Aviv Amirav (1) and David Benanou (2)
- The ratio of Cold EI fatty acids peak to the hydrocarbon peak that elutes just before it is higher by a factor of almost 20 times in Cold EI in comparison with standard EI. Thus, clearly the free fatty acids are mostly degraded and/or tailing in the standard EI ion source and their quantitative determination becomes practically impossible, while Cold EI with its fly-through ion source provides uniform response also to free acids.
- The Cold EI peak shows a clear shoulder of near co-elution of two acids while this almost separation is fully eroded in standard EI due to peak tailing.
- The Cold EI mass spectra resulted in clear NIST library identification of the acids as 9-Octadecenoic acid (C18H34O2) and linoleic acid (C18H32O2) although the identification at the isomer level can not be fully trusted. On the other hand, attempted NIST library identification of the standard EI mass spectra failed.
- Cold EI provides enhanced molecular ions but in its NIST identification the library finds a standard EI mass spectrum that was obtained at a low ion source temperature thus with higher than usual molecular ion hence with good fit.
- The Cold EI mass spectra are cleaner than the standard EI mass spectra and also exhibit useful high mass fragments.
- Few additional free fatty acids were detected at lower concentrations and identified while their detection and identification failed in standard EI. For example, the small peak eluting at 5.78 min was identified as octadecanoic acid.