Variable electron voltage (VeV) on Q Exactive GC and Exactive GC Orbitrap GC-MS systems

Importance of the Topic

Gas chromatography–mass spectrometry (GC-MS) routinely uses 70 eV electron ionization, which provides extensive fragmentation but often lacks molecular ion information. Softer ionization approaches can improve structural elucidation and sensitivity, especially in complex matrices such as biological fluids where low concentration analytes demand high confidence identification.

Objectives and Study Overview

This study demonstrates the application of variable electron voltage (VeV) on high-resolution Orbitrap GC-MS systems to achieve a softer electron ionization. Using a panel of 111 doping compounds spiked into urine, the work evaluates sensitivity, spectral simplification, and mass accuracy across a range of electron energies.

Methodology and Techniques

The VeV approach allows tuning of electron energy from 12 eV to 150 eV within the instrument control software. Samples of eleven urine blanks and four positive quality controls (0.02–200 ng/mL) containing 111 target analytes were analyzed in full-scan mode at 60 000 FWHM resolution. For each compound one quantitation and one confirming ion were selected. Sensitivity and signal-to-noise (S/N) were compared across energies of 12, 15, 20, 30, 50 and 70 eV to identify the optimum setting.

Used Instrumentation

• Thermo Scientific Q Exactive GC-MS/MS
• Thermo Scientific Exactive GC Orbitrap GC-MS

Main Results and Discussion

Lower electron energies significantly enhanced higher m/z fragment and molecular ion signals while reducing low-mass background ions. On average, 30 eV produced a 254 % increase in summed target ion intensity relative to 70 eV, making it the optimal setting for broad analyte sensitivity. At 12 eV, the molecular ion for 19-norandrosterone (m/z 420.28738) showed a marked increase in intensity and simplification of the spectrum, resulting in a 7790:1 S/N ratio compared to 1587:1 at 70 eV. Mass accuracy remained below 1 ppm for all 111 analytes even at lower electron energies, supporting reliable compound identification in complex urine matrices.

Benefits and Practical Applications

• Enhanced sensitivity for low concentration compounds due to stronger molecular and high-mass fragment signals.
• Improved spectral clarity by minimizing low-mass fragments, facilitating library matching and structural elucidation.
• Fully automated autotuning within 30 seconds simplifies operation and maintains instrument performance.
• Consistent sub-1 ppm mass accuracy enables strict extracted ion chromatogram tolerances, reducing false positives.

Future Trends and Opportunities

VeV ionization may be extended to environmental, forensic, metabolomics, and petrochemical analyses where trace detection and structural detail are critical. Integration with advanced data processing and expanding spectral libraries under variable electron energies will further enhance compound identification workflows.

Conclusion

Variable electron voltage on Orbitrap GC-MS platforms offers a versatile, softer ionization mode combining the benefits of electron and chemical ionization. It delivers higher sensitivity, superior spectral information, and robust mass accuracy, improving detection limits and confidence in compound identification for complex sample analyses.

References

1. Märk TD, Dunn GH. Electron Impact Ionization. Springer Science & Business Media; 2013.
2. Munson MSB, Field FH. Chemical Ionization Mass Spectrometry. I. General Introduction. J Am Chem Soc. 1966;88:2621–2630.