Positive chemical ionization using the Thermo Scientific ISQ 7610 single quadrupole GC-MS system

Significance of the Topic

Chemical ionization (CI) in gas chromatography–mass spectrometry (GC-MS) provides a softer ionization pathway compared to conventional electron ionization (EI), preserving molecular ions and enabling more reliable identification of unknowns, especially for compounds not represented in spectral libraries. Positive CI (PCI) broadens analytical capabilities in environmental, food, clinical, and petrochemical applications where molecular weight confirmation is critical.

Objectives and Study Overview

This technical note evaluates the performance of PCI using methane as reagent gas on the Thermo Scientific ISQ 7610 single quadrupole GC-MS with NeverVent technology. A standard mixture of 76 semi-volatile organic compounds (SVOCs) was analyzed in both EI and PCI modes to assess:

• Preservation and detection of molecular ions
• Effect of ion source temperature on spectral patterns
• Practical benefits of rapid mode switching without venting

Methodology and Instrumentation

Samples: Restek 8270 MegaMix (76 SVOCs at 10 ppm in dichloromethane).
Injection: 1 µL split (ratio 8) via TRACE 1610 GC with AI/AS 1610 autosampler.
GC Conditions: TraceGOLD™ TG-5MS column (30 m × 0.25 mm × 1.0 µm); He carrier at 1.8 mL/min; oven 60 °C (1 min) ramped to 265 °C at 10 °C/min, then to 315 °C at 2 °C/min.
MS Conditions: ISQ 7610 with NeverVent; CI ion volume; methane reagent; electron energy 70 eV; source temp 150-320 °C; scan range m/z 45–500.

Main Results and Discussion

Comparison of total ion chromatograms showed all SVOCs detectable in both modes. Key findings include:

• Pyridine: EI produced a weak molecular ion (m/z 79) with extensive fragmentation; PCI yielded strong [M+H]+ (m/z 80) and adducts [M+29]+ and [M+41]+.
• Nitrobenzene: PCI emphasized protonated molecule with minimal fragmentation versus EI.
• Hexachlorocyclopentadiene: Both modes relied on charge exchange; PCI match quality was lower but molecular ion traceable.
• Diethyl phthalate: EI lacked molecular ion; PCI clearly showed M+1+, M+29+, M+41+ patterns, confirming identity.
• Temperature effect: Phenanthrene at 150 °C produced prominent adducts and protonation; higher source temperatures reduced adducts and simplified spectra but improved source cleanliness.

Benefits and Practical Applications

PCI on the ISQ 7610 extends the analyst’s toolset by:

• Improving molecular ion detection for unknown compounds
• Complementing EI and enhancing library matching
• Enabling rapid switching between EI and PCI modes via NeverVent for mixed-mode workflows

Future Trends and Potential Applications

Advancements may include expanded reagent gases, automated mode selection based on target analytes, and integration with high-resolution MS for improved structural elucidation. PCI may find growing use in non-target screening, metabolomics, and trace-level environmental analysis.

Conclusion

Positive chemical ionization with methane on the Thermo Scientific ISQ 7610 GC-MS offers a robust approach to preserving molecular ions and confirming analyte identity. NeverVent technology minimizes downtime and simplifies mode transitions, making PCI an effective complementary technique to EI for a wide range of analytical applications.

References

1. Sparkman O.D., Penton Z.E., Kitson F.G. Gas Chromatography and Mass Spectrometry – A Practical Guide; Academic Press, 2011.
2. Harrison A.G. Chemical Ionization Mass Spectrometry; CRC Press, 1992.
3. Hübschmann H.-J. Handbook of GC-MS: Fundamentals and Applications; Wiley-VCH, 2015.