Ion source


An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.

Electron ionization

is widely used in mass spectrometry, particularly for organic molecules. The gas phase reaction producing electron ionization is
where M is the atom or molecule being ionized, e^- is the electron, and M^ is the resulting ion.
The electrons may be created by an arc discharge between a cathode and an anode.
An electron beam ion source is used in atomic physics to produce highly charged ions by bombarding atoms with a powerful electron beam. Its principle of operation is shared by the electron beam ion trap.

Electron capture ionization

Electron capture ionization is the ionization of a gas phase atom or molecule by attachment of an electron to create an ion of the form A−•. The reaction is
where the M over the arrow denotes that to conserve energy and momentum a third body is required.
Electron capture can be used in conjunction with chemical ionization.
An electron capture detector is used in some gas chromatography systems.

Chemical ionization

is a lower energy process than electron ionization because it involves ion/molecule reactions rather than electron removal. The lower energy yields less fragmentation, and usually a simpler spectrum. A typical CI spectrum has an easily identifiable molecular ion.
In a CI experiment, ions are produced through the collision of the analyte with ions of a reagent gas in the ion source. Some common reagent gases include: methane, ammonia, and isobutane. Inside the ion source, the reagent gas is present in large excess compared to the analyte. Electrons entering the source will preferentially ionize the reagent gas. The resultant collisions with other reagent gas molecules will create an ionization plasma. Positive and negative ions of the analyte are formed by reactions with this plasma. For example, protonation occurs by

Charge exchange ionization

Charge-exchange ionization is a gas phase reaction between an ion and an atom or molecule in which the charge of the ion is transferred to the neutral species.

Chemi-ionization

Chemi-ionization is the formation of an ion through the reaction of a gas phase atom or molecule with an atom or molecule in an excited state. Chemi-ionization can be represented by
where G is the excited state species, and M is the species that is ionized by the loss of an electron to form the radical cation.

Associative ionization

Associative ionization is a gas phase reaction in which two atoms or molecules interact to form a single product ion. One or both of the interacting species may have excess internal energy.
For example,
where species A with excess internal energy interacts with B to form the ion AB+.

Penning ionization

is a form of chemi-ionization involving reactions between neutral atoms or molecules. The process is named after the Dutch physicist Frans Michel Penning who first reported it in 1927. Penning ionization involves a reaction between a gas-phase excited-state atom or molecule G* and a target molecule M resulting in the formation of a radical molecular cation M+., an electron e−, and a neutral gas molecule G:
Penning ionization occurs when the target molecule has an ionization potential lower than the internal energy of the excited-state atom or molecule.
Associative Penning ionization can proceed via
Surface Penning ionization refers to the interaction of the excited-state gas with a bulk surface S, resulting in the release of an electron according to

Ion attachment

is similar to chemical ionization in which a cation is attached to the analyte molecule in a reactive collision:
Where M is the analyte molecule, X+ is the cation and A is a non-reacting collision partner.
In a radioactive ion source, a small piece of radioactive material, for instance 63Ni or 241Am, is used to ionize a gas. This is used in ionization smoke detectors and ion mobility spectrometers.

Gas-discharge ion sources

These ion sources use a plasma source or electric discharge to create ions.

Inductively-coupled plasma

Ions can be created in an inductively coupled plasma, which is a plasma source in which the energy is supplied by electrical currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields.

Microwave-induced plasma

Microwave induced plasma ion sources are capable of exciting electrodeless gas discharges to create ions for trace element mass spectrometry. A microwave plasma has high frequency electromagnetic radiation in the GHz range. It is capable of exciting electrodeless gas discharges. If applied in surface-wave-sustained mode, they are especially well suited to generate large-area plasmas of high plasma density. If they are both in surface-wave and resonator mode, they can exhibit a high degree of spatial localization. This allows to spatially separate the location of plasma generations from the location of surface processing. Such a separation may help reduce the negative effect, that particles released from a processed substrate may have on the plasma chemistry of the gas phase.

ECR ion source

The ECR ion source makes use of the electron cyclotron resonance to ionize a plasma. Microwaves are injected into a volume at the frequency corresponding to the electron cyclotron resonance, defined by the magnetic field applied to a region inside the volume. The volume contains a low pressure gas.

Glow discharge

Ions can be created in an electric glow discharge. A glow discharge is a plasma formed by the passage of electric current through a low-pressure gas. It is created by applying a voltage between two metal electrodes in an evacuated chamber containing gas. When the voltage exceeds a certain value, called the striking voltage, the gas forms a plasma.
A duoplasmatron is a type of glow discharge ion source that consists of a hot cathode or cold cathode that produces a plasma that is used to ionize a gas. They can produce positive or negative ions. They are used for secondary ion mass spectrometry, ion beam etching, and high-energy physics.

Flowing afterglow

In a flowing plasma afterglow, ions are formed in a flow of inert gas, typically helium or argon. Reagents are added downstream to create ion products and study reaction rates. Flowing-afterglow mass spectrometry is used for trace gas analysis for organic compounds.

Spark ionization

ionization is used to produce gas phase ions from a solid sample. When incorporated with a mass spectrometer the complete instrument is referred to as a spark ionization mass spectrometer or as a spark source mass spectrometer.
A closed drift ion source uses a radial magnetic field in an annular cavity in order to confine electrons for ionizing a gas. They are used for ion implantation and for space propulsion.

Photoionization

is the ionization process in which an ion is formed from the interaction of a photon with an atom or molecule.

Multi-photon ionization

In multi-photon ionization, several photons of energy below the ionization threshold may actually combine their energies to ionize an atom.
Resonance-enhanced multiphoton ionization is a form of MPI in which one or more of the photons accesses a bound-bound transition that is resonant in the atom or molecule being ionized.

Atmospheric pressure photoionization

uses a source of photons, usually a vacuum UV lamp, to ionize the analyte with single photon ionization process. Analogous to other atmospheric pressure ion sources, a spray of solvent is heated to relatively high temperatures and sprayed with high flow rates of nitrogen for desolvation. The resulting aerosol is subjected to UV radiation to create ions. Atmospheric-pressure laser ionization uses UV laser light sources to ionize the analyte via MPI.

Desorption ionization

Field desorption

Field desorption refers to an ion source in which a high-potential electric field is applied to an emitter with a sharp surface, such as a razor blade, or more commonly, a filament from which tiny "whiskers" have formed. This results in a very high electric field which can result in ionization of gaseous molecules of the analyte. Mass spectra produced by FI have little or no fragmentation. They are dominated by molecular radical cations and less often, protonated molecules

Particle bombardment

Fast atom bombardment

Particle bombardment with atoms is called fast atom bombardment and bombardment with atomic or molecular ions is called secondary ion mass spectrometry. Fission fragment ionization uses ionic or neutral atoms formed as a result of the nuclear fission of a suitable nuclide, for example the Californium isotope 252Cf.
In FAB the analytes is mixed with a non-volatile chemical protection environment called a matrix and is bombarded under vacuum with a high energy beam of atoms. The atoms are typically from an inert gas such as argon or xenon. Common matrices include glycerol, thioglycerol, 3-nitrobenzyl alcohol, 18-crown-6 ether, 2-nitrophenyloctyl ether, sulfolane, diethanolamine, and triethanolamine. This technique is similar to secondary ion mass spectrometry and plasma desorption mass spectrometry.

Secondary ionization

Secondary ion mass spectrometry is used to analyze the composition of solid surfaces and thin films by sputtering the surface of the specimen with a focused primary ion beam and collecting and analyzing ejected secondary ions. The mass/charge ratios of these secondary ions are measured with a mass spectrometer to determine the elemental, isotopic, or molecular composition of the surface to a depth of 1 to 2 nm.
In a liquid metal ion source, a metal is heated to the liquid state and provided at the end of a capillary or a needle. Then a Taylor cone is formed under the application of a strong electric field. As the cone's tip get sharper, the electric field becomes stronger, until ions are produced by field evaporation. These ion sources are particularly used in ion implantation or in focused ion beam instruments.