Trace Detection of Rhodamine B in Cayenne Powder

Significance of the Topic

Adulteration of spices with unauthorized dyes such as Rhodamine B poses a health risk and undermines food safety regulations. Rapid, on-site detection methods are essential for quality control in supply chains and for regulatory compliance.

Study Objectives and Overview

This study evaluated the Metrohm Instant SERS Analyzer (MISA) for trace detection of Rhodamine B in ground cayenne pepper. Goals included establishing a reference spectrum, determining the limit of detection in powdered spice, and developing a simple field test protocol.

Methodology and Instrumentation

A library entry for Rhodamine B was created using a 50 µg/g aqueous standard and gold nanoparticle–enhanced SERS.

• Sample preparation: Cayenne powder was spiked at concentrations from 1 to 1000 µg/g with Rhodamine B in methanol, dried, then extracted with 0.1 g sample in 400 µL methanol.
• SERS measurement: 50 µL extract mixed with 400 µL Au nanoparticle colloid and 50 µL 0.5 M NaCl, then analyzed in MISA.
• Instrument settings: 785 nm laser, 5 mW power, 10 s integration, 10 accumulations, orbital raster scan enabled.

Main Results and Discussion

Baseline subtraction of control pepper spectra enabled detection of Rhodamine B down to 50 µg/g. Implementing a fivefold extract concentration improved the detection limit to 10 µg/g. The SERS profiles showed distinct marker peaks consistent with the reference spectrum, validating the method’s specificity and sensitivity.

Benefits and Practical Applications

• High sensitivity: Detects low microgram-per-gram levels of illicit dye.
• Speed and ease: Total assay time under 15 minutes with minimal sample and reagent consumption.
• Portability: MISA’s compact design and battery operation support on-site screening at processing facilities or marketplaces.

Future Trends and Potential Applications

Integration of additional libraries could expand monitoring to multiple banned colorants. Automated data analysis and cloud connectivity would streamline reporting. Adaptation to other food matrices and emerging contaminants could broaden field deployment in regulatory and industrial settings.

Conclusion

The MISA SERS platform offers a cost-effective, rapid, and sensitive approach for detecting Rhodamine B in spices. Its portability and straightforward protocol make it suitable for high-throughput, on-site food safety screening.

Reference

1. Metrohm AG. AN-RS-014: Trace Detection of Rhodamine B in Cayenne Powder. Metrohm Application Note, 2023.