Multidimensional Characterization of Short Chain Chlorinated Paraffins (SCCPs) with GC-APCI and Cyclic Ion Mobility

Significance of the topic

Short-chain chlorinated paraffins (SCCPs, C10–C13) are industrially produced polychlorinated n-alkanes of environmental concern and are regulated as persistent organic pollutants. Their chemical complexity—hundreds of congeners per homologue group and extensive co-elution in gas chromatography—makes reliable qualitative and quantitative analysis difficult. Improved analytical workflows that provide molecular-ion information and an orthogonal separation dimension are therefore important for environmental monitoring, source apportionment, and regulatory compliance.

Objectives and study overview

This study evaluated gas chromatography coupled to atmospheric pressure chemical ionization (GC-APCI, APGC) combined with cyclic ion mobility spectrometry (Cyclic IMS) and high-resolution mass spectrometry (HRMS) for characterization of SCCPs. Goals were to: 1) optimize ionization to produce informative molecular ions for SCCP congeners, 2) assess how ion mobility adds separation power to complex technical mixtures, and 3) demonstrate the ability to resolve and quantify homologue groups in technical SCCP mixes with varying chlorine content.

Methodology

Samples tested included two authentic C10Cl6 congeners and three commercial SCCP technical mixtures (C10–C13) with chlorine contents of 51.5%, 55%, and 63%. GC separations used a nonpolar Rxi-5SilMS column and a modified Agilent 8890 injector. The APGC source was operated in different modes: positive APCI (charge transfer) under dry conditions—yielding extensive fragmentation—and APGC operated with a chloroform modifier to promote formation of [M+Cl]- adducts under negative-ion conditions, which produced dominant molecular ion signals. Ion mobility separations were performed on the SELECT SERIES Cyclic IMS with single- and multi-pass modes (2, 6, and 8 passes) to increase IMS resolving power.

Used instrumentation

• Gas chromatograph: modified Agilent 8890 with Rxi-5SilMS column (30 m × 0.25 mm ID, 0.25 µm film).
• Ion source: Waters APGC (APCI) with capability to introduce chloroform modifier for Cl-adduct formation.
• Mass spectrometer: SELECT SERIES Cyclic IMS with HRMS detection (mass range m/z 50–1200, resolving power ~60,000 FWHM).
• IMS: Cyclic ion mobility cell with multi-pass functionality (resolution reported up to ~184 Ω/ΔΩ for 8 passes).
• Key MS/GC settings: injection 1 µL pulsed splitless (injection port 280 °C), transfer line 290 °C, carrier He ~1.4 mL/min, make-up gas N2 350 mL/min, corona current 2 µA, cone/aux gases per source tuning.

Main results and discussion

• Ionization behavior: Positive APGC charge-transfer conditions caused extensive fragmentation of SCCP congeners and generally did not produce detectable intact molecular ions. Introducing chloroform into the APGC source enabled chloride-adduct [M+Cl]- formation and produced spectra dominated by molecular-ion envelopes for tested C10Cl6 congeners, facilitating both identification and potential quantitation.

• Chromatographic complexity: Technical mixtures showed substantial co-elution across homologues and chlorine-content variants. Extracted-ion chromatograms for homologue-specific m/z values revealed overlapping elution windows, illustrating why simple GC-MS often cannot resolve composition fully.

• Ion mobility contributions: Plotting drift time versus m/z produced distinct trendlines corresponding to homologue groups that share the same number of chlorine atoms. Using the Cyclic IMS with 2 passes allowed isolation of these trendlines and extraction of drift-time traces for individual homologue groups, enabling peak integration and estimation of relative homologue abundances in the mixtures.

• Multi-pass IMS: Increasing IMS passes (6–8) improved separation in the ion mobility dimension and permitted partial differentiation of individual congeners (examples C11Cl6 and C12Cl7). Complete baseline separation of all congeners was not achieved, but differences in drift-time behavior between early- and late-eluting congeners became observable, enhancing compositional insight beyond GC-MS alone.

• Quantitation and profiling: The combination of chloride-adduct formation and IMS-based trendline isolation enabled reproducible integration of homologue groups and relative quantification of their proportions in technical mixes (presented as percent contributions). Limited availability of authentic standards remains a barrier to absolute quantitation at the congener level.

Benefits and practical applications of the method

• Generation of stable molecular ions ([M+Cl]-) increases confidence in homologue assignment and aids quantitative workflows compared with charge-transfer fragmentation alone.
• Ion mobility adds an orthogonal separation that helps deconvolute co-eluting homologues and simplifies integration of homologue groups from complex technical mixtures.
• Multi-pass cyclic IMS provides tunable resolution enabling targeted improvement in separation for specific congested m/z regions.
• Approach supports environmental monitoring, fingerprinting of technical mixtures, and more informed risk assessment where SCCP composition matters.

Limitations and considerations

• Authentic standards are scarce for the multitude of possible SCCP congeners; this limits absolute congener-level identification and quantitation.
• Even with multi-pass IMS, full separation of all congeners in a homologue group is not yet achievable; interpretation often rests on grouping by carbon number and chlorine count.
• Method optimization (source modifiers, pass number, and data processing) is required for routine application and standardization across laboratories.

Future trends and potential applications

1. Continued development of IMS resolving power and multi-dimensional separations to approach congener-level resolution for SCCPs.
2. Expansion of authentic standards and isotope-labeled congeners to enable absolute quantitation and inter-laboratory harmonization.
3. Integration of advanced data-processing algorithms (deconvolution, isotope-pattern matching, machine-learning classification) to extract congener fingerprints from complex IMS–HRMS datasets.
4. Application of GC-APCI–Cyclic IMS–HRMS workflows to environmental matrices (water, sediment, biota) for monitoring, source identification, and regulatory compliance.

Conclusion

GC-APCI operated with a chloroform modifier to produce chloride adducts, combined with cyclic ion mobility and HRMS detection, provides a powerful multidimensional approach for characterizing SCCP technical mixtures. The workflow yields molecular-ion information and an ion-mobility separation dimension that together improve homologue-group resolution and sample profiling, although full congener separation and absolute quantitation remain constrained by standard availability and residual co-migration. This methodology strengthens qualitative characterization and relative quantitation of SCCPs and offers a promising platform for future method development in environmental and regulatory analyses.

References

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5. Waters Atmospheric Pressure GC (APGC) Application Notebook. June 2018;720006302EN.