Analytical Instrumentation

PEFTEC - Enhanced Molecular Weight Information in GC/MS by Cold EI

May 11 2016 Read 2017 Times

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Electron Ionization Gas Chromatography / Mass Spectrometry (EI GC/MS) is a powerful and information-rich technique for qualitative characterization and quantitative analysis of the compounds in a mixture. One of its most valuable functions is to provide the molecular weight of a compound. However, many compounds do not have a stable molecular ion under EI conditions and it is small or absent in the spectrum.

Cold Electron Ionization GC/MS (Cold EI GC/MS) can enhance the molecular ion abundance of most compounds while retaining the EI fragmentation pattern for spectral library searching.

The enhanced molecular ion intensity is achieved by supersonic expansion of the GC effluent into a molecular beam, where the analyte molecules are vibrationally and rotationally cooled adiabatically to ~10 - 30 K. When this vibrationally cold molecule is ionized by a 70 eV electron the final thermal energy is much lower than in EI, which ionizes a molecule at GC transferline temperature of several hundred °C. The reduced Cold EI internal energy produces much less fragmentation of the molecular ion. The ion source used in this work is of axial "fly-through" design, eliminating molecule and ion collisional scattering with ion source walls which could lead to additional fragmentation. This is demonstrated for compounds such as squalane, which shows a large molecular ion with Cold EI, but <1% by conventional EI.

When used in a novel q-TOF configuration the enhanced molecular ion increases sensitivity, selectivity, and information content.


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