Combination of Chemical Ionization (CI) and Low Energy Ionization (EI) Capabilities with High-Resolution Q-TOF GC/MS

Significance of the Topic

Environmental monitoring increasingly relies on robust analytical methods to detect trace contaminants in complex matrices

Objectives and Overview

This study evaluates the combined use of low-energy electron ionization (EI) and chemical ionization (CI) on a high-resolution gas chromatograph quadrupole time-of-flight mass spectrometer (GC/Q-TOF)
The main goals are to compare molecular ion abundance, fragmentation behavior, and detection limits across standard EI, low-energy EI, positive CI (PCI) and negative CI (NCI) modes for environmentally relevant compounds

Methodology and Instrumentation

All analyses used an Agilent 7890B GC coupled to a 7250 GC/Q-TOF

• Ion sources: interchangeable CI source (methane reagent gas) and low-energy EI source
• Column: 30 m × 0.25 mm i.d., 0.25 µm HP-5MS; helium carrier at 1.2 mL/min
• CI conditions: methane flow at 20 % for PCI and 40 % for NCI; source and quadrupole temperatures adjusted per mode
• EI energies: 70 eV standard and 9–17 eV low-energy EI
• Mass range: 50–1 200 m/z; acquisition rate: 5 Hz
• Software: Agilent MassHunter Qualitative B.08 and Quantitative B.09

Major Results and Discussion

Low-energy EI enhanced molecular ion signals compared to standard EI, improving selectivity without source changes
NCI exhibited highest sensitivity for organochlorine, organophosphate and pyrethroid pesticides with detection limits down to sub-fg levels of OFN and pg levels of benzophenone
PCI favored formation of protonated and adduct ions but showed more extensive fragmentation for some compound classes
Fragmentation comparisons revealed distinct patterns:

• CI modes preserved molecular ions and characteristic adducts facilitating elemental composition determination
• Low-energy EI balanced fragmentation and molecular ion yield, aiding library matching and quantitation

Negative CI allowed clear discrimination of cis and trans stereoisomers of triazole fungicides via unique fragment ions and high mass accuracy (≤1.5 ppm)

Benefits and Practical Applications

Combining low-energy EI and CI on a single high-resolution platform streamlines workflows for targeted pesticide screening and untargeted unknown identification
Key advantages include:

• Rapid source switching without downtime
• Enhanced molecular ion detection for reliable formula confirmation
• Sub-pg to fg detection capability in complex extracts
• Improved stereoisomer differentiation for regulatory and quality control contexts

Future Trends and Opportunities

Emerging applications may integrate automated switching between ionization modes to create hybrid acquisition strategies
Advances in data processing algorithms will leverage CI and low-energy EI fragmentation libraries for more comprehensive environmental screening
Possibilities exist for coupling soft ionization modes with tandem MS and high-resolution imaging for spatial contaminant mapping

Conclusion

The Agilent 7250 GC/Q-TOF with low-energy EI and interchangeable CI sources delivers a versatile, high-performance solution for environmental analysis, combining sensitivity, selectivity and accurate mass measurement in a single platform