A systematic study of molecular ion intensity and mass accuracy in low energy electron ionisation using gas chromatography-quadrupole time-of-flight mass spectrometry.

Molecular ions, which contain accurate mass information, are valuable for providing elemental composition elucidation. Under the most common electron ionisation (EI) condition (electron energy, 70 eV and temperature, 230 °C), molecular ions are often in relatively low intensities or completely unapparent. In this research, low energy EI source parameters (electron energy and temperature) in a gas chromatography-quadrupole time of flight (GC-QTOF) were systematically studied to evaluate their correlative impact on the intensity and mass accuracy of molecular ions. Lower temperatures were generally associated with higher molecular ion intensities under various EI energies. Apart from compounds with more chemically stable molecular ion structures, the lowest electron energy (12 eV) corresponded to higher intensities. On the other hand, mass accuracy appeared to be mostly constant (≤5 ppm) at different temperatures, while improvement was observed with the use of lower electron energies (12 eV). Moreover, the effect of compound concentration on molecular ion intensity and mass accuracy was studied from 50 to 5000 ppm, and the compound-specific concentration profiles were constructed. Finally, it was found that higher column flow rates corresponded to higher intensities, while the response under 12 eV was higher than that of 70 eV. Mass accuracy remained approximately constant across different flow rates. Therefore, these findings suggest that the use of low energy EI may be a viable approach for the preservation of molecular ions.