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

Significance of the Topic

The retention of molecular and high mass fragment ions during electron ionization is essential for confident compound identification and structural elucidation, especially in complex matrices such as biological fluids or environmental samples. Conventional 70 eV EI yields extensive fragmentation, often compromising molecular ion information and sensitivity at low analyte levels. A softer ionization approach that preserves useful structural ions while maintaining high mass accuracy and sensitivity is highly valuable in analytical chemistry applications ranging from doping control to forensic analysis.

Objectives and Study Overview

This study aims to demonstrate the implementation and performance advantages of variable electron voltage (VeV) ionization on Thermo Scientific Q Exactive GC-MS/MS and Exactive GC Orbitrap GC-MS systems. By tuning the electron energy below the standard 70 eV, the work evaluates enhancements in sensitivity, preservation of molecular ions, and mass accuracy for a panel of 111 doping-related analytes at concentrations near regulatory limits.

Methodology and Instrumentation

A fully automated autotuning routine was used to set electron energies between 12 and 70 eV. Samples comprised 11 blank urine matrices and four positive quality control samples spiked with 111 target compounds at concentrations from 0.02 to 200 ng/mL. Analyses were performed in full-scan mode at 60 000 FWHM resolution (m/z 200). Sensitivity and spectral signal-to-noise (S/N) ratios were compared across energies to identify optimal settings for maximum target ion response and minimal background fragmentation.

Used Instrumentation

• Thermo Scientific Q Exactive GC-MS/MS system
• Thermo Scientific Exactive GC Orbitrap GC-MS system
• High-resolution accurate-mass Orbitrap analyzer operating at 60 000 FWHM
• Automated VeV autotuning module allowing electron energy selection from 12 to 150 eV

Key Results and Discussion

Optimization revealed that lowering the electron voltage to 30 eV delivered an average 254 % increase in summed target ion intensity compared with standard 70 eV EI. At the extreme low energy of 12 eV, the molecular ion of 19-norandrosterone (m/z 420.28738) exhibited a S/N ratio improvement from 1587 (70 eV) to 7790, while low-m/z fragments decreased by approximately 20 %, simplifying spectra and enhancing selectivity. Mass accuracy remained consistently below 1 ppm for all 111 analytes at half the minimum required performance limit, demonstrating robust accurate-mass performance independent of ionization energy.

Benefits and Practical Applications

• Enhanced sensitivity for low-abundance compounds, lowering limits of detection
• Improved molecular ion and high mass fragment signals for reliable identification
• Fully automated tuning simplifies routine operation and reduces method development time
• Maintained sub-ppm mass accuracy enables tighter extracted ion chromatogram tolerances and fewer false positives

Future Trends and Applications

Variable electron voltage ionization is poised for broader adoption across fields requiring high sensitivity and structural insight, including environmental monitoring, food safety testing, metabolomics, and forensic toxicology. Integration with advanced spectral libraries and data-processing algorithms may further enhance compound identification. Continued refinement of low-energy EI protocols could expand applicability to even more labile or high-mass analytes.

Conclusion

The VeV technique on Q Exactive GC-MS/MS and Exactive GC Orbitrap GC-MS platforms offers a versatile, soft EI alternative that significantly improves sensitivity and molecular ion preservation while maintaining exceptional mass accuracy. The automated tuning workflow streamlines operation, making VeV a practical enhancement for routine high-resolution GC-MS analysis.

References

1. Märk, T. D.; Dunn, G. H. Electron Impact Ionization. Springer Science & Business Media, 2013.
2. Munson, M. S. B.; Field, F. H. Chemical Ionization Mass Spectrometry. I. General Introduction. J. Am. Chem. Soc., 1966, 88, 2621–2630.