Combination of Chemical Ionization (CI) and Low Energy Ionization Capabilities with High-resolution Time-of-Flight GC/MS

Importance of the Topic

High-resolution gas chromatography coupled with high-resolution time-of-flight mass spectrometry (GC/Q-TOF) using combined chemical ionization (CI) and low-energy electron ionization (EI) expands analytical capabilities, especially for environmental contaminants and untargeted screening. This approach enhances molecular ion detection and compound identification specificity without frequent source changes.

Study Objectives and Overview

The study evaluated the performance of a prototype CI source alongside a low-energy EI source on an Agilent 7250 GC/Q-TOF system. Target groups included chlorinated phenols, nitroaromatics, organochlorine and organophosphate pesticides, and conazole stereoisomers. The goals were to compare ionization modes, assess detection limits, and demonstrate isomer differentiation.

Methodology and Instrumentation

• Instrumentation: Agilent 7890B GC coupled to 7250 GC/Q-TOF with interchangeable low-energy EI (9–17 eV) and CI sources.
• Reagent gas: Methane, 20% flow for positive CI (PCI) and 40% for negative CI (NCI).
• Chromatography: HP-5MS column (30 m × 0.25 mm × 0.25 µm), He carrier at 1.2 mL/min, injector and interface at 280 °C.
• Conditions: Electron energy 9–17 eV (CI), emission current optimized per mode, source temps 200–280 °C (EI) and 150–280 °C (CI), quadrupole at 150 °C for PCI and NCI.
• Detection: Spectral acquisition at 5 Hz, mass range m/z 50–1200.
• Data processing: Agilent MassHunter Qualitative (vB.08) and Quantitative (vB.09) software.

Main Results and Discussion

Low-energy EI significantly increased molecular ion abundance compared to standard 70 eV EI, improving selectivity. Positive CI produced protonated and adduct ions, while negative CI yielded highly sensitive and selective detection of pesticides, achieving limits of detection as low as 0.5–2.3 fg on column for model compounds. Analysis of broccoli extracts demonstrated LODs of 0.9–2.9 pg for diverse pesticides. Negative CI also enabled clear distinction of cis/trans stereoisomers of conazole fungicides based on unique fragmentation patterns and accurate mass measurements.

Benefits and Practical Applications

• The combined ionization approach enhances compound identification confidence and sensitivity.
• Low-energy EI avoids downtime for source changes, facilitating seamless analysis.
• CI modes provide soft ionization for structural elucidation and isomer differentiation.
• Applicable to environmental monitoring, food safety testing, and industrial quality control.

Future Trends and Potential Applications

Advancements may include exploration of alternative reagent gases, integration with machine learning for spectral interpretation, expansion to broader analyte classes, coupling with chiral separation techniques, and development of portable high-resolution systems for on-site analysis.

Conclusion

The integration of low-energy EI and CI on a high-resolution GC/Q-TOF platform delivers a versatile, sensitive, and selective tool for targeted and untargeted analysis. This methodology addresses challenges in environmental and complex sample analysis by maximizing molecular ion detection and enabling isomer differentiation without instrument downtime.