A New Retention Index System for PCBs

Significance of the topic

Polychlorinated biphenyls (PCBs) remain persistent environmental contaminants with 209 possible congeners and many positional isomers within homologue groups. Mass spectrometry (MS) provides strong structural information, but MS spectra alone often cannot distinguish closely related PCB isomers. Retention indices (RIs) add an orthogonal, retention-based identifier that, when combined with MS, increases confidence in congener-specific assignments. Replacing conventional n-alkane markers with a resident series of PCB congeners as retention standards aligns the marker chemistry with analytes of interest, reduces sample manipulation, and improves selectivity in complex environmental matrices.

Objectives and overview of the study

The work proposes and tests a new RI system specifically for PCBs using a resident reference series of PCB congeners commonly present in industrial technical mixtures (Aroclors). The objectives were to: establish a linear retention scale for PCBs across typical GC stationary phases, implement the RI calculation in vendor software, and demonstrate the approach on Aroclor standard mixtures to evaluate identification accuracy and practical applicability in routine PCB analysis.

Methodology

• Concept: Use a nine-member series of PCB congeners (IUPAC numbers 1, 8, 31, 44, 101, 138, 180, 203, 206) as reference markers, assigning incremental RI anchor points (100, 200, ... 900) and interpolating RIs for other congeners using retention time relationships.
• RI calculation: Van den Dool and Kratz algorithm implemented within LECO ChromaTOF software was used to compute RIs from the resident PCB reference series.
• Validation: Retention behavior versus chlorine number (Cl#) was evaluated for linearity across multiple stationary phases; an Aroclor mix was analyzed to assign congeners by RI and confirm assignments by mass spectral matching and deconvolution.

Instrumentation used

• GC-TOFMS platform: LECO Pegasus III gas chromatograph—time-of-flight mass spectrometer controlled and processed by LECO ChromaTOF software.
• Column: DB-XLB, 40 m × 0.10 mm i.d. × 0.10 µm film thickness with a 4 m retention gap.
• Carrier gas/flow: Hydrogen, constant flow at 0.7 mL/min.
• Injection: 1 µL splitless injection at 260 °C.
• Oven program: 75 °C (1 min) ramp to 125 °C at 50 °C/min then to 260 °C at 5 °C/min.
• MS conditions: Electron ionization at 70 eV; source temperature 225 °C; mass range 120–520 u; acquisition rate 5 spectra/s.
• Standards: 1 ng/µL PCB mixes (AccuStandard) and Aroclor blends used for method development.

Main results and discussion

• Reference series behavior: The chosen nine PCB congeners showed linear retention behavior when plotted as retention time versus chlorine number on DB-XLB and similar linearity across a range of stationary phases from nonpolar to moderately polar. This linearity supports interpolation of RIs for other congeners.
• Implementation: The RI routine was incorporated into ChromaTOF, enabling automated peak finding, deconvolution, and RI assignment using the resident PCB markers—avoiding the need to spike each sample with external alkane standards.
• Aroclor test: Analysis of an Aroclor mixture located the resident reference peaks and produced RI assignments for many congeners. Comparison of derived RIs against standard RIs showed excellent agreement (most differences on the order of 0.0x–0.x RI units), indicating robust transferability and precision for congener identification on DB-XLB under the stated GC conditions.
• Tables and figures summary: Tabulated RI values (Table 1) provide RIs for numerous congeners on DB-XLB anchored to the 100–900 resident-series scale. Table 2 demonstrates congener assignments in an Aroclor mix and reports small RI differences versus reference standards. A graphical regression (RT vs Cl#) confirms linear retention across the reference series.

Benefits and practical applications of the method

• Improved isomer discrimination: RI adds an orthogonal criterion to MS spectra, aiding reliable assignment among co-eluting or isomeric congeners that produce similar mass spectra.
• Cleaner marker chemistry: Using PCB congeners as markers reduces potential confusion from n-alkane contamination and provides marker peaks with chlorine-rich spectral signatures distinct from common matrix interferences.
• Workflow efficiency: A resident marker series that occurs in Aroclors and many environmental samples eliminates the need for routine spiking of each sample with external standards, simplifying sample preparation and reducing consumable costs.
• Software integration: Built-in RI calculation and automated deconvolution streamline identification in routine GC-TOFMS analysis, improving throughput in monitoring and research laboratories.

Future trends and potential applications

• Cross-platform standardization: Expanding RI databases for PCBs across different columns and instrument platforms to promote harmonized identification criteria across laboratories.
• Expanded resident marker sets: Developing resident RI reference series for other halogenated pollutants to replicate the benefits shown for PCBs.
• Integration with high-resolution MS and chemometrics: Combining RI data with exact-mass spectral libraries and multivariate pattern recognition to further resolve complex mixtures and low-abundance congeners.
• Automated library matching and reporting: Enhancing software to automatically flag RI/MS concordance and provide confidence metrics for regulatory reporting and long-term environmental monitoring programs.

Conclusion

A resident PCB-based retention index system is a practical and robust complement to mass spectrometry for congener-specific PCB analysis. The chosen nine-congener reference series exhibits linear retention behavior across a range of columns, can be found natively in Aroclors, and supports reliable RI assignment with minimal deviation from standards when analyzed on a DB-XLB column under the tested GC-TOFMS conditions. Implementation within vendor software streamlines adoption and improves confidence in PCB congener identification for environmental monitoring and research.

References

• J.W. Cochran and G.M. Frame, Recent developments in the high-resolution gas chromatography of polychlorinated biphenyls, Journal of Chromatography A 843 (1999) 323–368.
• S. Chu, X. Miao, and X. Xu, Retention index system for identification of polychlorinated biphenyl congeners in gas chromatographic analysis, Journal of Chromatography A 724 (1996) 392–397.
• G. Castello and G. Testini, Gas chromatographic retention index system for polychlorinated biphenyls: possibilities and limitations, Journal of Chromatography A 787 (1997) 215–225.
• LECO Corporation application note: A New Retention Index System for PCBs, LECO Pegasus III GC-TOFMS platform documentation (2008).