Enhanced Sensitivity in Multiresidue Pesticide Analysis

Significance of the topic

Determination of multiresidue pesticides in food matrices (fruits, vegetables) is critical for food safety, regulatory compliance and public health. Common QuEChERS extraction workflows produce high-organic (acetonitrile) extracts that can cause severe solvent‑mismatch effects in reversed‑phase LC/MS/MS, negatively impacting peak shape, quantitation and limits of detection. Addressing injection-induced peak distortion without adding complex sample manipulation or changing chromatographic separations improves throughput and robustness for routine pesticide monitoring laboratories.

Objectives and study overview

This application note evaluates the impact of Feed Injection (sample introduced directly into the mobile phase stream) versus classical flow‑through injection on peak shape, linearity and sensitivity for a comprehensive multiresidue pesticide workflow. The study targeted 764 pesticide analytes (dynamic MRM >1,500 transitions) in QuEChERS spinach extracts and assessed injection volumes from 0.2 to 2.0 µL under UHPLC conditions to determine how Feed Injection mitigates solvent‑strength related peak broadening and splitting and how it affects analytical sensitivity and automated data processing.

Methodology and instrumentation

Key experimental design points:

• Sample: QuEChERS extract from homogenized organic spinach (10.0 g sample, extracted with 10 mL acetonitrile, salts added, centrifuged). Post‑extraction spike to 1 ng/mL (equivalent to 10 µg/kg) for multiresidue standard mixture.
• Chromatography: Agilent ZORBAX Rapid Resolution HD Eclipse Plus C18, 2.1 × 150 mm, 1.8 µm; flow 0.4 mL/min; column temperature 40 °C; gradient 5% B to 100% B (total run 20 min, 3 min post time).
• Injection modes compared: classical flow‑through injection vs Feed Injection (feed speed fixed at 10 µL/min, automatic overfeed ~4.4 µL, effective dilution approx. 1:40 of sample with mobile phase during injection).
• MRM: dynamic MRM method developed from Agilent pesticide tMRM database; total MRMs ~1,590 for 764 compounds.

Used instrumentation

The study employed an ultrahigh‑pressure LC/MS/MS stack capable of >1,000 bar operation:

• Agilent 1290 Infinity III High‑Speed Pump (G7120A)
• Agilent 1290 Infinity III Hybrid Multisampler (G7137B) — supports Feed Injection and operation up to 1,300 bar
• Agilent 1290 Infinity III Multicolumn Thermostat (G7116B)
• Agilent 6495D Triple Quadrupole LC/MS (G6495D) with AJS source
• Software: Agilent MassHunter Acquisition and Quantitative Analysis (version 12)

Results and discussion

Key findings from comparative experiments with spinach QuEChERS extracts:

• Classical flow‑through injection: At low injection volumes (0.2–1.0 µL) most compounds showed acceptable peak shape, but some early‑eluting polar pesticides (e.g., acephate, aminocarb, dinotefuran, pymetrozine) already exhibited early signs of distortion. At 2.0 µL pronounced peak broadening and peak splitting occurred for early eluters due to the high elution strength of acetonitrile extracts.
• Quantitative behavior in flow‑through mode: Peak area scaled approximately linearly with injection volume, but peak height showed saturation between 1 and 2 µL, indicating loss of peak capacity and reduced signal intensity per unit injected mass (peak broadening / breakthrough).
• Feed Injection: Introducing the sample directly into the mobile phase at a feed speed of 10 µL/min (approx. 1:40 dilution) eliminated solvent‑mismatch effects. Even at 2.0 µL injected volume, early‑eluting pesticides displayed symmetrical, well‑defined peaks with no splitting or broadening.
• Quantitative behavior in Feed Injection mode: Both peak area and peak height increased linearly across the tested injection range (0.2–2.0 µL). Doubling injection volume from 1 to 2 µL produced an approximately proportional doubling in peak height, indicating absence of saturation and improved sensitivity.
• Operational impact: Improved peak shapes reduce manual reintegration needs and enable reliable automated integration for large panels (>1,500 transitions), simplifying data review and increasing throughput.

Benefits and practical applications

Practical advantages demonstrated by Feed Injection in multiresidue pesticide workflows:

• Mitigation of solvent‑strength related peak distortion for high‑organic QuEChERS extracts without modifying chromatographic gradient or performing extra sample cleanup/concentration.
• Ability to inject larger sample volumes while maintaining peak shape, thereby increasing method sensitivity and lowering limits of quantification for early‑eluting, polar pesticides.
• Enhanced robustness for comprehensive multiresidue methods (hundreds of analytes) under UHPLC conditions using sub‑2 µm columns and high backpressure operation.
• Reduced manual data handling because of improved peak symmetry and stable linearity of both peak area and peak height.

Future trends and potential applications

Potential developments and broader uses of Feed Injection technology include:

• Wider implementation in routine pesticide monitoring labs to improve LOQs for polar residues and to simplify workflows for high‑throughput screening.
• Extension to other high‑organic extracts (e.g., environmental, biological) where solvent mismatch causes injection artifacts.
• Integration with advanced dynamic MRM scheduling and smart data review (machine learning/AI) to manage very large analyte panels more efficiently.
• Further optimization of feed parameters and valve designs to increase dilution control, reduce carryover and adapt the approach to diverse chromatographic columns and gradients.
• Standardization efforts and interlaboratory validation to foster regulatory acceptance for compliance testing.

Conclusion

Feed Injection implemented on an ultrahigh‑pressure capable multisampler provides an effective, practical strategy to overcome solvent‑mismatch effects from QuEChERS acetonitrile extracts in comprehensive pesticide LC/MS/MS methods. By diluting the sample with mobile phase prior to column entry, Feed Injection preserves peak shape at larger injection volumes, delivers linear increases in both peak area and peak height, increases sensitivity (lower LOQs) and reduces manual data handling. The approach maintains chromatographic conditions and method simplicity while significantly enhancing analytical performance for large multiresidue panels.

References

1. Kornas P.; Wu L.; Zhao H. Analysis of 764 Pesticides in Tomato Using an Agilent 6495D Triple Quadrupole LC/MS System; Agilent Technologies application note, publication number 5994-8004EN, 2025.
2. Agilent Technologies. Quantitative Analysis of Multiresidue Pesticides in Food Matrices Using Agilent 6470 Triple Quadrupole LC/MS System—Method Protocol; 2022.
3. Naegele E. Improved Peak Shape and Lower LOQs in Pesticide Analysis; Agilent Technologies application note, publication number 5994-6125EN, 2024.
4. Naegele E. Feed Injection at Elevated Pressure Using the Agilent 1290 Infinity III Hybrid Multisampler; Agilent Technologies application note, publication number 5994-8478EN, 2025.