Electron ionization

Electron ionization

[
toluene [http://webbook.nist.gov/cgi/cbook.cgi?Name=toluene&Units=SI] ] Electron ionization (EI, formerly known as electron impact) is an ionization technique widely used in mass spectrometry, particularly for organic molecules.

How it works

The gas phase reaction producing electron ionization is

:M + e^- o M^{+ullet} + 2e^-

where M is the atom of molecule being ionized, e^- is the electron, and M^{+ullet} is the resulting ion.

In an EI source, electrons are produced through thermionic emission by heating a wire filament that has electric current running through it. The electrons are accelerated through the ionization space towards an anode; in the ionization space, they interact with analyte molecules in the gas phase, causing them to ionize to a radical ion, and frequently causing numerous cleavage reactions that give rise to fragment ions, which can convey structural information about the analyte.

The efficiency of ionization and production of fragment ions depends strongly on the chemistry of the analyte and the energy of the electrons. 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.

References

*cite book | author = Edmond de Hoffman | coauthors = Vincent Stroobant | title = Mass Spectrometry: Principles and Applications | edition = 2nd ed. | publisher = John Wiley and Sons | year = 2001 | id = ISBN 0-471-48566-7


Wikimedia Foundation. 2010.

Игры ⚽ Поможем сделать НИР

Look at other dictionaries:

  • Electron gun — An electron gun (also called electron emitter) is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in televisions and monitors which use cathode ray tube technology, as well as in… …   Wikipedia

  • Ionization — is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions.This process works slightly differently depending on whether an ion with a positive or a negative… …   Wikipedia

  • Electron configuration — Electron atomic and molecular orbitals Simp …   Wikipedia

  • Electron cyclotron resonance — is a phenomenon observed both in plasma physics and condensed matter physics. An electron in a static and uniform magnetic field will move in a circle due to the Lorentz force. The circular motion may be superimposed with a uniform axial motion,… …   Wikipedia

  • Electron beam ion trap — (or its acronym EBIT) is used in physics to denote an electromagnetic bottle that produces and confines highly charged ions. It was invented by R. Marrs [Levine et al, 1988] and M. Levine at LLNL.An EBIT uses an electron beam focused by means of… …   Wikipedia

  • Ionization — I on*i*za tion, n. (Elec. Chem.) the process of converting neutral atoms or molecules into ions. The process may occur by dissolving an ionic substance in a dissociating solvent, such as water, or by adding or subtracting an electron to or from… …   The Collaborative International Dictionary of English

  • Electron binding energy — (BE) is the energy required to release an electron from its atomic or molecular orbital. Binding energy values are normally reported as positive values with units of eV . The binding energies of 1 s electrons are roughly proportional to (Z 1)²… …   Wikipedia

  • Ionization potential — The ionization potential, ionization energy or EI of an atom or molecule is the energy required to remove an electron from the isolated atom or ion. More generally, the nth ionization energy is the energy required to strip it of the n th electron …   Wikipedia

  • ionization energy — ▪ chemistry also called  ionization potential,    in chemistry, the amount of energy required to remove an electron from an isolated atom or molecule. There is an ionization energy for each successive electron removed; the ionization energy… …   Universalium

  • Electron avalanche — An electron avalanche is a process in which a number of free electrons in a medium (usually a gas) are subjected to strong acceleration by an electric field, ionizing the medium s atoms by collision (called impact ionization), thereby forming new …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”