DETERMINATION OF THE RELATIVE CCS OF PESTICIDE PROTOMERS AND CONFORMERS USING LINEAR AND MULTI-PASS CYCLIC TWIM ION MOBILITY SPECTROMETRY

Importance of Topic

The increasing need to detect and characterize pesticide residues in complex matrices drives the development of robust analytical methods. Ion mobility spectrometry provides an additional dimension of separation based on collision cross section (CCS), improving specificity by distinguishing isomeric and conformeric species. Accurate determination of CCS can enhance pesticide identification in laboratories, food safety, and environmental monitoring, reducing false positives and improving confidence in screening workflows.

Goals and Overview of Study

This study aims to compare linear traveling wave ion mobility (TWIM) and multi-pass cyclic ion mobility (cIM) methods to determine relative CCS values for pesticide protomers, sodimers and potassimers, as well as to explore the formation of conformers. The objectives include cross-platform correlation of CCS measurements, development of an exploratory calibration strategy, and evaluation of the impact of these species on product ion spectra.

Methodology and Instrumentation

• Sample preparation: Fortified extracts of rice, green tea, black tea, orange, coriander, and leek.
• Chromatography: Reversed-phase C18 column (1.7 μm, 2.1×100 mm), 17-minute gradient of 0.1% formic acid in water and acetonitrile at 0.45 mL/min, 5 μL injection.
• Ionization: Positive electrospray ionization (ESI).
• Mass spectrometry: Quadrupole-IM-TOF mass spectrometer for linear TWIM; cyclic IM device for high-resolution, multi-pass separations (2–5 passes, resolving power ~92–145 Ω/ΔΩ).
• Calibration: CCS calibration following established protocols using a Major Mix IMS/ToF Calibration Kit and an in-house multi-pass strategy.

Main Results and Discussion

• Protomers and adducts: Linear TWIM resolved pesticide protomers, sodimers, and potassimers, achieving cross-platform agreement of CCS values within ΔTWCCSN2 <1.69% (RMS=0.85%, R²=0.99).
• Enantiomers: E/Z isomers of metaflumizone and chiral enantiomers of indoxacarb were separated and assigned distinct CCS values.
• Conformers: cIM resolved multiple conformers for spinosyn A, spinosyn D, avermectin B1a and epoxiconazole, with CCS differences ranging from ~0.2% to ~1.9% relative to TWIM values.
• CCS differences: The maximum CCS spread between conformers reached ~17 Å2 for spinosad (sum of spinosyn A and D), ~11 Å2 for avermectin B1a, and ~8 Å2 for epoxiconazole.
• Product ion impact: Conformer-specific product ion ratios revealed shifts in fragmentation patterns, exemplified by avermectin B1a.

Benefits and Practical Applications

• Enhanced specificity: Charge-site isomer and conformer CCS fingerprints serve as robust descriptors to improve pesticide identification.
• Screening workflows: Integration of high-resolution CCS data into routine analyses reduces ambiguity in complex matrices.
• Quality control: Improved discrimination of structurally similar species supports regulatory compliance in food safety and environmental monitoring.

Future Trends and Possibilities

• Broader adoption of cIM: Wider implementation of cyclic ion mobility across laboratories to leverage higher resolving power.
• Automated CCS libraries: Development of comprehensive databases encompassing isomeric and conformeric CCS profiles for various pesticide classes.
• Integration with AI: Use of machine learning models to predict CCS values and streamline identification in high-throughput settings.
• Multidimensional analytics: Combining ion mobility with advanced fragmentation and chromatographic techniques for deeper molecular characterization.

Conclusion

This investigation demonstrates that linear TWIM and multi-pass cIM can reliably measure CCS of pesticide protomers, adducts, and conformers with high cross-platform consistency. The ability to resolve and characterize isomeric and conformeric species offers an additional identification metric, enhancing analytical specificity in complex sample screening. The exploratory calibration strategy proved robust, paving the way for routine use of CCS-based descriptors in pesticide residue analysis.

Reference

• McCullagh M., Goscinny S., Palmer M., Ujma J. Investigations into pesticide charge-site isomers using conventional IM and cIM systems. Talanta 2021, 234, 122604.
• Bush M.F., Campuzano I.D.G., Robinson C.V. Ion mobility mass spectrometry of peptide ions: effects of drift gas and calibration strategies. Anal. Chem. 2012, 84, 7124–7130.
• McCullagh M., Fox J., Palmer M., Muck A., Higton D. Further investigations into charged isomer species of the fluoroquinolone class of antibiotics using linear TWIM and cyclic ion mobility. ASMS Poster FP 418, 69th ASMS Conference, 2021.