Use of SPE and Twister (SBSE) to enrich Heptachlor and Heptachlor epoxide from water with a GC/QTOF in NCI

Importance of the Topic

Organochlorine pesticides such as heptachlor and its epoxide are persistent environmental contaminants in water at ultra-trace levels. Detecting these compounds at parts-per-trillion concentrations is critical for regulatory compliance and assessing ecological and human health risks. Integrating solid-phase extraction (SPE) with stir bar sorptive extraction (SBSE or Twister™) and high-resolution GC/QTOF in negative chemical ionization mode enhances analyte enrichment and detection sensitivity for hydrophobic pollutants.

Aims and Study Overview

The study aimed to develop an automated enrichment procedure combining SPE and PDMS-coated Twister™ extraction to achieve ultra-trace detection of heptachlor and heptachlor epoxide in aqueous samples. Performance was evaluated using spiked standards at 20 ng/L, low-level SPE extracts at 1 pg/L, and blank matrices to assess sensitivity, precision, and background contamination.

Methodology

Extraction and Enrichment:

• Prepared six 10 mL aqueous standards at 20 ng/L in 5 % methanol.
• Conducted SPE on water samples, then added PDMS-coated Twister bars for SBSE.
• Agitated samples for one hour, removed bars, and dried before analysis.

Detection:

• Thermally desorbed analytes from Twister bars into the cooled inlet system (CIS) using a GERSTEL TDU/CIS setup.
• Analytes separated on an Agilent 7890B GC and detected with an Agilent 7200 QTOF in NCI mode using ammonia reagent gas.

Used Instrumentation

• GERSTEL MultiPurpose Sampler (MPS) with dual-head configuration
• GERSTEL Thermal Desorption Unit (TDU) and Cooled Inlet System (CIS4)
• Agilent 7890B Gas Chromatograph
• Agilent 7200 QTOF Mass Spectrometer in NCI mode

Key Results and Discussion

Extraction Efficiency and Sensitivity:

• Heptachlor and its epoxide, with Log Kow values above 5, exhibited recoveries above 90% on PDMS Twister bars.
• For 20 ng/L twister extracts, signal-to-noise ratios were 91.6 (heptachlor) and 1114.9 (epoxide).
• At 1 pg/L (SPE twister extracts), S/N ratios of 35.7 (heptachlor) and 99.5 (epoxide) were achieved.

Precision:

• Six replicate 20 ng/L extractions yielded RSDs of 9.4 % for heptachlor and 8.8 % for the epoxide.
• Using each analyte as an internal standard for the other could improve precision to around 6 % RSD.

Background Levels:

• Trace contamination detected in blank SPE twister extracts indicates the need for further background reduction.

Benefits and Practical Applications

• Automated SPE-SBSE workflow offers high throughput and reproducibility.
• High-resolution QTOF detection in NCI mode provides selective soft ionization and accurate mass measurement (<3 ppm error).
• Method achieves ppt-level sensitivity suitable for environmental monitoring, water quality assessment, and regulatory testing.

Future Trends and Opportunities

Further optimization to reduce blank background will enhance method robustness. Extending this automated SPE-SBSE-GC/QTOF approach to other organochlorine pesticides and emerging hydrophobic pollutants can broaden environmental surveillance. Incorporating dedicated internal standards and advanced data processing will improve quantitation and screening capabilities.

Conclusion

The developed automated SPE and Twister™ SBSE method coupled with GC/QTOF in NCI mode delivers a sensitive, selective, and robust protocol for trace-level determination of heptachlor and heptachlor epoxide in water. Achieving ppt detection limits with acceptable precision supports stringent water quality analyses and can be adapted to other hydrophobic contaminants.