Analysis of PCBs using NCI and EI mode

Importance of the Topic

Gas chromatography coupled with comprehensive two-dimensional separation (GC×GC) and quadrupole mass spectrometry under negative chemical ionization (NCI) provides exceptional resolving power and sensitivity for trace-level analysis of polychlorinated biphenyls (PCBs) in complex matrices. This capability is critical for environmental monitoring, food safety, and regulatory compliance, where coeluting matrix interferences often hinder accurate quantitation.

Objectives and Article Overview

• Demonstrate the application of GC×GC-QMS in both electron ionization (EI) and NCI modes for PCB analysis.
• Compare selectivity, sensitivity, and quantitative precision between EI and NCI approaches.
• Illustrate matrix suppression challenges and strategies to enhance target analyte detection.

Methodology

• Two-dimensional separation achieved by thermal modulation using a single hot/cold jet loop modulator (Zoex Corp.) with an 8 s modulation period.
• First-dimension column: 30 m BPX-1 (0.25 mm ID × 0.25 µm df); second-dimension column: 1 m BPX-50 (0.15 mm ID × 0.15 µm df).
• Injection of PCB standards at 25 pg each; QA standard of bovine fat extract spiked with PCBs and dioxins at 10–250 pg/g.
• Acquisition of >10 data points per modulated peak (base width ~250 ms) for robust quantitation.

Used Instrumentation

• Loop modulator (Zoex Corp.) providing two-stage thermal modulation (single hot and cold jet).
• GC×GC system with two capillary columns (BPX-1 and BPX-50).
• Shimadzu GCMS-QP2010 Plus high-speed quadrupole mass spectrometer operated at 25–50 scans/s and 10,000 amu/s scanning speed.
• Capability to switch instantly between EI and NCI ionization modes via software control.

Main Results and Discussion

• NCI mode yielded high selectivity for electrophilic PCBs, minimizing background signals from the bovine fat matrix, as shown by the near absence of non-target peaks.
• EI mode produced total ion chromatograms dominated by matrix interferences, illustrating the benefit of NCI for halogenated analytes.
• Extracted ion images (e.g., m/z 324–329 amu for penta-chlorinated PCBs) further isolated target congeners from residual noise.
• Quantitative precision was confirmed across 10–250 pg/g levels in the spiked fat standard, demonstrating method robustness.

Benefits and Practical Applications

• Enhanced chromatographic peak capacity and resolution mitigate coelution challenges.
• NCI selectively amplifies halogenated compound signals, reducing sample cleanup and improving detection limits.
• Rapid switching between EI and NCI extends method versatility without hardware modifications.
• Applicable to environmental analysis, food safety testing, quality assurance, and regulatory compliance for persistent organic pollutants.

Future Trends and Opportunities

• Integration with high-resolution mass analyzers (e.g., time-of-flight, orbitrap) to improve mass accuracy and compound identification.
• Development of automated deconvolution software for complex GC×GC data to accelerate result interpretation.
• Extension of NCI-based approaches to other electrophilic pollutants, such as polybrominated diphenyl ethers (PBDEs) or dioxin-like compounds.
• Advancements in portable or miniaturized GC×GC systems for in-field environmental monitoring.

Conclusion

The study highlights GC×GC-QMS with NCI as a powerful analytical platform for trace PCB determination in challenging matrices. By coupling high-capacity chromatographic separation with selective ionization and rapid data acquisition, the method achieves superior sensitivity, precision, and throughput, meeting the demands of environmental and food safety laboratories.