The use of Kendrick mass defect plots, a new feature in GC Image™ software for GC x GC/high resolution mass spectrometric data analysis: an application on the identification of halogenated contaminants in electronic waste

Significance of the topic

Advances in environmental and industrial analysis demand robust approaches for resolving and identifying trace-level halogenated contaminants in complex matrices. The combination of comprehensive two-dimensional gas chromatography (GC×GC) with high-resolution mass spectrometry (HRMS) generates highly detailed data sets, but interpreting these can be daunting. Implementing Kendrick mass defect plots tailored for hydrogen/chlorine substitutions streamlines the visualization and classification of halogenated homologues, enhancing sensitivity, selectivity and throughput in environmental monitoring and quality control.

Objectives and overview

This study demonstrates a non-traditional Kendrick mass defect approach, substituting hydrogen with chlorine, to simplify detection of chlorinated and brominated compounds in electronic waste dust. The goals were to implement H/Cl Kendrick plotting within GC Image™ software version 2.5 and to evaluate its performance for rapid visual identification and automated extraction of halogenated homologous series from GC×GC/HR-TOF data.

Methodology and instrumentation

Sample preparation: A dust sample from an e-waste recycling facility was directly analyzed without derivatization.

GC×GC setup:

• Thermal modulation: Zoex ZX2 two-stage modulator
• 1st column: Rxi-5SilMS, 30 m × 0.25 mm, 0.25 µm
• 2nd column: Rxi-17SilMS, 2 m × 0.15 mm, 0.15 µm
• Oven program: 50 °C (1 min) → 5 °C/min → 320 °C (5 min)
• Inlet: splitless, constant flow

HR-TOFMS:

• Instrument: JEOL AccuTOF GCv 4G
• Ionization: EI+, 70 eV, 300 μA
• Source temperature: 250 °C; transfer line: 280 °C
• Mass range: m/z 45–800, acquisition at 50 Hz

Data processing:

• External calibration: single-point using C5H15O3Si3+ (m/z 207.0329)
• Generation of an average mass spectrum over the full chromatogram
• Calculation of H/Cl Kendrick mass defect: IUPAC mass × (34/33.96102)
• Plotting nominal mass vs. H/Cl mass defect in GC Image™ v2.5

Results and discussion

The GC×GC total ion chromatogram revealed hundreds of co-eluting features. The average mass spectrum exhibited numerous halogenated peaks. When transformed to an H/Cl Kendrick defect plot, each homologous series of chlorinated or brominated species aligned horizontally, enabling instant visual grouping. Key findings:

• Tetrachlorinated phosphate ester (TCPP) and polychlorinated terphenyls (Cl8–Cl11) formed distinct horizontal clusters.
• Brominated flame retardants such as tetrabromobisphenol A, polybrominated diphenyl ethers (Br2–Br7) and hexabromobenzene were similarly grouped.
• Vertical polygons drawn around clusters allowed automated extraction of selective high-resolution mass chromatograms.
• Library matching (NIST) and exact mass measurements confirmed compound identities with mass errors typically below ±1.5 mmu.

Benefits and practical applications

By harnessing non-traditional Kendrick defect plotting, analysts can:

• Instantly recognize and differentiate halogenated compound classes in complex mixtures.
• Generate targeted chromatograms without manual peak picking.
• Reduce false positives by focusing on homologous series rather than individual m/z values.
• Streamline workflows for environmental screening, regulatory compliance and forensic studies.

Future trends and potential uses

Expanding Kendrick mass defect analyses may include other element substitutions (e.g., H/Br, H/F) and integration with machine-learning algorithms for automated pattern recognition. Real-time data processing and cloud-based sharing of Kendrick plots could further enhance collaborative environmental monitoring. Applications will extend into petrochemical profiling, metabolomics, food safety, and materials characterization.

Conclusion

The implementation of H/Cl Kendrick mass defect plotting in GC Image™ v2.5 greatly simplifies the identification of chlorinated and brominated contaminants in electronic waste dust. Coupled with GC×GC/HR-TOFMS, this approach enables rapid visual classification, automated extraction of homologous series and high-confidence confirmation via accurate mass and library searches. The methodology promises to enhance throughput and reliability in complex mixture analysis.

References

1. Kendrick E., A mass scale based on CH2 = 14.000 for high resolution mass spectrometry of organic compounds, Anal. Chem. 1963, 35:2146–2154
2. Jobst K.J. et al., The use of mass defect plots for the identification of (novel) halogenated contaminants in the environment, Anal. Bioanal. Chem. 2013, 405:3289–3297