Comparison of matrix effects in multi-residue pesticide analysis when using online SPE or direct injection in Liquid Chromatography-tandem Mass Spectrometry

Importance of the Topic

Multi-residue analysis of pesticides in environmental waters is critical given the potential health risks associated with long-term exposure to trace levels of diverse agrochemicals. Stricter regulations worldwide demand methods with lower detection limits, faster turnaround, and high precision without increasing costs. Matrix effects arising from complex water samples pose a significant challenge, highlighting the need to compare sample introduction techniques that can mitigate signal suppression or enhancement.

Study Objectives and Overview

This study aimed to develop and validate two rapid, sensitive, and cost-effective liquid chromatography-tandem mass spectrometry (LC-MS/MS) workflows for the simultaneous determination of 94 pesticides. The approaches under comparison were direct injection and automated online solid-phase extraction (SPE). Key performance metrics included limits of quantification, linearity, matrix effects, and accuracy across different water matrices.

Methodology and Instrumentation

Samples were prepared by spiking spring, surface, deep, and tap water with the target pesticides at various concentrations (5–300 ng/L), along with ascorbic acid, phosphoric acid, and three deuterated internal standards (d5-Atrazine, d6-Bentazone, d6-Diuron). Analyses were performed on a Nexera X2 UHPLC system coupled to an LCMS-8050 triple quadrupole mass spectrometer operating in fast polarity-switching MRM mode.

Used Instrumentation

• UHPLC: Shimadzu Nexera X2
• Mass Spectrometer: Shimadzu LCMS-8050 triple quadrupole
• Direct Injection Column: Zorbax Eclipse Plus C18, 2.1×150 mm, 3.5 μm
• Online SPE Column: Restek Raptor ARC-18, 100×2.1 mm, 2.7 μm; PLRP-S SPE cartridge, 10×2 mm, 15 μm

Main Results and Discussion

Both methods demonstrated excellent linearity (r>0.99). Online SPE achieved limits of quantification below 5 ng/L for 97% of compounds, whereas direct injection reached LOQs below 20 ng/L for 70%. Chromatograms revealed higher signal intensity and reduced baseline noise with SPE. Evaluation of matrix effects across spring, surface, deep, and tap waters showed that SPE generally minimized ion suppression compared with direct injection. Accuracy studies yielded recoveries within 70–130% and RSDs below 15% for most pesticides, confirming robustness in diverse matrices.

Benefits and Practical Applications of the Method

• Online SPE enhances sensitivity and lowers quantification limits, enabling reliable trace-level monitoring.
• Direct injection offers faster sample throughput and simplified workflow for compounds with minor matrix effects.
• Both approaches provide comprehensive multi-residue coverage suitable for regulatory compliance in water quality laboratories.

Future Trends and Opportunities

Advances in high-resolution mass spectrometry and micro-flow LC could further improve sensitivity and selectivity. Integration of online SPE with real-time data processing and automated sample tracking will enhance throughput. Expanding the method to novel pesticide classes and transformation products, as well as coupling with suspect screening workflows, represents promising directions for environmental monitoring.

Conclusion

This comparative study demonstrated that online SPE-LC-MS/MS significantly improves sensitivity and mitigates matrix effects for multi-residue pesticide analysis in water, while direct injection remains a viable high-throughput option for less challenging samples. The validated workflows provide reliable, cost-effective solutions for meeting stringent regulatory requirements.