Detection of molecular ions of fluorine compounds by GC/FI-TOFMS

Importance of the topic

Field ionization offers a soft ionization approach that preserves molecular ions of volatile fluorinated compounds, overcoming limitations of electron ionization and chemical ionization which often fragment these molecules. This capability is vital for accurate molecular weight determination and structural elucidation of environmental contaminants and industrial by-products.

Objectives and study overview

• Demonstrate the effectiveness of GC/FI-TOFMS for detecting molecular ions in fluorinated compounds that fail to produce molecular ions under EI or CI.
• Apply the method to two test samples: a fluorinated ether (MW 174) and a fluorocarbon mixture AK225.
• Compare field ionization results with conventional ionization techniques to highlight improvements in molecular ion observation.

Methodology and instrumentation

• Mass spectrometer: JEOL JMS-T100GC configured for positive field ionization (FI+).
• Gas chromatograph: Agilent 6890N using helium carrier gas at 0.8 mL/min and split injection (10:1) at 200 °C.
• Oven temperature program: 40 °C hold for 2 min, ramp to 60 °C at 5 °C/min, then to 200 °C at 15 °C/min.
• Mass range scanned: m/z 35–450 with 0.4 s spectrum intervals.

Main results and discussion

Field ionization produced clear molecular ion peaks at m/z 174 for the fluorinated ether and m/z 202 for AK225, alongside characteristic fragment ions. Total ion chromatograms confirmed compound retention times, and high-resolution accurate mass measurements of both molecular and fragment ions yielded sub-5 mDa deviations between observed and theoretical masses. This precision supports unambiguous elemental composition assignments.

Benefits and practical applications

• Enables reliable molecular weight determination for fluorinated compounds that lack molecular ions under EI/CI.
• Facilitates identification of unknown impurities or by-products in environmental and industrial samples.
• Combines soft ionization with high-resolution mass accuracy to enhance structural analysis workflows in QA/QC and research settings.

Future trends and opportunities

Advances may include coupling FI-TOFMS with automated data interpretation tools, extending the technique to other classes of labile molecules, and integrating with tandem MS for detailed structural characterization. Potential applications span environmental monitoring, pharmaceutical impurity profiling, and polymer additive analysis.

Conclusion

GC/FI-TOFMS effectively detects molecular ions of fluorinated compounds that evade EI and CI, providing accurate mass measurements and clear elemental compositions. This method enhances the analytical toolkit for researchers and industries dealing with volatile and environmentally sensitive fluorinated molecules.

References

• T. Isemura, R. Kakita, A. Tamaoki, S. Yonemori, J. Fluor. Chem., 80 (1996) 81–85.