ElectronIonizationlabhandout

ElectronIonizationlabhandout - Electron Ionization:...

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Electron Ionization: Fundamentals and Quantitation Appearance energies and bond dissociations Introduction: Electron ionization (EI) is the source of choice for analyzing volatile and semi-volatile species. The reaction for EI is given below: (1) M + e - Æ M +• + 2e - Where M is any gaseous molecule and M +• is the radical cation of M (also called the molecular ion). Equation 1 is not a complete description of the process, however. If the ionizing electron has significantly more energy than the ionization potential of the molecule M, a fragmentation reaction can occur: (2) M + e- Æ M +•* + 2e- Æ (M-F) + + F + 2e - Where M +•* is an excited molecular ion, (M-F) + is the original molecular ion after the loss of a neutral fragment, and F is the neutral fragment (in this case, a radical fragment is shown although even electron fragments that give rise to radical product ions are possible as well). This process can continue and a molecule can fragment multiple times until all the excess energy is expended and or distributed. Ionization can happen any time the incoming electron’s kinetic energy exceeds the ionization potential of the analyte molecule (usually 2-25 eV). Any excess energy can be coupled into the newly formed ion as electronic or vibrational energy. It is this excess energy that causes fragmentation. In modern practice, the electron energy is set to 70 eV and the resulting fragmentation pattern can be used to identify molecules. 70 eV has been chosen because the vast majority of ionization cross sections for organic molecules have maxima near 70 eV. Searchable databases containing the 70 eV EI mass spectra of tens of thousands of compounds are commercially available. The electron energy can be used to probe the ionization potential of a gas as well as the energy required to dissociate chemical bonds. This technique involves determining the “appearance potentials” of ions and fragments. The appearance potential is the minimum energy required to generate the ion in question (see chapter 2 in Gross). Appearance potentials are determined by recording mass spectra of the analyte as a function of the ionizing electron energy. The energies at which particular ions appear are then used to calculate the energetics of the reactions that created them. The “threshold energy” is the energy required for the incoming electron to just complete the process in question with no energy left over. Typically, the true threshold energy cannot be reliably measured, so the intensity of the ion is plotted as a function of electron
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energy and the threshold energy is determined by extrapolation. 1-3 You will use the MAT-95 XP magnetic sector mass spectrometer in the Mass Spectrometry Facility to measure the ionization energies and appearance energies for argon, methylamine, and a couple of fragment ions from methylamine.
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This note was uploaded on 01/18/2012 for the course C 613 taught by Professor Davide.clemmer during the Spring '08 term at Indiana.

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ElectronIonizationlabhandout - Electron Ionization:...

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