Doubly Charged Ions in Mass Spectrometry with Cold EI

Ksenia J. Margolin-Eren, School Of Chemistry, Tel Aviv University, Tel Aviv , Israel (kseniak@mail.tau.ac.il)
Alexander B. Fialkov, School Of Chemistry, Tel Aviv University, Tel Aviv , Israel
Uri Keshet, School Of Chemistry, Tel Aviv University, Tel Aviv , Israel
Svetlana Tsizin, School Of Chemistry, Tel Aviv University, Tel Aviv , Israel
Aviv Amirav, School Of Chemistry, Tel Aviv University, Tel Aviv , Israel

Doubly charged ions (M⁺²) are observed in some rare cases in 70 eV EI mass spectra, but are practically limited to doubly charged atoms and polycyclic aromatic hydrocarbons (PAH). We have discovered that the relative abundance of the doubly charged molecular ions can be the same as the singly charged molecular ion (M⁺). Also we report on the new observation of triply charged molecular ions (M⁺³). This discovery was achieved with the help of a single quadrupole based GC-MS with Electron Ionization (EI) of vibrationally cool molecules in Supersonic Molecular Beams (SMB), known as "Cold EI". The latter enables to significantly enhance the relative abundance of the doubly charged molecular ion in the mass spectrum as it is believed to have a correlation with the stability of the molecular ion.  

For Agidol 40, C₅₄H₇₈O₃, m/z=774, we observed a doubly charged ion that is more abundant than the molecular ion itself. In order to rule out that it is a fragment, we made sure that the doubly charged ion appears at exactly half m/z of the molecular ion and doesn’t exhibit the isotopic pattern of unit mass spacing. This was confirmed via a Raw Scan which exhibited an isotopic mass spacing much smaller than m/z=1 and closer to m/z=0.5.

We found that working with increasing Helium gas flow rates, up to 100 ml/min, improved the vibrational cooling, even in hydrocarbons that form highly fragile molecular ions. Based on these observations, we suggest a new mechanism that involves two separate electron ionization processes, most likely with the same electron but at two different atoms in the same molecule. These surprising findings are important in shedding new light on the fragmentation mechanisms of electron ionization mass spectra and doubling the upper mass range of GC-MS with Cold EI.