Ion and Ionization

Ionization is the process in which one or more electrons are removed from an atom or molecule, thereby creating an ion. The word ionization is also used for the process in which an ionic solid, such as a salt, dissociates into its component ions upon solution. In order to remove an electron from an atom, enough energy must be supplied to break the bond between the negatively charged electron and the positively charged nucleus; this is the ionization energy. Ionization can be induced by high energy radiation such as x rays and ultraviolet light (photoionization), bombardment by high energy electrons (electron impact ionization) or small molecular ions (chemical ionization) and by exposure to high electric fields (field ionization). Ionization is employed in many important analytical techniques used to study the character of atoms and molecules including mass spectrometry, photoelectron and Auger electron spectroscopy, and multiphoton ionization spectroscopy.

Electron impact source

The most common method of producing ions for mass spectrometry is by bombarding a gaseous sample with a stream of fast moving electrons. The stream of electrons, produced by an electron gun (a heated tungsten wire from which electrons are emitted—thermionic emission), bombard the sample and "kick" out additional electrons. The process of electron impact ionization is not very efficient. Because of the very small size of electrons and the relatively low density of electrons around molecules, electron-electron impacts are rare. Nonetheless, electron impact is the most widely used ion source in commercial mass spectrometers. Electron guns can produce vast quantities of electrons, so even if one in a million is successful, enough ions can be generated. When the impact is effective in producing ionization, typically there is more energy supplied by the impact than is needed to remove the electron. The excess energy may result in the ion breaking up into smaller fragment ions. The intact molecular ion is referred to as the parent ion, and the fragment ions are called daughter ions.

Field ionization

Ionization may also be produced by subjecting a molecule to a very intense electric field. This process is called field ionization. A familiar example of field ionization is the small blue spark that jumps from the tip of your finger to any grounded surface on a dry day when static electricity can build up. The strong electrostatic field actually pulls electrons out of your finger. Electric fields are strongest at the tips of pointed conductors. To produce electric fields of sufficient magnitude to ionize molecules, very fine, sharpened wires are used. Field ionization sources are relatively gentle compared to electron impact sources in that they do not deposit as much excess energy into the parent ion. Therefore, field ionization sources are usually employed when we do not want to damage the ionized specimen too much.

Chemical ionization

Chemical ionization is similar to electron impact ionization except that a beam of positively charged molecular ions, rather than electrons, is used to bombard and ionize the sample. The bombarding ions are usually small molecules such as methane, propane, or ammonia. Because of the much larger size of a molecular ion compared to an electron, these collisions are highly reactive and generally produce less fragmentation than electron impact ionization with comparable efficiency. Chemical ionization is widely used in commercial mass spectrometers, and many instruments are equipped with a source which is capable of both electron impact and chemical ionization.

Photoionization

If the ionization energy is supplied by electromagnetic radiation, the ionization is called photoionization, referring to the fact that a photon of radiation produces the ionization. However, not all electromagnetic radiation has sufficient energy to cause ionization. Generally, only radiation with wavelengths shorter than visible light, that is, radiation in the ultraviolet, x ray, and gamma ray regions of the electromagnetic spectrum can produce ionization.

Ultraviolet radiation can cause ionization of many small molecules, including oxygen, O₂. In fact, short wavelength solar radiation causes ionization of molecular oxygen and molecular nitrogen found in the upper atmosphere; these processes are important to the chemistry of the earth's atmosphere. In the laboratory, ultraviolet light from special lamps or lasers is used to ionize molecules in order to study them. Ultraviolet photoelectron spectroscopy (UPS) measures the energy of the departing electron.

The high energy carried by x rays can easily cause ionization of isolated atoms. X rays are therefore frequently referred to as ionizing radiation. X ray photoelectron spectroscopy and Auger spectroscopy are two techniques which, like ultraviolet photoelectron spectroscopy, study the ejected electron to gain information about the atom from which it came.

Flame ionization

Probably the simplest way of supplying energy for ionization is by subjecting the atoms or molecule to a flame. However, temperatures of several thousand degrees are usually required to achieve an appreciable degree of ionization. Specialized flames, such as an electrical arc, spark or plasma, can produce the necessary temperatures in a controlled manner.