Determination of trace (pg/L) levels of Cypermethrin in water using Stir Bar Sorptive Extraction

Importance of the Topic

Aquatic toxicity concerns and increasingly strict regulatory limits for pesticide residues in water demand analytical methods capable of detecting cypermethrin at sub-nanogram per liter levels. Traditional liquid–liquid extraction approaches require large sample volumes and extensive cleanup steps. Stir bar sorptive extraction (SBSE), combined with sensitive GC-MS/MS detection, offers a streamlined workflow with high enrichment factors, enabling trace-level quantitation in small water samples.

Objectives and Overview of the Study

This work aims to develop and validate an SBSE–GC-MS/MS method for measuring cypermethrin in drinking water at detection limits as low as 0.01 ng/L. The study describes optimization of extraction conditions, thermal desorption parameters and selective multiple reaction monitoring (MRM) transitions to achieve good precision, linearity and sensitivity while minimizing sample volume requirements.

Methodology and Used Instrumentation

The extraction phase consists of a polydimethylsiloxane (PDMS)-coated magnetic stir bar (Twister™) placed into a 50 mL water aliquot. Key steps include:

• Addition of sodium chloride and 5 % methanol to enhance analyte partitioning.
• Stirring for 2 hours to allow cypermethrin uptake onto PDMS (theoretical recovery > 99% for Log Kow = 5.3).
• Drying of the Twister and thermal desorption in a GERSTEL Thermal Desorption Unit directly into a GC inlet equipped with a cooled injection system (CIS) for refocusing.
• Analysis on an Agilent 7890B GC coupled to an Agilent 7000C triple quadrupole MS in MRM mode (transitions 163>127, 163>91, 164.9>91).

Main Results and Discussion

Calibration over 0.1 to 0.5 ng/L yielded excellent linearity (R² = 0.993). Six replicate extractions of 0.1 ng/L cypermethrin produced an RSD of 9 % without internal standard. By increasing sample volume to 250 mL, the method achieved detection at 0.05 ng/L. Chromatographic data confirmed selective and efficient transfer of analyte peaks through optimized thermal desorption and CIS refocusing.

Practical Benefits and Applications

The SBSE–GC-MS/MS approach offers several advantages:

• High enrichment from small sample volumes, reducing labware and solvent use.
• Improved sensitivity and selectivity via MRM detection, minimizing interferences.
• Simplified sample preparation without laborious liquid–liquid extraction.
• Robust precision and linearity suitable for routine monitoring of ultra-trace pesticide levels.

Future Trends and Applications

Further optimization will focus on extraction time, salt and organic modifier concentrations, and inclusion of an isotopically labeled internal standard to enhance quantitation accuracy. Evaluation of method performance in diverse, “dirty” water matrices will address pH effects and potential sorption to particulates or sediment. Integration of SBSE with automated workflows and high-throughput MS platforms could expand applications to broader classes of hydrophobic contaminants.

Conclusion

This study demonstrates that stir bar sorptive extraction coupled with GC-MS/MS delivers the required sensitivity, precision and throughput for routine monitoring of cypermethrin at pg/L levels in water. The streamlined protocol avoids traditional solvent-intensive steps and can be adapted for other lipophilic analytes.

References

• Ridgway K., Carrier D., Maury A. Determination of trace levels of Cypermethrin in water using Stir Bar Sorptive Extraction. Anatune Ltd, Cambridgeshire, UK.