Editing Electron ionization (section)

==Principle of operation==
[[File:Electron Ionization - Born Oppenheimer Potential Curves.png|300px|thumb|Electron Ionization of Methanol - Born Oppenheimer Potential Curves]]

In this process, an electron from the [[analyte]] molecule (M) is expelled during the collision process to convert the molecule to a positive ion with an odd number of electrons. The following [[Phase (matter)|gas phase]] reaction describes the electron ionization process<ref>R. Davis, M. Frearson, (1987). ''Mass Spectrometry – Analytical Chemistry by Open Learning'', John Wiley & Sons, London.</ref>

:<chem>M{} + e^- -> M^{+\bullet}{} + 2e^-</chem>

where M is the analyte molecule being ionized, e<sup>−</sup> is the electron and M<sup>+•</sup> is the resulting [[molecular ion]].

In an EI [[ion source]], electrons are produced through [[thermionic emission]] by heating a wire filament that has [[electric current]] running through it. The kinetic energy of the bombarding electrons should have higher energy than the [[ionization energy]] of the sample molecule.  The electrons are accelerated to 70 [[Electronvolt|eV]] in the region between the filament and the entrance to the ion source block. The sample under investigation which contains the neutral molecules is introduced to the ion source in a perpendicular orientation to the electron beam. Close passage of highly energetic electrons in low pressure (ca. 10<sup>−5</sup> to 10<sup>−6</sup> torr) causes large fluctuations in the electric field around the neutral molecules and induces ionization and fragmentation.<ref>J. Robinson ''et al.'' Undergraduate Instrumental Analysis, 6th ed. Marcel Drekker, New York, 2005</ref> The fragmentation in electron ionization can be described using Born Oppenheimer potential curves as in the diagram. The red arrow shows the electron impact energy which is enough to remove an electron from the analyte and form a molecular ion from non- dissociative results. Due to the higher energy supplied by 70 eV electrons other than the molecular ion, several other bond dissociation reactions can be seen as dissociative results, shown by the blue arrow in the diagram. These ions are known as second-generation product ions. The [[radical ion|radical cation]] products are then directed towards the mass analyzer by a repeller electrode.  The ionization process often follows predictable cleavage reactions that give rise to fragment ions which, following detection and signal processing, convey structural information about the analyte.

=== The efficiency of EI ===
Increasing the electron ionization process is done by increasing the [[ionization efficiency]]. In order to achieve higher ionization efficiency there should be an optimized filament current, emission current, and ionizing current. The current supplied to the filament to heat it to incandescent is called the filament current. The emission current is the current measured between the filament and the electron entry slit. The ionizing current is the rate of electron arrival at the trap. It is a direct measure of the number of electrons in the chamber that are available for ionization.

The sample ion current (I<sup>+</sup>) is the measure of the ionization rate. This can be enhanced by manipulation of the ion extraction efficiency (β), the total ionizing cross section (Q<sub>i</sub>), the effective ionizing path length (L), the concentration of the sample molecules([N]) and the ionizing current (I<sub>e</sub>). The equation can be shown as follows:

:<math chem>I^+=\beta Q_iL [\ce N] I_e</math>

The ion extraction efficiency (β) can be optimized by increasing the voltage of both repeller and acceleration. 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 [[Matter wave|de Broglie wavelength]] of the electrons matches the length of typical bonds in organic molecules (about 0.14 [[Nanometre|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. The effective ionizing path length (L) can be increased by using a weak magnetic field. But the most practical way to increase the sample current is to operate the ion source at higher ionizing current (I<sub>e</sub>).<ref name=":0" />