Ion source

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Mass spectrometer EI/CI ion source

An ion source is an electro-magnetic device that is used to create charged particles such as atomic and molecular ions.


This article summarizes the many different types of ion sources used in different analytical instruments and applications.

Electron and chemical ionization[edit]

Electron ionization[edit]

Main article: Electron ionization


M + e^- \to M^{+\bullet} + 2e^-

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

The electrons may be created by an arc discharge between a cathode and an anode.

Chemical ionization[edit]

Main article: Chemical ionization

Chemical ionization is a lower energy process than electron ionization.[1] The lower energy yields less fragmentation, and usually a simpler spectrum. A typical CI spectrum has an easily identifiable molecular ion.[2]


CH_4 + e^- \to CH_4^+ + 2e^- (primary ion formation),
CH_4 + CH_4^+ \to CH_5^+ + CH_3 (reagent ion formation),
M + CH_5^+ \to CH_4 + [M + H]^+ (product ion formation, e.g. protonation).

Radioactive ion sources[edit]

A small piece of radioactive material, for instance 63Ni or 241Am, can be used to ionize a gas. This is used in ionization smoke detectors and ion mobility spectrometers.

Ion-attachment ionization[edit]




Gas discharge ion sources[edit]

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

Inductively coupled plasma[edit]


Microwave induced plasma[edit]

Microwaves are capable of exciting electrodeless gas discharges to create ions.

Glow discharge[edit]

Main article: Glow discharge

Ions can be created in an electric glow discharge.

Spark ionization[edit]

Main article: Spark ionization


Closed drift ion sources[edit]

These ion sources use 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 (Hall effect thrusters).

Desorption ionization[edit]

Aerosol laser desorption and ionization[edit]

In aerosol time-of-flight mass spectrometry, micrometer sized solid aerosol particles extracted from the atmosphere are simultaneously desorbed and ionized by a precisely timed laser pulse as they pass through the center of a time-of-flight ion extractor.[6][7]

Desorption ionization on silicon[edit]

Main article: Soft laser desorption

Desorption ionization on silicon (DIOS) refers to laser desorption/ionization of a sample deposited on a porous silicon surface.

Fast atom bombardment[edit]

Main article: Fast atom bombardment

In fast atom bombardment (FAB) the analytes is mixed with a non-volatile chemical protection environment called a matrix and is bombarded under vacuum with a high energy (4000 to 10,000 electron volts) beam of atoms.[8] The atoms are typically from an inert gas such as argon or xenon. Common matrices include glycerol, thioglycerol, 3-nitrobenzyl alcohol (3-NBA), 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.

Field desorption[edit]

Main article: 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.[9] 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 M+. and less often, protonated molecules [M+H]^+.

Liquid metal ion sources[edit]

In a Liquid metal ion source (LMIS), a metal (typically gallium) 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.

Matrix-assisted laser desorption/ionization[edit]

Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique. The sample is mixed with a matrix material. Upon receiving a laser pulse, the matrix absorbs the laser energy and it is thought that primarily the matrix is desorbed and ionized (by addition of a proton) by this event. The analyte molecules are also desorbed. The matrix is then thought to transfer proton to the analyte molecules (e.g., protein molecules), thus charging the analyte.

Surface-enhanced laser desorption/ionization[edit]

Surface-enhanced laser desorption/ionization (SELDI) is a variant of MALDI that is used for the analysis of protein mixtures that uses a target modified to achieve biochemical affinity with the analyte compound.[10]

Spray ionization[edit]

Atmospheric pressure chemical ionization[edit]

Atmospheric pressure chemical ionization is a form of chemical ionization which takes place at atmospheric pressure.[11] A spray of solvent is heated to relatively high temperatures (above 400 degrees Celsius), sprayed with high flow rates of nitrogen and the entire aerosol cloud is subjected to a corona discharge that creates ions. APCI is not as "soft" an ionization technique as ESI.[12]

Atmospheric pressure photoionization[edit]

Atmospheric pressure photoionization uses a source of photons, usually a vacuum UV (VUV) lamp, to ionize the analyte with single photon ionization process. Analogous to other AP ion sources a spray of solvent is heated to relatively high temperatures (above 400 degrees Celsius) and sprayed with high flow rates of nitrogen for desolvation. The resulting aerosol cloud is subjected to UV radiation to create ions.

Atmospheric pressure laser ionization[edit]

Atmospheric pressure laser ionization uses UV laser light sources to ionize the analyte via a multi photon ionization process (1+1 REMPI). The method is selective towards particular molecules like PAHs.

Electrospray ionization[edit]


Probe electrospray ionization[edit]

Probe electrospray ionization (PESI) is a modified version of electrospray, where the capillary for sample solution transferring is replaced by a sharp-tipped solid needle with periodical motion.[14]

Sonic spray ionization[edit]

Sonic spray ionization is method for creating ions from a liquid solution, for example, a mixture of methanol and water.[15] A pneumatic nebulizer is used to turn the solution into a supersonic spray of small droplets. Ions are formed when the solvent evaporates and the statistically unbalanced charge distribution on the droplets leads to a net charge and complete desolvation results in the formation of ions.

Thermospray ionization[edit]


Surface ionization[edit]

Main article: Thermal ionization

Surface ionization (also known as thermal ionization, or contact ionization) involves spraying vaporized, neutral atoms onto a hot surface, from which the atoms re-evaporate in ionic form. To generate positive ions, the atomic species should have a low ionization energy, and the surface should have a high work function. This technique is most suitable for alkali atoms (Li, Na, K, Rb, Cs) which have low ionization energies and are easily evaporated.[17]

To generate negative ions, the atomic species should have a high electron affinity, and the surface should have a low work function. This second approach is most suited for halogen atoms Cl, Br, I, At.[18]

Ambient ionization[edit]

Main article: Ambient ionization

In ambient ionization, ions are formed outside the mass spectrometer without sample preparation or separation.[19]

Desorption electrospray ionization[edit]


Direct analysis in real time[edit]

Main article: DART ion source


Matrix-assisted laser desorption electrospray ionization[edit]

Matrix-assisted laser desorption electrospray ionization (MALDESI)[21] is an atmospheric pressure ionization source for generation of multiply charged ions. An ultraviolet or infrared laser is directed onto a solid or liquid sample containing the analyte of interest and matrix (UV = organic acid, IR = sacrificial analyte or water of hydration) desorbing neutral analyte molecules which are postionized[clarification needed] by interaction with electrosprayed solvent droplets generating multiply charged ions.

Laser Ablation Electrospray Ionization[edit]

Laser Ablation Electrospray Ionization (LAESI) is an ambient ionization method for mass spectrometry that combines laser ablation from a mid-infrared (mid-IR) laser with a secondary electrospray ionization (ESI) process.

Particle accelerators[edit]

In particle accelerators an ion source creates a particle beam at the beginning of the machine, the source. The technology to create ion sources for particle accelerators depends strongly on the type of particle that needs to be generated: electrons, protons, H- ion or a Heavy ions.






Heavy ions can also be generated with an Ion Gun which typically uses the thermionic emission of electrons to ionize a substance in its gaseous state. Such instruments are typically used for surface analysis.

Theory of Operation[edit]


Ion source applications[edit]


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See also[edit]