In standard GC-MS analysis Electron Impact (EI) ionisation is performed at 70eV, producing established fragmentation patterns that are used in many commercial libraries. In some cases, the molecular ion is weak, or absent altogether, making unequivocal identification challenging. Using a specially designed source, the Agilent 7250 Q-TOF is capable of performing a ‘low energy’ EI ionisation, providing the opportunity to increase the relative intensity of the molecular ion and help with structural elucidation. This application note describes the optimisation of Low Energy EV using Design of Experiments and subsequent analysis of Farnesol isomers for comparison with standard 70eV EI ionisation. Results demonstrated this approach for characterisation of isomers and identification of unknowns in a customer sample. The ability to perform ‘low energy’ EI alongside standard EI, without changing the source offers considerable advantages compared to other structural elucidation approaches, such as Chemical Ionisation (CI).
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The power of low energy EI ionisation in GC/Q-TOF MS