Extraction and Analysis of Pesticides from Water by Solid Phase Extraction and GC/MS

Importance of the topic

Solid-phase extraction (SPE) followed by gas chromatography–mass spectrometry (GC/MS) is a cornerstone technique for trace-level monitoring of pesticides in drinking and environmental waters. Reliable, sensitive, and validated methods are essential for regulatory compliance, public-health protection, and routine laboratory screening. The described approach implements EPA Method 525.2 adapted with Agilent SPE cartridges and Agilent 8890/5977 GC/MS instrumentation to quantify 22 semivolatile and organochlorine pesticides at sub-ppb concentrations, addressing regulatory needs such as those defined by Bureau of Indian Standards (BIS) and other international guidelines.

Objectives and study overview

The main goal was to demonstrate a cartridge-based SPE procedure coupled with GC/MS for accurate, precise, and sensitive determination of 22 pesticides in water. Specific objectives included: establishing matrix-matched calibration (5–80 ng/L post-extraction equivalents 5–80 ng/mL after 1,000× concentration), evaluating linearity, determining recoveries and precision at low fortification levels (5–80 ng/L), and illustrating practical workflow suitable for regulatory laboratories.

Methodology and instrumentation

The method follows EPA Method 525.2 with practical adaptations for cartridge SPE and GC/MS analysis. Key procedural steps and parameters are summarized below.

• Sample volume and conditioning: 1 L water samples adjusted to ~pH 2; reagent (blank) water used for controls.
• SPE cartridges: Agilent Bond Elut C18 cartridges (1 g, 3 mL) conditioned sequentially with ethyl acetate (5 mL), methylene chloride (5 mL), and methanol (10 mL).
• Sample loading: Samples drawn through cartridges under vacuum (~13 cm Hg); 1 L passes in ~2 hours; avoid cartridge drying during loading; post-sample air/N2 purge for 10 minutes.
• Elution and drying: Cartridge eluates collected using 5 mL ethyl acetate and 5 mL methylene chloride rinses. Residual water removed by passage through anhydrous sodium sulfate (2–3 g) and rinsed with methylene chloride.
• Concentration: Solvent extracts concentrated under a gentle nitrogen stream (low-volume concentrator) to 1 mL (1,000× concentration factor) and transferred to GC vials.
• Calibration: Working mix prepared in ethyl acetate; blank water fortified before extraction to produce extracted calibration standards corresponding to 5, 10, 20, 40, and 80 ng/L (resulting in 5–80 ng/mL after concentration).

Instrumentation used

• GC/MS: Agilent 8890 gas chromatograph coupled to Agilent 5977 mass spectrometer operated in selected ion monitoring (SIM) mode.
• Column and flow: Agilent J&W HP-5ms UI, 30 m × 0.25 mm, 0.25 μm; helium carrier gas, constant flow ~1.2 mL/min.
• Inlet and injection: Agilent Multimode Inlet (MMI) at 280 °C; splitless injection, 1 µL.
• Oven program and temperatures: initial ~60 °C (hold 1 min), ramp (≈10 °C/min) to mid-temperature (≈170 °C) and then to 310 °C with final hold (~3 min); transfer line ~310 °C.
• MS conditions: source ~300 °C (Extractor source with 3 mm lens), quadrupole ~150 °C; SIM acquisition using specific quantifier/qualifier ions per compound.
• Consumables: non-stick septa, ultra-inert splitless liners (with glass wool), graphite/Vespel ferrules, and appropriate syringes as listed in the original setup.

Main results and discussion

• Linearity: Matrix-matched calibration curves across the extracted-equivalent range 5–80 ng/mL produced correlation coefficients (R2) greater than 0.995 for all 22 pesticides, demonstrating excellent linearity under SIM conditions.
• Retention window: Compounds eluted in a predictable retention-time window spanning roughly 7.8–13.2 minutes, enabling targeted SIM windows and reliable chromatographic separation on the HP-5ms column.
• Accuracy and precision: Recoveries were within 80–120% for the evaluated pesticides at fortification levels down to 5 ng/L; associated relative standard deviations (RSDs) were generally below 15% across replicate runs, meeting common regulatory acceptance criteria.
• Blank performance: Reagent blanks showed no measurable contamination for the target analytes at the method detection levels, indicating adequate procedural cleanliness and solvent selection.
• Practical throughput: A single 1 L sample required approximately 2 hours to pass through the SPE manifold under the stated vacuum, with post-extraction concentration and instrumental runtime supporting routine batch analysis in regulatory laboratory settings.

Benefits and practical applications of the method

• Regulatory suitability: Sensitivity and precision allow detection and quantitation below typical maximum residue limits (MRLs) set by BIS and similar agencies, supporting compliance testing for drinking, packaged, and surface waters.
• Robustness: Cartridge SPE with C18 sorbent and dual-solvent elution provides broad coverage for moderately polar to nonpolar pesticides included in Method 525.2.
• Reproducibility: The use of matrix-matched extracted standards and SIM acquisition minimizes matrix effects and improves quantitation accuracy for trace-level analytes.
• Scalability: The procedure is compatible with vacuum manifolds and semi-automated SPE workflows, enabling higher sample throughput in monitoring or routine QA/QC labs.

Future trends and applications

• Method integration with high-resolution MS: Adoption of high-resolution mass spectrometry (HRMS) could expand screening capability to additional pesticide transformation products and non-target screening while retaining trace sensitivity.
• Automation and online SPE: Increased use of automated cartridge handling and online SPE–GC/MS interfaces can reduce manual handling, minimize variability, and improve laboratory throughput.
• Miniaturized and green workflows: Method optimization to reduce solvent consumption (e.g., microextraction, dispersive SPE) and replace hazardous solvents would improve sustainability and safety.
• Expanded analyte scope: Adapting extraction chemistries (mixed-mode sorbents) and complementary LC–MS methods will be necessary to cover more polar pesticides that are not amenable to GC/MS.

Conclusion

The implemented SPE–GC/MS procedure based on EPA 525.2 combined with Agilent Bond Elut C18 cartridges and Agilent 8890/5977 instrumentation delivers a sensitive, precise, and accurate workflow for determination of 22 pesticides in water at levels as low as 5 ng/L. The method demonstrated robust linearity (R2 > 0.995), acceptable recoveries (80–120%), and RSDs typically under 15%, making it well suited for regulatory monitoring and routine laboratory applications focused on drinking and environmental water quality.

References

1. Bureau of Indian Standards — Drinking water specification (IS 10500).
2. Bureau of Indian Standards — Packaged drinking water specification (IS 14543).
3. Bureau of Indian Standards — Natural mineral water specification (IS 13428).
4. United States Environmental Protection Agency — Method 525.2: Determination of organic compounds in drinking water by liquid–solid extraction and GC/MS.