Trace Detection of Acetamiprid on Raisins

Significance of the topic

Raisins are a widely consumed dried fruit snack often presumed safe, yet recent surveys have revealed frequent contamination with multiple pesticide residues. The long-term health impact of chronic exposure to pesticide cocktails in foods such as raisins remains uncertain. Rapid, on-site analytical methods capable of detecting trace levels of contaminants are therefore critical to ensure consumer safety and maintain confidence in food supply.

Objectives and overview of the study

The primary aim of this work was to evaluate the Metrohm Instant SERS Analyzer (MISA) for its ability to rapidly detect the systemic insecticide acetamiprid on commercially available raisins. The study demonstrates both sensitivity and speed advantages of surface-enhanced Raman scattering (SERS) over conventional laboratory techniques for screening of food contaminants.

Used methodology and instrumentation

The analysis workflow comprised the following steps:

• Spiking 1 g raisin samples with known acetamiprid concentrations (100, 25, 5, 2 μg/mL and 500 ng/mL) in methanol.
• Drying the samples, extracting residues with 0.2 mL dichloromethane per aliquot, vortexing and phase separation.
• Evaporative drying of extract, addition of 0.9 mL silver colloid and 0.1 mL 500 mmol/L NaCl, and gentle mixing.
• Insertion of the prepared vial into the MISA analyzer for SERS measurement.

The MISA Advanced system includes a high-efficiency spectrograph with Orbital-Raster-Scan technology and attachments for vial-based SERS. Silver colloid served as the enhancing substrate.

Main results and discussion

SERS spectra of acetamiprid extracts from raisins showed clear signature peaks matching the pure reference spectrum down to 500 ng/mL. Peak intensities scaled with nominal concentration. At the regulatory limit of 500 ng/mL, some bands were weak but still identifiable. Two strategies improved detection at this level:

• Combining multiple DCM extract aliquots (three to four) prior to drying, which boosted signal intensity.
• Extending integration times manually from default values to four–eight seconds, enhancing signal-to-noise.

Non-linear response at trace levels, variability in extraction efficiency, and analyte binding to raisin surfaces were noted as factors affecting quantification.

Benefits and practical applications

The MISA platform offers:

• Rapid on-site screening without complex sample preparation or lab infrastructure.
• Sensitivity down to low parts-per-billion levels suitable for maximum residue limit compliance.
• Portable operation with battery power, Bluetooth or USB connectivity.
• Reusable workflow adaptable to a wide range of pesticides and food matrices using dedicated nanoparticle substrates.

Such capabilities support quality assurance, regulatory compliance, and food safety monitoring in field or mobile lab settings.

Future trends and potential applications

Advancements likely include:

• Integration of automated extraction and multiplexed sample handling to increase throughput.
• Development of broader SERS substrate libraries covering emerging agrochemicals.
• Machine learning-driven spectral interpretation for improved quantification and compound identification.
• Miniaturization and cloud connectivity for real-time data sharing and centralized monitoring.

These innovations will expand SERS screening to a wider array of contaminants and faster decision-making in food safety workflows.

Conclusion

The Metrohm Instant SERS Analyzer demonstrates a powerful, user-friendly approach for rapid detection of acetamiprid residues on raisins at trace levels. Its portability, speed, and sensitivity position it as a valuable tool for proactive food safety screening, reducing the risk of contaminated products reaching consumers.

Used Instrumentation

• MISA Advanced Metrohm Instant SERS Analyzer with Orbital-Raster-Scan spectrograph.
• Vial attachment and P-SERS strip attachment.
• Silver colloid solution for SERS (50 mL).
• MISA Cal software and ASTM calibration standard.

References

• Agricultural Marketing Service, Raisin Grades Standards, US Department of Agriculture, accessed July 26, 2022.
• Environmental Working Group, EWG’s 2022 Shopper’s Guide to Pesticides in Produce, accessed July 26, 2022.
• European Food Safety Authority, National Summary Reports on Pesticide Residue Analysis Performed in 2020, EFSA Support Publications 2022; EN-7216.