Improved Peak Shape and Lower LOQs in Pesticide Analysis

Significance of the Topic

Polar pesticides are often challenging to analyze by reversed-phase LC/MS when samples are dissolved in strong organic solvents, as in QuEChERS extracts. High elution strength and large injection volumes can lead to peak distortion, breakthrough, and poor quantitation.

Study Objectives and Overview

This study compares Agilent’s 1260 Infinity II Hybrid Multisampler operating in Feed Injection mode with conventional flow-through injection for multiresidue LC/MS analysis of eight early-eluting polar pesticides in pure acetonitrile extracts. Key performance parameters include peak shape, signal precision, linearity, limits of quantification (LOQs), and recoveries.

Methodology and Instrumentation

• Instrumentation:
  Agilent 1260 Infinity II Hybrid Multisampler (G7167C), 1260 Infinity II Flexible Pump (G7104C), 1290 Infinity II Multicolumn Thermostat (G7116B), Agilent Ultivo triple quadrupole LC/MS (G6465B), ZORBAX StableBond C18 column.
• LC Conditions:
  Gradient from water + 5 mM ammonium formate + 0.1% formic acid to methanol + same additives at 0.5 mL/min, column temperature 40 °C, injection volumes 0.5–2.0 µL. Feed Injection speed 35 µL/min with 6 µL overfeed and inner wash cycles.
• MS Parameters:
  Jet Stream source, gas temperature 120 °C, sheath gas 325 °C, nebulizer 45 psi, capillary +3,500 V. MRM transitions optimized for quantifier and qualifier ions of eight pesticides.
• Sample Preparation:
  QuEChERS extraction of strawberries in acetonitrile, dispersive SPE cleanup, filtration through 0.2 µm syringe filter. Calibration standards 0.2–100 ppb; recovery evaluated at 10 ppb spike.

Main Results and Discussion

• Peak Shape and Precision:
  Feed Injection produced sharp, symmetrical peaks up to 2.0 µL injection with area RSDs <3%. Flow-through injection showed peak splitting and breakthrough ≥1.0 µL, with area RSDs rising to 14% for early eluters and up to 30% for later compounds at 2.0 µL.
• Linearity:
  Feed Injection calibrations maintained R2 >0.999 across 0.2–100 ppb and all injection volumes. Flow through mode R2 values degraded to 0.93–0.99 at ≥1.5 µL.
• Limits of Quantification:
  Feed Injection achieved LOQs of 30–40 ppt across the tested volume range. Flow-through injection LOQs increased to 70–120 ppt at 1.5 µL and ~200 ppt at 2.0 µL.
• Recovery:
  Feed Injection recoveries from spiked strawberry extracts ranged 85–105%, meeting 80–120% guidelines. Flow-through mode showed similar trends but suffered from peak distortions at higher volumes.

Benefits and Practical Applications

Feed Injection mode allows direct analysis of extracts in strong organic solvents without additional mixing hardware, delivering enhanced sensitivity, precision, chromatography, and automated integration. This streamlines routine multiresidue pesticide testing in food safety and QA/QC laboratories.

Future Trends and Potential Applications

Integration of Feed Injection with ultra-high-throughput LC/MS systems, expanded use for other polar or challenging analytes, and coupling with automated sample prep and robotics. Potential extensions include environmental monitoring and clinical trace analysis where solvent effects limit chromatographic performance.

Conclusion

The Agilent 1260 Infinity II Hybrid Multisampler in Feed Injection mode significantly improves LC/MS analysis of polar pesticides in strong organic matrices by maintaining superior peak shape, precision, linearity, and lower quantification limits compared to conventional flow-through injection, enhancing data quality and laboratory throughput.

Reference

1. Herschbach H., Naegele E. Performance Characteristics of the Agilent 1260 Infinity II Hybrid Multisampler. Agilent Technologies Technical Overview, Publication 5994-5952EN, 2023.
2. European Commission. Method Validation and Quality Control Procedures for Pesticide Residues Analysis in Food and Feed. SANTE/12682/2019.