Screening of Pesticide Residues in Water by Sequential Stir Bar Sorptive Extraction-Thermal Desorption with GC/MS

Significance of the Topic

Monitoring pesticide residues in water is crucial due to the potential risks these chemicals pose to human health and ecosystems. Pesticides can persist and bioaccumulate in aquatic environments, often at ultra-trace levels. Traditional extraction methods can struggle to recover both hydrophilic and hydrophobic compounds efficiently, creating a demand for techniques that offer broad-ranging, sensitive screening in a simple and solvent-free workflow.

Objectives and Study Overview

This application note evaluates a novel workflow combining sequential stir bar sorptive extraction (SBSE), thermal desorption (TD), and retention time locked gas chromatography–mass spectrometry (RTL-GC/MS) for comprehensive pesticide screening in water. The goal is to achieve uniform enrichment across a wide polarity spectrum, enabling reliable detection of trace-level contaminants in river water samples.

Methodology

The sequential SBSE procedure comprises two consecutive extractions on a single 5 mL water sample:

• First extraction: ambient water sample without salt, using a PDMS stir bar (24 µL) for 60 min at 1500 rpm.
• Second extraction: same sample after adding 30 % NaCl, using a fresh stir bar under identical conditions.

Both stir bars are transferred into one glass thermal desorption liner, then processed in a thermal desorption unit. Desorbed analytes are cryo-focused in a PTV inlet before scan‐mode GC/MS analysis with a locked retention time method for 926 target compounds.

Instrumentation Used

• GERSTEL Thermal Desorption Unit (TDU) with MPS 2 autosampler
• GERSTEL CIS 4 programmed temperature vaporization inlet
• Agilent 7890 GC coupled to an Agilent 5975 MSD triple-axis detector
• HP-5ms capillary column (30 m × 0.25 mm × 0.25 µm)

Results and Discussion

Recoveries of pesticides with log Kow < 4 rose dramatically with salt addition (e.g., pirimicarb from 15 % to 74 %, fenobucarb from 41 % to 90 %), while hydrophobic compounds (log Kow > 4) maintained high recovery without salt effect using the sequential approach. Signal‐to‐noise ratios of 40:1 were achieved for diazinon at 20 ng/L in scan mode. The RTL library search returned match factors up to 94, reducing false positives. In real river water, symetryn was quantified at 240 ng/L (RSD 2.9 %, n = 6) and pyributicarb at 25 ng/L (RSD 8.9 %, n = 6) via standard addition.

Benefits and Practical Applications

• Broad polarity range coverage in a single workflow
• High enrichment factors and enhanced sensitivity at ng/L levels
• Solvent-free, simple sample preparation
• Automated screening with locked retention times for confident identification

Future Trends and Opportunities

Advances may include expansion of RTL libraries, integration with real-time monitoring systems, coupling with high-resolution mass spectrometry, and applying sequential SBSE to other environmental matrices. Data analytics improvements could further streamline identification and quantification of emerging contaminants.

Conclusion

The sequential SBSE-TD-RTL-GC/MS workflow delivers comprehensive, sensitive screening of a wide spectrum of pesticide residues in water. This solvent-free, automated method ensures uniform extraction efficiency, reliable identification, and practical applicability for environmental monitoring and quality control.

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