Soft ionization GC-HRMS of n-Alkanes C8 - C20 (Thermo LTQ Orbitrap XL)

Importance of the Topic

Mineral oil saturated hydrocarbons (MOSH), notably n-alkanes C8–C20, are critical analytes in environmental and food controls. Reliable detection and profiling of these nonpolar molecules is essential to assess contamination and ensure consumer safety.

Study Objectives and Overview

This work presents a novel coupling of gas chromatography with soft plasma ionization (SICRIT) and high-resolution mass spectrometry for comprehensive trace analysis of n-alkanes. The goal is to achieve confident identification and quantification of C8–C20 alkanes, overcoming limitations of traditional GC–EI–MS and nonapplicability of LC–MS for such analytes.

Methodology and Instrumentation

A plug-and-play SICRIT cold plasma source was adapted to a Thermo Trace GC and an LTQ Orbitrap XL to enable soft ionization after chromatographic separation. Key operating parameters included:

• Column: RXI-5ms, 30 m × 0.25 mm, 0.25 µm film
• Injector: Splitless injection of 1 µL n-alkane mixture (40 mg/L in hexane)
• GC oven program: 40 °C (2 min), ramp at 20 °C/min to 280 °C (6 min hold)
• Carrier gas: Helium flow at 1 mL/min
• Plasma conditions: 1.5 kV, 10 kHz
• Mass spectrometer: Orbitrap full scan at high resolution

Main Results and Discussion

All thirteen n-alkanes were baseline-resolved by GC and produced characteristic soft-ionized adducts ([M+O−3H]+ and [M+2O−H]+) with minimal fragmentation. High-resolution mass data enabled exact mass confirmation within sub-ppm accuracy. Calibration curves for C16–C18 alkanes exhibited linearity across three orders of magnitude (R2 > 0.999) and detection limits in the low parts-per-billion range, demonstrating both sensitivity and quantitation reliability.

Benefits and Practical Applications

• Soft ionization reduces fragmentation, simplifying spectral interpretation.
• High mass resolution ensures unambiguous compound identification.
• Wide dynamic range and low detection limits suit trace-level analysis.
• Compatibility with standard LC–MS platforms expands analyte scope.

Future Trends and Potential Applications

Integration of soft-plasma ionization with GC–HRMS may extend to other nonpolar contaminants and complex hydrocarbon mixtures. Ongoing improvements in plasma source design and MS instrumentation could enable ambient analysis and real-time screening in environmental, food, and petrochemical sectors.

Conclusion

The GC–SICRIT–HRMS approach offers a robust and sensitive solution for identification and quantification of trace n-alkanes C8–C20. Its soft ionization mechanism combined with high-resolution detection addresses challenges of conventional methods, marking a significant advance in hydrocarbon analytics.

Reference

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