Thiram is used extensively as a fungicide and parasiticide to prevent disease in crops and as an animal repellent to protect trees and ornamental plants. However, extensive toxicological studies conclude that chronic, high-level exposure can cause considerable organ damage to land and aquatic species. While only moderately toxic to humans through skin exposure and ingestion, thiram is highly toxic if inhaled. To reduce its negative effects on health and the environment, the US defines maximum residue limits that vary for different food crops. In contrast, the EU recently banned thiram and is moving to use pesticides that carry reduced health risks.

Using Misa (Metrohm Instant SERS Analyzer), low level detection of thiram on apples is achieved with guided workflows adapted for use by diverse testers. Misa Cal software automatically processes results and achieves chemical identification by matching to either proprietary, user-created, or commercially available spectral libraries. Results can be geotagged and shared immediately with the accompanying chemical hazard information.

This application note describes a simulated test procedure for detecting thiram residue on apple skin. The assay is based on the acquisition of SERS-specific spectra for thiram using Misa and Metrohm Raman’s proprietary silver P-SERS substrates.

To establish a reference spectrum, a pure thiram standard at a concentration of 500 μg/mL in ethanol was analyzed using silver (Ag) P-SERS strips. The unique SERS spectrum shown in Figure 1 can be used to create a library entry for thiram.

Figure 1. Standard SERS reference spectrum of thiram.
https://s7e5a.scene7.com/is/image/metrohm/195000010_1?ts=1641884617110&dpr=off

To simulate rapid testing, 25 μL aliquots of 1000, 100, and 10 μg/mL thiram in ethanol were applied to 2 cm2 sections of apple skins washed with water and dried. After thorough drying of the test sections, Au P-SERS test strips were dipped in ethanol and immediately swabbed in a circular motion within treated sections on the apple skin. Each strip was dried for 5 minutes and then placed in the P-SERS attachment on Misa for measurement.

Table 1. Experimental parameters
Instrument Acquisition
Firmware 0.9.33 Laser Power 5
Software Misa Cal V1.0.15 Int. Time 5 s
Misa Vial Attachment 6.07505.030 Averages 10
ID Kit - Ag P-SERS 6.07506.470 Raster ON

Overlaid and baseline-corrected spectra acquired for swabs of an apple skin treated with different concentrations of thiram show detection down to 10 μg/mL (Figure 2).

Figure 2. Overlaid baseline-corrected spectra acquired from Ag P-SERS swabs show detection of thiram on apple skins to 10 μg/mL.

Detection of thiram in the field

Dip the printed end of a P-SERS strip into ethanol. Touch only the unprinted back side of the swab with your gloved hands. Use the moistened, printed side of the strip to swab the peel of an apple. Rub the strip in a circular motion. After drying for 5 minutes, insert the Ag P-SERS strip, with the printed portion facing down and toward the instrument, into the P-SERS attachment for measurement.

Table 2. Requirements for field test protocol
ID Kit - Ag P-SERS 6.07506.470
includes: Silver P-SERS
Scoop
Disposable pipettes
2 mL glass vials
Reagents Ethanol
Test settings Use ID Kit OP on MISA

Misa is a highly sensitive, cost-effective system for the unambiguous identification of pesticide residue on fruit. Its user-friendly analyses paired with smart software in a compact, portable format make it a powerful solution for field detection of food adulterants.

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Metrohm AG

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9100 Herisau

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