I KNEW YOU WERE TROUBLE: EXPANDING LC METHODS TO INCLUDE DIFFICULT GC COMPOUNDS USING A NOVEL IONIZATION TECHNIQUE

Importance of the Topic

Reliable quantification of pesticide residues in food is essential for consumer safety and regulatory compliance. Traditional GC-MS methods can lead to degradation of unstable compounds such as captan and folpet, compromising data quality and reproducibility. Implementing advanced LC-MS approaches with innovative ionization techniques offers a unified workflow for a broad spectrum of analytes.

Study Objectives and Overview

This study aimed to develop and validate an LC-MS/MS method for thiophthalimide fungicides (captan, folpet) and their degradation products (THPI, PI) using two ionization strategies. Key objectives:

• Compare Electrospray ionization (ESI) and UniSpray ionization (USI).
• Assess repeatability and robustness in challenging food matrices (kale, celery).
• Ensure method sensitivity meets EU maximum residue level requirements.

Methodology and Instrumentation

Sample Preparation involved extraction and cleanup of fortified food matrices. Analytical conditions:

• LC System: ACQUITY UPLC I-Class
• MS System: Xevo TQ-XS
• Column: ACQUITY BEH C18, 2.1×50 mm, 1.7 μm
• Ionization: Electrospray (3 kV capillary) vs. UniSpray (3 kV impactor pin)
• Mobile Phase A: Water + 0.1% formic acid + 0.05% ammonia
• Mobile Phase B: Methanol + 0.1% formic acid + 0.05% ammonia
• Gradient: 10→100% B over 8 min; flow rate 0.45 mL/min
• Injection Volume: 10 μL; Column Temp: 45 °C; Sample Temp: 4 °C

Main Results and Discussion

Key findings:

• GC-MS splitless injections of captan showed rapid degradation and poor repeatability.
• LC-MS/MS with both ESI and USI yielded consistent results across 25 injections (RSD <10%).
• Calibration curves in kale and celery matrices exhibited excellent linearity (R² >0.995).
• UniSpray enhanced ionization efficiency, producing higher peak areas compared to ESI.
• Limits of detection for all targets were well below EU MRLs (e.g., captan LOD <0.5 ppb in both matrices).

Benefits and Practical Applications

Advantages of the developed method include:

• A single LC-MS/MS workflow covering both polar and non-polar pesticides.
• Superior robustness and reproducibility compared to GC-MS approaches.
• Enhanced sensitivity via UniSpray suitable for low-level residue monitoring.
• Streamlined analysis for regulatory and quality-control laboratories.

Future Trends and Applications

The adoption of advanced ionization sources like UniSpray is expected to broaden compound coverage and further improve sensitivity. Future developments may include coupling with high-resolution mass spectrometry, automation of sample handling, and extension to other challenging analyte classes.

Conclusion

The LC-MS/MS methods developed with Electrospray and UniSpray ionization deliver robust, sensitive, and reproducible analysis of captan, folpet, THPI, and PI in food matrices. UniSpray notably increases ionization efficiency, providing a powerful alternative to traditional GC-MS for pesticide residue analysis.

Reference

• Berthet A., Bouchard M., Schupfer P., Vernez D., Danuser B., Huynh C.K. Liquid chromatography-tandem mass spectrometry (LC/APCI-MS/MS) method for the quantification of captan and folpet phthalimide metabolites in human plasma and urine. Anal Bioanal Chem. 2011;399.
• Lubin A., Bajic S., Cabooter D., Augustijns P., Cuyckens F. Atmospheric pressure ionization using a high voltage target compared to electrospray ionization. J Am Soc Mass Spectrom. 2016;28.
• Lubin A., De Vries R., Cabooter D., Augustijns P., Cuyckens F. An atmospheric pressure ionization source using a high voltage target compared to electrospray ionization for the LC/MS analysis of pharmaceutical compounds. J Pharm Biomed Anal. 2017;142.
• European Commission. Commission Regulation (EU) 2016/156 amending Annexes II and III to Regulation (EC) No 396/2005 regarding maximum residue levels for certain pesticides. 2016.
• European Commission. Commission Regulation (EU) 2016/452 amending Annexes II and III to Regulation (EC) No 396/2005 regarding maximum residue levels for certain pesticides. 2016.