Sensitive and Reproducible Determination of Organochlorine Pesticides in Water

Importance of the topic

The analysis of organochlorine pesticides (OCPs) remains a high-priority task in environmental monitoring because these compounds are persistent, bioaccumulative and toxic. Regulatory compliance and public-health decision making require analytical methods that combine low detection limits, reliable quantification and confirmatory identification. Gas chromatography with electron capture detection (GC/ECD) is widely used for OCPs due to the strong electron-capturing nature of halogenated analytes; coupling GC/ECD with large-volume injection (LVI) and a dual-column confirmation strategy improves sensitivity while meeting regulatory confirmation criteria such as USEPA Method 8081B.

Study objectives and overview

This application note evaluated an Agilent GC/ECD system equipped with a Multimode Inlet (MMI) operating in solvent-vent LVI mode for the quantitative determination of 31 organochlorine pesticides in a water-analysis context. Specific goals were to: demonstrate sensitive detection using a 5 µL LVI in hexane, verify calibration linearity across a practical regulatory range, determine instrument detection limits (IDLs), assess multiday stability (retention time and peak-area repeatability), and confirm analyte identity using two orthogonal GC columns (DB-5ms and DB-1701) per EPA 8081 recommendations.

Methodology

Sample preparation and calibration:

• Calibration standards prepared in hexane at six concentrations: 5, 10, 20, 50, 100 and 200 ppb.
• IDLs estimated from seven replicate injections of the 10 ppb standard using a 99% confidence approach (Student’s t, n–1 = 6; t = 3.14) and the %RSD of replicate peak areas.

Large-volume injection strategy:

• An Agilent Multimode Inlet (MMI) solvent‑vent LVI method deposited 5.0 µL of sample onto the inlet liner while venting solvent; after venting the inlet switched to splitless for analyte transfer to the column.
• Method optimization considered solvent type, inlet temperature ramps, vent flow/time, inlet pressure during evaporation, and liner geometry to avoid solvent flooding, peak distortion and tailing.

Chromatographic conditions (summary):

• Two columns were used for primary and confirmatory separations: DB‑5ms (30 m × 0.25 mm × 0.25 µm) and DB‑1701 (30 m × 0.25 mm × 0.25 µm).
• Carrier gas: helium with constant flow (1.1 mL/min on DB‑5ms; 0.75 mL/min on DB‑1701).
• Injector: MMI solvent‑vent mode with initial low inlet temperatures and rapid ramp to 325 °C; Agilent Ultra Inert dimpled splitless liner used.
• Detector: Electron Capture Detector (ECD) at 320 °C with 15 mL/min make‑up gas.
• Oven programs were tailored per column to provide effective peak focusing during venting and baseline separation of all 31 OCPs.

Instrumentation used

Key system components and consumables used in the study:

• Agilent gas chromatograph coupled to an Electron Capture Detector (ECD).
• Agilent Multimode Inlet (MMI) run in solvent‑vent (LVI) mode.
• Analytical columns: DB‑5ms (p/n 122‑5532) and DB‑1701 (p/n 122‑0732), both 30 m × 0.25 mm, 0.25 µm film thickness.
• Agilent Ultra Inert dimpled splitless inlet liner (p/n 5190‑2297).
• Carrier gas: helium; ECD make‑up gas flow 15 mL/min.

Key results and discussion

Calibration and linearity:

• Six‑point calibrations (5–200 ppb) for all 31 pesticides produced excellent linearity: all analytes achieved R² > 0.992, most exceeding 0.997 on both columns, indicating robust quantitative performance across the working range.

Instrument detection limits (IDLs):

• IDLs were calculated from seven replicate injections of the 10 ppb standard using the 99% confidence multiplier (t = 3.14) and the observed %RSD.
• For both DB‑5ms and DB‑1701 columns, IDLs were 2.39 ppb or lower for all analytes; many compounds showed substantially better IDLs (≈0.7–1.5 ppb on DB‑1701 for numerous targets).

Chromatography and confirmation:

• Representative chromatograms demonstrated baseline separation of the 31 OCPs on both phases, enabling primary identification and orthogonal confirmation per EPA 8081B requirements.

System stability and reproducibility:

• Multiday stability was assessed with 15 replicate injections over five days (three injections/day).
• Retention time repeatability was excellent, with RT %RSD ≤ 0.07% for all analytes.
• Peak area reproducibility across the test sequence showed %RSD ≤ 7.83% (many compounds substantially lower), confirming method robustness for routine monitoring.

Practical considerations and method optimization:

• MMI solvent‑vent mode simplified LVI method development by enabling controlled solvent venting and analyte refocusing on the column; proper vent timing, vent flow and initial oven temperature were critical to avoid solvent overloading or analyte loss.
• Column selection influenced sensitivity and IDLs; DB‑1701 provided slightly lower IDLs for several analytes, illustrating the value of a confirmatory column with different polarity.

Benefits and practical applications

The combined MMI solvent‑vent LVI and GC/ECD workflow offers several operational advantages for environmental pesticide analysis:

• Improved sensitivity without complex sample concentration steps, enabling detection near or below regulatory screening levels.
• High reproducibility (retention time and area), supporting reliable quantification and trending in routine monitoring and QA/QC programs.
• Dual‑column confirmation aligns with EPA 8081B requirements, increasing confidence in identification and reducing false positives.
• Compatibility with existing GC/ECD infrastructure, facilitating adoption in regulatory and contract laboratories handling water and environmental matrices.

Future trends and applications

Potential directions to extend and apply this workflow include:

• Integration of automated sample-prep techniques (e.g., online extraction or SPE‑GC interfaces) to increase throughput while preserving LVI sensitivity.
• Coupling LVI with mass spectrometric detectors (GC‑MS/MS) for enhanced selectivity and unequivocal confirmation, particularly in complex matrices.
• Method adaptation for trace analysis in other matrices (soil, sediment, biota) where matrix effects require additional cleanup or matrix-matched calibration.
• Exploration of alternative inlet liners and venting algorithms to further reduce IDLs and increase robustness for low‑volatility or thermally labile compounds.

Conclusion

The Agilent GC/ECD system with Multimode Inlet solvent‑vent LVI successfully delivered a sensitive, reproducible and regulatory‑compatible method for 31 organochlorine pesticides. The approach achieved excellent calibration linearity (R² > 0.992), low IDLs (≤ 2.39 ppb), and strong multiday stability (RT %RSD ≤ 0.07%, area %RSD ≤ 7.83%). Dual‑column confirmation using DB‑5ms and DB‑1701 met EPA 8081B confirmation requirements, providing a practical solution for routine environmental monitoring programs.

Reference

Method 8081B: Organochlorine Pesticides by Gas Chromatography, part of Test Methods for Evaluating Solid Waste, Physical/Chemical Methods