Adjusting strength of first MS in Electron Ionization mode.

I would like to know if the strength of the first ionization (typically 70EV) in electron ionization mode can be adjusted to less than 70EV to increase sensitivity for fragile analytes. If yes what is the lowest acceptable initial ionization strength? I use a Agilent 7010 triple quadrupole mass spectrometer . 

  • This process has various names depending on the manufacturer - Markes call it soft EI, Thermo call it Variable Electron Voltage and Agilent call it low-energy EI. I believe it's only an option on the latest generation GC QToF (7250), not on any of the QQQs. An application note is available from Agilent.

  • Amazingly, Wikipedia has a good explanation for 70eV: "Since the ionization cross section depends on the chemical nature of the sample and the energy of ionizing electrons a standard value of 70 eV is used. At low energies (around 20 eV), the interactions between the electrons and the analyte molecules do not transfer enough energy to cause ionization. At around 70 eV, the de Broglie wavelength of the electrons matches the length of typical bonds in organic molecules (about 0.14 nm) and energy transfer to organic analyte molecules is maximized, leading to the strongest possible ionization and fragmentation. Under these conditions, about 1 in 1000 analyte molecules in the source are ionized. At higher energies, the de Broglie wavelength of the electrons becomes smaller than the bond lengths in typical analytes; the molecules then become "transparent" to the electrons and ionization efficiency decreases."

     

    • Lower ionization energies may result in strange and inconsistent results.
    • Higher ionization energies will reduce overall sensitivity.  This may be a good thing, actually, and is being explored for some applications.
    • If you need to do any sort of library searching, all of the commercial EI libraries are created using 70eV.

     

    Fragile analytes are typically either run using Chemical Ionization, by derivatizing them before injection, or on other types of instrumentation.

  • This question has been marked as assumed answered.

Was this helpful?