Using variable ionisation energy time-of-flight mass spectrometry with comprehensive GC×GC to identify isomeric species

Although GC×GC-ToF-MS allows the separation of thousands of peaks, many of these peaks are not positively identified owing to the lack of mass spectral library data and/or standard materials, leading to a substantial amount of information being inaccessible. The fragmentation patterns of molecules in mass spectrometers using electron impact ionisation at 70 eV can be useful for molecule identification, provided a match is available in a published EI MS library, but are indistinguishable for many isomeric organic compounds (for example, aliphatic and branched alkanes). Lower ionisation energies have been exploited leading to organic compounds being ionised with lower excess internal energy and less fragmentation, retaining the molecular ion and maximising its relative signal. This has enabled the identification of a large number of isomeric organic compounds, both aliphatic and aromatic, between C12-C36, in the previously unresolved complex mixture (UCM) of two motor oil samples.

This technique also demonstrates problems associated with separation of co-eluting isomers, particularly for the n-alkanes, which are routinely measured by 1D GC/MS and may be overestimated, due to co-elution. As a consequence retention times in 2 dimensions and mass spectra at variable ionisation energies are shown to give unparalleled power to identify specific isomers.