Soft ionization GC-HRMS of n-Alkanes C8 - C20 (SCIEX TripleTOF® 5600)

Importance of the Topic

Mineral oil saturated hydrocarbons pose analytical challenges due to complex mixtures. MOSH fractions in environmental and food samples require reliable separation and identification. Traditional GC-FID and EI-MS methods struggle with isomer differentiation and produce extensive fragmentation, limiting structural confirmation. A soft ionization approach for non-polar n-alkanes can improve sensitivity and specificity in quantitative trace analysis.

Objectives and Study Overview

This study demonstrates a novel coupling of gas chromatography with Soft Ionization by Chemical Reaction In Transfer (SICRIT) and high-resolution mass spectrometry (GC-SICRIT-HRMS) on a TripleTOF 5600 platform. The method targets trace analysis of straight-chain alkanes (C8–C20), aiming to achieve minimal fragmentation, accurate mass measurement, and clear isomeric differentiation under fast and straightforward setup conditions.

Instrumentation and Methodology

The analytical setup integrated standard GC and LC-MS components through a plug-and-play plasma-based ion source. Key parameters included:

• Gas chromatograph: Agilent 7890 with Rxi-5ms column (30 m × 0.25 mm, 0.25 µm film) and deactivated split liner.
• SICRIT ion source: cold plasma at 1.5 kV and 15 kHz enabling flow-through soft ionization.
• Mass spectrometer: SCIEX TripleTOF 5600 operated in high-resolution full scan mode.
• Carrier gas: helium at constant flow 2 mL/min; GC temperature program from 40 °C to 280 °C.

Standard solutions of n-alkanes C8–C20 at 40 mg/L in hexane were injected (1 µL, split 1:10). The method generated oxidized molecular ions with minimal fragmentation, facilitating precise mass assignment.

Main Results and Discussion

The GC-SICRIT-HRMS approach successfully separated all 13 n-alkanes from octane to eicosane. High-resolution mass spectra were dominated by [M+O–3H]+ and [M+2O–H]+ species while conventional EI fragments were nearly absent. Exact mass data allowed unambiguous identification of each alkane isomer. Preliminary tests suggest detection limits in the low parts-per-billion range, indicating high sensitivity. This soft ionization strategy overcomes the unresolved complex mixture issue common in EI-MS and enhances confidence in compound assignment.

Benefits and Practical Applications

• Reduced fragmentation yields cleaner spectra and simplifies data interpretation.
• High resolution mass accuracy ensures reliable identification of homologous series.
• Low detection limits enable trace-level analysis in environmental and food matrices.
• Easy integration with existing GC and LC-MS instruments via plug-and-play ion source.

Future Trends and Opportunities

The soft ionization GC-HRMS concept can be extended to other non-polar compound classes, including polyolefins and complex hydrocarbon mixtures. Ongoing improvements in plasma ion source design may further enhance sensitivity and ionization efficiency. Standardization of soft ionization techniques for MOSH analysis could support regulatory monitoring and quality control in industry and environmental laboratories.

Conclusion

GC-SICRIT-HRMS offers a robust and versatile method for the analysis of short-chain n-alkanes, combining gentle ionization with high-resolution mass spectrometry. The technique provides clear molecular ion signals, low detection limits, and straightforward instrument coupling, making it a promising solution for MOSH profiling and trace hydrocarbon analysis.

References

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