant SERS Analyzer (MISA). The goals are to establish reference spectra, validate sensitivity across a range of concentrations, and provide a straightforward protocol for non-specialist users.

Methodology

Sample preparation and analysis followed these steps:

• Spiking: Apply 25 µL aliquots of thiram solutions (1000, 100, and 10 µg/mL in ethanol) onto 2 cm² apple skin sections.
• Drying: Allow spiked areas to air dry thoroughly.
• Swabbing: Dip gold P-SERS strips in ethanol, then swab each treated area in a circular motion.
• Measurement: After a 5 min drying period, insert strips into the MISA P-SERS attachment for spectral acquisition.
• Spectral processing: Perform baseline correction and compare acquired spectra against a pre-established thiram library entry.

Instrumentation Used

The analysis employed the MISA Advanced system, featuring:

• Orbital-Raster-Scan spectrograph for enhanced signal collection.
• Laser class 1 attachments with 5 mW power.
• MISA vial and P-SERS attachments for flexible sampling.
• P-SERS substrates (silver and gold) and proprietary MISA Cal software (version 1.0.15).
• Acquisition settings: 5 s integration time, 10 averages, ORS activated.

Main Results and Discussion

Baseline-corrected SERS spectra revealed characteristic thiram peaks down to 10 µg/mL on apple skin samples. Overlaid spectra demonstrated consistent peak positions and intensities proportional to concentration. The reference library created from a 500 µg/mL thiram standard enabled reliable spectral matching and rapid identification. The portable workflow allowed data geotagging and immediate sharing of hazard information.

Benefits and Practical Applications

This MISA-based approach offers multiple advantages:

• High sensitivity for low-level thiram detection.
• Compact, battery-powered design suitable for field use.
• Guided software workflows tailored for non-experts.
• Immediate data processing and spectral library matching.
• Capability to integrate geolocation metadata for traceability.

Future Trends and Applications

Advancements may include:

• Extension to a broader range of pesticides and contaminants.
• Development of more robust and selective nanoparticle substrates.
• Integration with cloud-based spectral libraries and machine learning for automated interpretation.
• Wireless connectivity and real-time monitoring platforms.
• Enhanced miniaturization for in-line quality control in processing facilities.

Conclusion

The Metrohm Instant SERS Analyzer delivers a rapid, sensitive, and user-friendly solution for detecting thiram residues on fruit surfaces. Its portability, coupled with streamlined software and reliable spectral matching, makes it an effective tool for field screening and food safety assurance.