GC-TOFMS and GCxGC-TOFMS of Organophosphate Pesticides in Ash Leaves

Significance of the Topic

An accurate assessment of organophosphate pesticides in plant matrices is essential for environmental monitoring, food safety, and regulatory compliance. Ornamental trees treated with Chlorpyrifos, Diazinon, and Malathion present analytical challenges due to low residual concentrations and complex leaf matrices. This study highlights advanced mass spectrometric approaches that improve sensitivity, selectivity, and spectral quality in such demanding samples.

Objectives and Study Overview

The primary goal was to compare one-dimensional GC-TOFMS and comprehensive two-dimensional GCxGC-TOFMS for detecting three common organophosphates in ash leaf extracts. Key objectives included:

• Evaluating automated peak finding and spectral deconvolution performance.
• Assessing the separation of pesticide signals from coeluting matrix interferences.
• Demonstrating improvements in library matching and identification confidence.

Used Methodology and Instrumentation

An ash leaf extract provided by USDA Gulfport was analyzed by two configurations:

1. GC-TOFMS
   
   • Agilent 6890 GC with Rtx-TNT column (20 m × 0.25 mm × 0.71 µm)
   • Carrier gas: helium at 2 mL/min (constant flow)
   • Injection: 1 µL splitless at 250 °C
   • Oven ramp: 80 °C (1 min) to 340 °C at 20 °C/min
   • LECO Pegasus TOFMS: EI 70 eV, source 225 °C, mass 45–550 u, 20 spectra/s
2. GCxGC-TOFMS
   
   • Agilent 6890 GC with LECO thermal modulator and secondary oven
   • Primary column: Rtx-5 (10 m × 0.18 mm × 0.20 µm)
   • Secondary column: Rtx-PCB (2 m × 0.10 mm × 0.10 µm)
   • Carrier gas: helium at 0.7 mL/min
   • Injection: 1 µL splitless at 275 °C
   • Modulation: 3 s, +35 °C offset
   • LECO Pegasus TOFMS: EI 70 eV, mass 45–550 u, 100 spectra/s

Main Results and Discussion

The study demonstrated:

• One-dimensional GC-TOFMS allowed detection of Diazinon (~50 pg) and Malathion (~125 pg) despite strong matrix peaks at m/z 124 and 167, respectively. Spectral deconvolution recovered library-quality spectra with similarity scores of ~800/999.
• GCxGC-TOFMS markedly increased separation power, shifting pesticide peaks away from matrix interferences in the second dimension. Both Chlorpyrifos and Malathion matched library spectra with similarity >900/999.
• Narrow GCxGC peak widths (100–200 ms) required high acquisition rates, making TOFMS uniquely suited to leverage thermal focusing and comprehensive separations.

Contributions and Practical Applications

This work offers:

• Demonstrated workflows for trace pesticide screening in botanical matrices.
• Validation of automated peak finding and spectral deconvolution in complex samples.
• Enhanced identification confidence via high-quality library matches.
• Guidance for environmental monitoring labs seeking to implement comprehensive GCxGC-TOFMS.

Future Trends and Potential Applications

Emerging directions include:

• Integration of faster detectors and intelligent software for real-time deconvolution.
• Expansion of comprehensive GCxGC libraries to broaden pesticide and contaminant coverage.
• Miniaturized or portable GCxGC-TOFMS systems for in-field testing.
• Application to other challenging matrices such as soils, biota, and food products.

Conclusion

Both GC-TOFMS and GCxGC-TOFMS effectively detect organophosphate pesticides in complex ash leaf extracts. Comprehensive two-dimensional separations significantly reduce matrix interference, yield superior spectra, and improve identification confidence. TOFMS with its high acquisition speed remains the gold standard for comprehensive GCxGC analyses.