Screening and Quantitation of Pesticides in Jonagold Apples by Comprehensive Two-Dimensional Gas Chromatography- Time-of-Flight Mass Spectrometry (GCxGC-TOFMS)

Significance of the Topic

Screening and quantitation of pesticide residues in fruits and vegetables is essential to ensure public health and compliance with regulatory standards. Complex commodity matrices can contain thousands of compounds that cause analytical interferences, making reliable detection and measurement of trace-level pesticides challenging.

Objectives and Study Overview

This work demonstrates the application of comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GCxGC-TOFMS) combined with a rapid QuEChERS extraction for effective screening and quantitation of pesticide residues in Jonagold apples from Southwest Michigan. Both washed and unwashed samples were evaluated to assess residue reduction through washing.

Methodology and Instrumentation

Sample preparation used the QuEChERS approach with dispersive solid-phase extraction (PSA and C18) to remove matrix interferences. Instrumental conditions included:

• Gas chromatograph: Agilent 6890 with LECO dual-stage quad-jet thermal cryogen-free modulator and secondary oven
• Column 1: Rxi-5Sil MS, 15 m × 0.25 mm × 0.25 µm; Column 2: Rtx-200, 1.25 m × 0.18 mm × 0.18 µm
• Oven program: 90 °C hold 1 min, ramp 5 °C/min to 300 °C, hold 10 min; secondary oven offset +25 °C
• Modulation: 5 s period with 1.2 s hot pulse, modulator offset +25 °C, chiller at −80 °C
• Injection: 1 µL splitless at 250 °C; Carrier gas helium at 1.5 mL/min
• Mass spectrometer: LECO Pegasus 4D TOFMS, acquisition rate 100 spectra/s over m/z 45–550, source temperature 225 °C

Main Results and Discussion

Chromatographic separation in the second dimension successfully resolved target pesticides from abundant matrix components such as sugars and fatty acids. Phosmet (insecticide) and chlorpropham (growth regulator) were identified by reference library matching. Key findings:

• Retention and mass spectra enabled clear identification of over 1000 analytes in the apple extract
• GCxGC contour plots confirmed separation of coeluting compounds that would overlap in one-dimensional GC
• Quantitation using calibration curves (60–600 ng/mL) with triphenyl phosphate as internal standard yielded correlation coefficients of 0.997 for phosmet and 0.999 for chlorpropham
• Residue levels decreased from 18 ng/g to 13 ng/g for phosmet and were below detection for chlorpropham after washing

Benefits and Practical Applications

The combination of GCxGC and TOFMS offers:

• Enhanced chromatographic peak capacity to reduce matrix interferences
• Rapid full-range mass spectral acquisition without sensitivity loss
• Capability to detect both targeted pesticides and emerging contaminants in complex food matrices

Future Trends and Opportunities

Further developments may include integration with high-resolution mass spectrometry for improved identification confidence, automation of sample preparation workflows, expansion of reference libraries for new pesticide chemistries, and adoption of greener solvent and energy-efficient strategies in multiresidue screening.

Conclusion

The implemented GCxGC-TOFMS method, coupled with a quick and efficient QuEChERS extraction, provides a robust platform for comprehensive screening and accurate quantitation of pesticide residues in apples. This approach addresses the challenges posed by complex sample matrices and supports food safety monitoring and regulatory compliance.