Investigations into Pesticide Charge State Isomers with Ion Mobility and High-Resolution Mass Spectrometry

Importance of the Topic

Pesticide residues in food and environmental samples pose health risks and demand precise detection methods. Traditional LC-MS approaches rely on retention time, m/z, and ion ratios, yet variability in ion ratios can compromise regulatory compliance. Charge site isomers (protomers and metal adduct isomers) introduce additional complexity, altering fragmentation patterns and ion abundances. Integrating ion mobility spectrometry (IMS) with high-resolution mass spectrometry (HRMS) provides an orthogonal separation based on ion shape and collision cross section (CCS), enhancing specificity and reliability in pesticide analysis.

Objectives and Study Overview

This study aimed to:

• Characterize charge site isomer and conformer formation for common pesticide residues.
• Examine the influence of isomer formation on multiple reaction monitoring (MRM) ion ratios.
• Demonstrate how CCS measurements can serve as an additional identification parameter.

Methodology and Instrumentation

Samples of pesticides including indoxacarb, fenpyroximate, spinosad, epoxiconazole, metaflumizone, and avermectin were analyzed using liquid chromatography coupled to an IMS-HRMS platform. A SELECT SERIES™ Cyclic™ IMS device provided high-resolution ion mobility separation through multiple passes, resolving isomeric species based on their distinct CCS values. Collision cross sections were determined under reproducible source and drift conditions, and fragmentation spectra were acquired for each mobility-resolved component.

Main Findings and Discussion

• Fenpyroximate displayed two baseline-resolved conformers after two cyclic IMS passes, both yielding equivalent product spectra despite differing shapes.
• Indoxacarb protomers were separated after three passes, with each isomer showing unique fragmentation profiles consistent with alternative protonation sites.
• Spinosyn A and D exhibited four charged isomer species, fully resolved after five cyclic IMS passes, while avermectin B1a showed three distinct isomers.
• CCS differences correlated with structural configurations and directly impacted observed ion ratios in MRM experiments.

Benefits and Practical Applications

Incorporating IMS-HRMS into routine pesticide screening offers:

• An independent identification point based on CCS, unaffected by chromatographic or matrix variability.
• Improved resolution of isomeric species, leading to more consistent ion ratios and reduced false positives/negatives.
• Enhanced confidence in compound identification for regulatory and quality-control laboratories.

Future Trends and Potential Applications

• Development of comprehensive CCS libraries for pesticides and metabolites.
• Automated workflows combining IMS separation with targeted and untargeted screening.
• Expansion of cyclic IMS technology to other classes of contaminants and biomolecules.
• Increased regulatory acceptance of CCS-based criteria for confirmatory analysis.

Conclusion

This investigation demonstrates that high-resolution cyclic IMS adds a critical dimension to pesticide analysis by resolving charge site isomers and providing reproducible CCS values. The approach enhances specificity, reduces variability in ion ratios, and supports more robust identification workflows in food safety and environmental testing.

Reference

1. McCullagh M., Goscinny S. Discovery of pesticide protomers using routine ion mobility screening. Waters Appl. Note 2014, 720005028E.
2. McCullagh M., Goscinny S., Palmer M., Ujma J. Investigations into pesticide charge site isomers using conventional IM and cIM systems. Talanta 234 (2021) 122604.