Defining the Retention Times of 209 PCB Congeners Using GCxGC-TOFMS

Importance of the topic

Polychlorinated biphenyls (PCBs) are persistent organic pollutants recognized for their toxicity, potential carcinogenicity, and environmental persistence.
Their 209 congeners have very similar structures and mass spectra, making complete separation and identification a significant analytical challenge.
The ability to resolve and assign retention times accurately is critical for environmental monitoring, food safety, and regulatory compliance.

Study objectives and overview

This study aimed to determine the primary (tr′) and secondary (tr″) GCxGC retention times for all 209 PCB congeners using a LECO Pegasus 4D GCxGC-TOFMS system.
It also evaluated the software’s ability to distinguish and correctly identify difficult "critical pairs"—congeners with nearly identical elution behavior.
The work includes constructing a custom user library of PCB spectra and applying ion ratio calibration to improve identification confidence.

Methodology and instrumentation

A solution of each PCB congener (20µg/mL in isooctane) was individually analyzed, with critical pairs tested in combined solutions (~10µg/mL each).
The GCxGC configuration comprised:

• Primary column: 40m×0.18mm i.d., 0.18µm RTX-PCB film
• Secondary column: 1m×0.10mm i.d., 0.10µm RTX-17 film
• Temperature program: 70°C hold, ramps to 150°C at 10°C/min, to 250°C at 1°C/min, to 275°C at 4°C/min with post-run extension
• Modulator offset 25°C, 4.00s period (1.10s hot, 0.90s cold)
• Split inlet ratios adjusted (100:1 to 20:1) based on elution time
• Helium carrier gas at 1.2mL/min, transfer line and inlet at 280°C
• TOFMS range 45–550m/z at 100 spectra/s, ion source at 200°C, 70eV

Data acquisition and initial retention processing used LECO ChromaTOF software; additional retention calculations were managed in Excel.

Main results and discussion

The primary and secondary retention times for all 209 congeners were established and organized by chlorine count.
Retention windows overlapped for many congener groups, except for 1-Cl and 10-Cl PCB congeners which were isolated in tr′.
Fifteen critical pairs with identical tr′ and 63 pairs within one or two modulation periods were identified.
Preliminary identification using a user-created library often returned the correct chlorine number but misassigned exact congener structure due to spectral similarity.
Applying ChromaTOF’s single-point ion ratio calibration improved match scores and correctly prioritized the target congener in at least one test case (PCB 43/69).
The study showed that combining two-dimensional retention information with ion ratio data enhances the reliability of PCB congener assignments.

Benefits and practical applications

This retention database enables analysts to:

• Narrow the list of candidate congeners to one to three possibilities based on tr′ and tr″
• Improve identification confidence when spectra are highly similar
• Optimize GCxGC methods for routine monitoring of environmental or food samples
• Support regulatory compliance by providing definitive retention benchmarks

Future trends and opportunities

Further integration of comprehensive two-dimensional chromatography with advanced data processing, such as machine learning classifiers, could target remaining critical pairs.
Development of automated retention time alignment and dynamic ion ratio libraries may streamline congener identification.
Expansion of the approach to other challenging analyte classes (e.g., brominated flame retardants) could broaden the impact.

Conclusion

This work delivered a robust retention time atlas for all 209 PCB congeners using GCxGC-TOFMS and demonstrated that leveraging two-dimensional separation alongside ion ratio calibration significantly improves congener identification.
The generated data and methods form a foundation for more accurate environmental and food safety analyses of PCBs.

Reference

Safe S Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action Crit Rev Toxicol 1984;13(4):319–395