Editing Ion source

{{About|ionization devices|the physical processes|Ionization}}
{{Use American English|date = February 2019}}
{{Short description|Device that creates charged atoms and molecules (ions)}}
[[File:Mass spectrometer ei ci ion source.jpg|thumb|300px|Mass spectrometer EI/CI ion source]]

An '''ion source''' is a device that creates atomic and molecular [[ion]]s.<ref name="Wolf1995">{{cite book|author=Bernhard Wolf|title=Handbook of Ion Sources|url=https://books.google.com/books?id=qoQHMSBNFsAC|date=31 August 1995|publisher=CRC Press|isbn=978-0-8493-2502-1}}</ref> Ion sources are used to form ions for [[Mass spectrometry|mass spectrometers]], [[optical emission spectrometer]]s, [[particle accelerator]]s, [[Ion implantation|ion implanters]] and [[Ion thruster|ion engines]].

==Electron ionization==
{{Main|Electron ionization}}
[[File:Schematic diagram of an EI ion source.jpg|thumb|300px|Electron ionization source schematic]]
[[Electron ionization]] is widely used in mass spectrometry, particularly for [[organic chemistry|organic]] molecules. The [[Phase (matter)|gas phase]] reaction producing electron ionization is

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

where M is the atom or molecule being ionized, <chem>e^-</chem> is the electron, and <chem>M^{+\bullet}</chem> is the resulting ion.

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

An electron beam ion source (EBIS) is used in [[atomic physics]] to produce highly charged [[ion]]s by bombarding [[atom]]s with a powerful [[electron beam]].<ref name="Brown2006">{{cite book|author=Ian G. Brown|title=The Physics and Technology of Ion Sources|url=https://books.google.com/books?id=cdjGzG_zBLwC|date=6 March 2006|publisher=John Wiley & Sons|isbn=978-3-527-60454-8}}</ref><ref name="BeyerBeyer1997">{{cite book|author1=Heinrich Beyer|author2=Heinrich F. Beyer|author3=H.-Jürgen Kluge|author4=H.-J. Kluge |author5=Vi͡acheslav Petrovich Shevelʹko |title=X-Ray Radiation of Highly Charged Ions|url=https://books.google.com/books?id=8_AJFBC2-BsC|date=14 August 1997|publisher=Springer Science & Business Media|isbn=978-3-540-63185-9}}</ref> Its principle of operation is shared by the [[electron beam ion trap]].

===Electron capture ionization===
Electron capture ionization (ECI) is the ionization of a gas phase [[atom]] or [[molecule]] by attachment of an [[electron]] to create an ion of the form A<sup>−•</sup>. The reaction is

:<chem>A + e^- ->[M] A^-</chem>

where the M over the arrow denotes that to conserve [[energy]] and [[momentum]] a third body is required (the [[molecularity]] of the reaction is three).

Electron capture can be used in conjunction with [[chemical ionization]].<ref>{{Citation | doi = 10.1021/ac50035a017 | title = Electron capture negative ion chemical ionization mass spectrometry | year = 1978 |author1=Donald F. Hunt |author2=Frank W. Crow | journal = Analytical Chemistry | volume = 50 | issue = 13 | pages = 1781–1784}}</ref>

An [[electron capture detector]] is used in some [[gas chromatography]] systems.<ref>{{GoldBookRef|title=electron capture detector (in gas chromatography)|file= E01981}}</ref>

==Chemical ionization==
{{Main|Chemical ionization}}
[[Chemical ionization]] (CI) is a lower energy process than [[electron ionization]] because it involves ion/molecule reactions rather than electron removal.<ref>{{Cite journal |doi = 10.1021/ja00964a001|title = Chemical Ionization Mass Spectrometry. I. General Introduction|journal = Journal of the American Chemical Society|volume = 88|issue = 12|pages = 2621–2630|year = 1966|last1 = Munson|first1 = M. S. B.|last2 = Field|first2 = F. H.| bibcode=1966JAChS..88.2621M }}</ref>  The lower energy yields less [[fragmentation (chemistry)|fragmentation]], and usually a simpler [[spectrum]].  A typical CI spectrum has an easily identifiable molecular ion.<ref name="MSPA">{{cite book|last=de Hoffmann|first=Edmond|author2=Vincent Stroobant|title=Mass Spectrometry: Principles and Applications|publisher=John Wiley & Sons, Ltd.|location=Toronto|date=2003|edition=Second|page=14| isbn = 978-0-471-48566-7 }}</ref>

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 (physics)|plasma]].  Positive and negative ions of the analyte are formed by reactions with this plasma. For example, [[protonation]] occurs by

:<chem>CH4 + e^- -> CH4+ + 2e^-</chem> (primary ion formation),

:<chem>CH4 + CH4+ -> CH5+ + CH3</chem> (reagent ion formation),

:<chem>M + CH5+ -> CH4 + [M + H]+</chem> 	(product ion formation, e.g. protonation).

===Charge exchange ionization===
{{See also|Charge exchange}}
Charge-exchange ionization (also known as charge-transfer 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.<ref>{{GoldBookRef|title=charge-exchange ionization|file= C00989}}</ref>

:<chem>A+ + B -> A + B+</chem>

===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]].<ref>{{GoldBookRef|title=chemi-ionization|file= C01044}} C01044</ref><ref name=Klucharev1993>{{citation | last = Klucharev | first =  A. N. | year = 1993 | title = Chemi-ionization processes | journal = Physics-Uspekhi | volume = 36 | pages = 486–512 | doi = 10.1070/PU1993v036n06ABEH002162 |bibcode = 1993PhyU...36..486K | issue = 6 }}</ref> Chemi-ionization can be represented by
:<chem>G^\ast{} + M -> G{} + M^{+\bullet}{} + e^-</chem>
where G is the excited state species (indicated by the superscripted asterisk), and M is the species that is ionized by the loss of an [[electron]] to form the [[Radical (chemistry)|radical]] [[cation]] (indicated by the superscripted "plus-dot").

===Associative ionization===
Associative ionization is a gas phase reaction in which two atoms or molecules interact to form a single product ion.<ref>{{GoldBookRef|title=associative ionization|file= A00475}}</ref><ref>*{{cite journal |vauthors=Jones DM, Dahler JS |title=Theory of associative ionization  |date=April 1988 |pmid=9900022 |journal=[[Physical Review A]]|volume=37|issue=8 |pages=2916–2933|doi= 10.1103/PhysRevA.37.2916|bibcode = 1988PhRvA..37.2916J }}</ref><ref>{{Cite journal | last = Cohen | first =  James S. | date = 1976 | title = Multistate curve-crossing model for scattering: Associative ionization and excitation transfer in helium | journal = Physical Review A | volume = 13 | issue =  1 | pages = 99–114 | doi = 10.1103/PhysRevA.13.99 |bibcode = 1976PhRvA..13...99C }}</ref> One or both of the interacting species may have excess [[internal energy]].

For example,

:<chem>A^\ast{} + B -> AB^{+\bullet}{} + e^-</chem>

where species A with excess internal energy (indicated by the asterisk) interacts with B to form the ion AB<sup>+</sup>.

===Penning ionization===
{{Main|Penning ionization}}
[[Penning ionization]] is a form of chemi-ionization involving reactions between neutral atoms or molecules.<ref name="pmid17155624">{{cite journal |vauthors=Arango CA, Shapiro M, Brumer P |title=Cold atomic collisions: coherent control of penning and associative ionization |journal=Phys. Rev. Lett. |volume=97 |issue=19 |page=193202 |date=2006 |pmid=17155624 |doi=10.1103/PhysRevLett.97.193202 |bibcode=2006PhRvL..97s3202A|arxiv = physics/0610131 |s2cid=1480148 }}</ref><ref name="pmid17016831">{{cite journal |vauthors=Hiraoka K, Furuya H, Kambara S, Suzuki S, Hashimoto Y, Takamizawa A |title=Atmospheric-pressure Penning ionization of aliphatic hydrocarbons |journal=Rapid Commun. Mass Spectrom. |volume=20 |issue=21 |pages=3213–22 |date=2006 |pmid=17016831 |doi=10.1002/rcm.2706|bibcode=2006RCMS...20.3213H }}</ref> The process is named after the Dutch physicist [[Frans Michel Penning]] who first reported it in 1927.<ref>Penning, F. M. ''Die Naturwissenschaften'', 1927, '''15''', 818.  ''Über Ionisation durch metastabile Atome.''</ref> Penning ionization involves a reaction between a gas-phase excited-state atom or molecule G<sup>*</sup> and a target molecule M resulting in the formation of a radical molecular cation M<sup>+.</sup>, an electron e<sup>−</sup>, and a neutral gas molecule G:<ref>{{GoldBookRef|title=Penning gas mixture|file= P04476}}</ref>

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

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

:<chem>G^\ast{} + M -> MG^{+\bullet}{} + e^-</chem>

Surface Penning ionization (also known as Auger deexcitation) refers to the interaction of the excited-state gas with a bulk surface S, resulting in the release of an electron according to

:<chem>G^\ast{} + S -> G{} + S{} + e^-</chem>.

===Ion attachment===
{{Main|Ion-attachment mass spectrometry}}
[[Ion-attachment mass spectrometry|Ion-attachment ionization]] is similar to [[chemical ionization]] in which a cation is attached to the analyte molecule in a reactive collision:
:<chem>M + X+ + A -> MX+ + A</chem>

Where M is the analyte molecule, X<sup>+</sup> is the cation and A is a non-reacting collision partner.<ref>{{cite journal|title=Lithium ion attachment mass spectrometry: Instrumentation and features|journal=Review of Scientific Instruments|volume=72|issue=5|page=2248|bibcode=2001RScI...72.2248S|last1=Selvin|first1=P. Christopher|last2=Fujii|first2=Toshihiro|date=2001|doi=10.1063/1.1362439}}</ref>

In a radioactive ion source, a small piece of radioactive material, for instance <sup>63</sup>[[Nickel|Ni]] or <sup>241</sup>[[Americium|Am]], is used to ionize a gas.{{citation needed|date=July 2014}} This is used in ionization [[smoke detector]]s and [[ion mobility spectrometer]]s.

==Gas-discharge ion sources==
[[File:NASA NEXT Ion thruster.712983main NEXT LDT Thrusterhi-res full.jpg|thumb|300px|NASA's [[NEXT (ion thruster)]] space craft propulsion system]]
These ion sources use a [[plasma (physics)|plasma]] source or [[electric discharge]] to create ions.

===Inductively-coupled plasma===
{{Main|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 current]]s which are produced by [[electromagnetic induction]], that is, by time-varying [[magnetic field]]s.<ref>{{cite journal |last1=Broekaert |first1=J. A. C. |title=Inductively Coupled Plasmas in Analytical Atomic Spectrometry Eds.: A. Montaser and D. W. Golightly VCH, Weinheim, 2nd Edition. 1992, ISBN 3-527-28339-0, 984 pp., Hardcover, DM 296,— |journal=Acta Hydrochimica et Hydrobiologica |date=January 1993 |volume=21 |issue=6 |pages=327–328 |doi=10.1002/aheh.19930210610}}</ref>


===Microwave-induced plasma===
Microwave induced plasma ion sources are capable of exciting electrodeless gas discharges to create ions for trace element mass spectrometry.<ref name="Okamoto1994">{{cite journal|last1=Okamoto|first1=Yukio|title=High-sensitivity microwave-induced plasma mass spectrometry for trace element analysis|journal=Journal of Analytical Atomic Spectrometry|volume=9|issue=7|date=1994|page=745|issn=0267-9477|doi=10.1039/ja9940900745}}</ref><ref name="DouglasFrench1981">{{cite journal|last1=Douglas|first1=D. J.|last2=French|first2=J. B.|title=Elemental analysis with a microwave-induced plasma/quadrupole mass spectrometer system|journal=Analytical Chemistry|volume=53|issue=1|date=1981|pages=37–41|issn=0003-2700|doi=10.1021/ac00224a011}}</ref> A microwave plasma has high frequency [[electromagnetic radiation]] in the [[GHz]] range. It is capable of exciting electrodeless [[gas discharge]]s. 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 (together with an appropriate gas-flow scheme) 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===
{{Main|electron cyclotron resonance#ECR ion sources}}
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===
{{Main|glow discharge}}
[[File:Quarzkapillaritron.Betrieb.jpg|thumb|right|[[Capillaritron]] with quartz capillary in operation within a vacuum chamber: On the left the glowing capillary with the plasma up to the extraction cathode and on the right behind it the bluish glowing ion beam.]]
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 [[electrode]]s 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.<ref name="Wolf1995"/><ref name="Lejeune1974">{{cite journal|last1=Lejeune|first1=C.|title=Theoretical and experimental study of the duoplasmatron ion source|journal=Nuclear Instruments and Methods|volume=116|issue=3|date=1974|pages=417–428|issn=0029-554X|doi=10.1016/0029-554X(74)90821-0|bibcode=1974NucIM.116..417L}}</ref> THey can produce positive or negative ions.<ref name="Aberth1967">{{cite journal|last1=Aberth|first1=William|last2=Peterson|first2=James R.|title=Characteristics of a Low Energy Duoplasmatron Negative Ion Source|journal=Review of Scientific Instruments|volume=38|issue=6|date=1967|page=745|issn=0034-6748|doi=10.1063/1.1720882|bibcode=1967RScI...38..745A}}</ref> They are used for secondary ion mass spectrometry, ion beam etching, and high-energy physics.<ref name="CoathLong1995">{{cite journal|last1=Coath|first1=C. D.|last2=Long|first2=J. V. P.|title=A high-brightness duoplasmatron ion source for microprobe secondary-ion mass spectrometry|journal=Review of Scientific Instruments|volume=66|issue=2|date=1995|page=1018|issn=0034-6748|doi=10.1063/1.1146038|bibcode=1995RScI...66.1018C|doi-access=free}}</ref><ref name="Mahoney2013">{{cite book|author=Christine M. Mahoney|title=Cluster Secondary Ion Mass Spectrometry: Principles and Applications|url=https://books.google.com/books?id=hVSqwK0uqfsC&pg=PA65|date=9 April 2013|publisher=John Wiley & Sons|isbn=978-1-118-58925-0|pages=65–}}</ref><ref name="Humphries2013">{{cite book|author=Stanley Humphries|title=Charged Particle Beams|url=https://books.google.com/books?id=1GjCAgAAQBAJ&pg=PA309|date=25 July 2013|publisher=Dover Publications|isbn=978-0-486-31585-0|pages=309–}}</ref>

===Flowing afterglow===
{{Main|plasma afterglow}}
In a flowing plasma afterglow, ions are formed in a flow of inert gas, typically [[helium]] or [[argon]].<ref name="FergusonFehsenfeld1969">{{Cite book|last1=Ferguson|first1=E. E.|title=Chemical Reactions in Electrical Discharges|last2=Fehsenfeld|first2=F. C.|last3=Schmeltekopf|first3=A. L.|volume=80|date=1969|pages=83–91|issn=0065-2393|doi=10.1021/ba-1969-0080.ch006|chapter=Ion-Molecule Reaction Rates Measured in a Discharge Afterglow|series=Advances in Chemistry|isbn=978-0-8412-0081-4}}</ref><ref name="Ferguson1992">{{cite journal|last1=Ferguson|first1=Eldon E.|title=A Personal history of the early development of the flowing afterglow technique for ion-molecule reaction studies|journal=Journal of the American Society for Mass Spectrometry|volume=3|issue=5|date=1992|pages=479–486|issn=1044-0305|doi=10.1016/1044-0305(92)85024-E|pmid=24234490|url=https://zenodo.org/record/1258658|type=Submitted manuscript|doi-access=free|bibcode=1992JASMS...3..479F }}</ref><ref name="Bierbaum2014">{{cite journal|last1=Bierbaum|first1=Veronica M.|title=Go with the flow: Fifty years of innovation and ion chemistry using the flowing afterglow|journal=International Journal of Mass Spectrometry|date=2014|issn=1387-3806|doi=10.1016/j.ijms.2014.07.021|volume=377|pages=456–466|bibcode = 2015IJMSp.377..456B }}</ref> 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.<ref name="SmithŠpaněl2005">{{cite journal|last1=Smith|first1=David|last2=Španěl|first2=Patrik|title=Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis|journal=Mass Spectrometry Reviews|volume=24|issue=5|date=2005|pages=661–700|issn=0277-7037|doi=10.1002/mas.20033|pmid=15495143|bibcode = 2005MSRv...24..661S }}</ref><ref>{{Cite journal|last1=Dhooghe|first1=Frederik|last2=Vansintjan|first2=Robbe|last3=Schoon|first3=Niels|last4=Amelynck|first4=Crist|date=2012-08-30|title=Studies in search of selective detection of isomeric biogenic hexen-1-ols and hexanal by flowing afterglow tandem mass spectrometry using [H3O]+ and [NO]+ reagent ions|journal=Rapid Communications in Mass Spectrometry|language=en|volume=26|issue=16|pages=1868–1874|doi=10.1002/rcm.6294|pmid=22777789|issn=1097-0231}}</ref>

===Spark ionization===
{{Main|spark ionization}}
Electric spark ionization is used to produce gas phase [[ion]]s 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 (SSMS).<ref>{{cite journal |author1=H. E. Beske |author2=A. Hurrle |author3=K. P. Jochum | title = Part I. Principles of spark source mass spectrometry (SSMS) | date = 1981 | journal = [[Fresenius' Journal of Analytical Chemistry]] | volume = 309 | issue = 4 | pages = 258–261 | doi = 10.1007/BF00488596|s2cid=92433014 }}</ref>

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 ([[Hall-effect thruster]]s).

==Photoionization==
{{Main|Photoionization}}
[[Photoionization]] is the ionization process in which an ion is formed from the interaction of a [[photon]] with an atom or molecule.<ref>{{GoldBookRef|title=photoionization|file=P04620}}</ref>

===Multi-photon ionization===
In multi-photon ionization (MPI), several photons of energy below the ionization threshold may actually combine their energies to ionize an atom.

[[Resonance-enhanced multiphoton ionization]] (REMPI) 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===
{{Main|Atmospheric pressure photoionization}}
[[Atmospheric pressure photoionization]] (APPI) uses a source of photons, usually a vacuum UV (VUV) 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 (above 400 degrees Celsius) 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===
{{Main|Field desorption}}
[[File:Field desorption.gif|thumb|300px|Field desorption schematic]]
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.<ref>{{cite journal | last1 = Beckey | first1 = H.D. | date =1969 | title = Field ionization mass spectrometry |journal=Research/Development | volume = 20 | issue = 11| page = 26 }}</ref>  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 {{chem2|M^{+.} }} and less often, protonated molecules {{chem2|[M + H]+}}

===Particle bombardment===

====Fast atom bombardment====
{{Main|fast atom bombardment}}
Particle bombardment with atoms is called fast atom bombardment (FAB) and bombardment with atomic or molecular ions is called [[secondary ion mass spectrometry]] (SIMS).<ref name="WilliamsFindeis1987">{{cite journal|last1=Williams|first1=Dudley H.|last2=Findeis|first2=A. Frederick|last3=Naylor|first3=Stephen|last4=Gibson|first4=Bradford W.|title=Aspects of the production of FAB and SIMS mass spectra|journal=Journal of the American Chemical Society|volume=109|issue=7|date=1987|pages=1980–1986|issn=0002-7863|doi=10.1021/ja00241a013|bibcode=1987JAChS.109.1980W }}</ref> 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 <sup>252</sup>Cf.

In FAB the analytes is mixed with a non-volatile chemical protection environment called a [[Matrix Isolation|matrix]] and is bombarded under vacuum with a high energy (4000 to 10,000 [[electron volts]]) beam of atoms.<ref name="pmid7306100">{{cite journal |vauthors=Morris HR, Panico M, Barber M, Bordoli RS, Sedgwick RD, Tyler A | title = Fast atom bombardment: a new mass spectrometric method for peptide sequence analysis | journal = Biochem. Biophys. Res. Commun. | volume = 101 | issue = 2 | pages = 623–31 | date = 1981 | pmid = 7306100 | doi =10.1016/0006-291X(81)91304-8 }}</ref> 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.

====Secondary ionization====
{{Unreferenced section|date=October 2024}}
Secondary ion mass spectrometry (SIMS) 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&nbsp;nm.

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.

====Plasma desorption ionization====
[[File:pdms inst.gif|thumb|300px|Schematic representation of a plasama desorption time-of-flight mass spectrometer]]
Plasma desorption ionization mass spectrometry (PDMS), also called fission fragment ionization, is a mass spectrometry technique in which ionization of material in a solid sample is accomplished by bombarding it with ionic or neutral atoms formed as a result of the [[nuclear fission]] of a suitable [[nuclide]], typically the [[californium]] isotope <sup>252</sup>Cf.<ref name="MacfarlaneTorgerson1976">{{cite journal|last1=Macfarlane|first1=R.|last2=Torgerson|first2=D.|title=Californium-252 plasma desorption mass spectroscopy|journal=Science|volume=191|issue=4230|date=1976|pages=920–925|issn=0036-8075|doi=10.1126/science.1251202|pmid=1251202|bibcode=1976Sci...191..920M}}</ref><ref name="Hilf1993">{{cite journal|last1=Hilf|first1=E.R.|title=Approaches to plasma desorption mass spectrometry by some theoretical physics concepts|journal=International Journal of Mass Spectrometry and Ion Processes|volume=126|date=1993|pages=25–36|issn=0168-1176|doi=10.1016/0168-1176(93)80067-O|bibcode=1993IJMSI.126...25H}}</ref>

===Laser desorption ionization===
{{Unreferenced section|date=October 2024}}
[[File:Maldi.PNG|thumb|300px|Diagram of a MALDI ion source]]
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-assisted laser desorption/ionization====
Surface-assisted laser desorption/ionization (SALDI) is a [[soft laser desorption]] technique used for analyzing [[biomolecule]]s by [[mass spectrometry]].<ref name="SunnerDratz1995">{{cite journal|last1=Sunner|first1=Jan.|last2=Dratz|first2=Edward.|last3=Chen|first3=Yu-Chie.|title=Graphite surface-assisted laser desorption/ionization time-of-flight mass spectrometry of peptides and proteins from liquid solutions|journal=Analytical Chemistry|volume=67|issue=23|date=1995|pages=4335–4342|issn=0003-2700|doi=10.1021/ac00119a021|pmid=8633776}}</ref><ref name="DattelbaumIyer2006">{{cite journal|last1=Dattelbaum|first1=Andrew M|last2=Iyer|first2=Srinivas|title=Surface-assisted laser desorption/ionization mass spectrometry|journal=Expert Review of Proteomics|volume=3|issue=1|date=2006|pages=153–161|issn=1478-9450|doi=10.1586/14789450.3.1.153|pmid=16445359|s2cid=39538990|url=https://zenodo.org/record/1235756|type=Submitted manuscript}}</ref> In its first embodiment, it used [[graphite]] matrix.<ref name="SunnerDratz1995" /> At present, laser desorption/ionization methods using other [[inorganic]] matrices, such as [[nanomaterials]], are often regarded as SALDI variants. A related method named "ambient SALDI" - which is a combination of conventional SALDI with ambient mass spectrometry incorporating the [[DART ion source]] - has also been demonstrated.<ref name="ZhangLi2012">{{cite journal|last1=Zhang|first1=Jialing|last2=Li|first2=Ze|last3=Zhang|first3=Chengsen|last4=Feng|first4=Baosheng|last5=Zhou|first5=Zhigui|last6=Bai|first6=Yu|last7=Liu|first7=Huwei|title=Graphite-Coated Paper as Substrate for High Sensitivity Analysis in Ambient Surface-Assisted Laser Desorption/Ionization Mass Spectrometry|journal=Analytical Chemistry|volume=84|issue=7|date=2012|pages=3296–3301|issn=0003-2700|doi=10.1021/ac300002g|pmid=22380704}}</ref>

====Surface-enhanced laser desorption/ionization====
{{Main|Surface-enhanced laser desorption/ionization}}
Surface-enhanced laser desorption/ionization (SELDI) is a variant of MALDI that is used for the analysis of [[protein]] [[mixture]]s  that uses a target modified to achieve biochemical [[Receptor affinity|affinity]] with the analyte compound.<ref>{{cite journal |vauthors=Tang N, Tornatore P, Weinberger SR |title=Current developments in SELDI affinity technology |journal=Mass Spectrometry Reviews |volume=23 |issue=1 |pages=34–44 |date=2004 |pmid=14625891 |doi=10.1002/mas.10066|bibcode = 2004MSRv...23...34T }}</ref>

====Desorption ionization on silicon====
{{Main|Desorption ionization on silicon}}
Desorption ionization on silicon (DIOS) refers to laser desorption/ionization of a sample deposited on a porous silicon surface.<ref name="BuriakWei1999">{{cite journal|last1=Buriak|first1=Jillian M.|title=Desorption-ionization mass spectrometry on porous silicon|last2=Wei|first2=Jing|author-link3=Gary Siuzdak|last3=Siuzdak|first3=Gary|journal=Nature|volume=399|issue=6733|date=1999|pages=243–246|issn=0028-0836|doi=10.1038/20400|pmid=10353246|bibcode=1999Natur.399..243W|s2cid=4314372}}</ref>

====Smalley source====
A laser vaporization cluster source produces ions using a combination of laser desorption ionization and supersonic expansion.<ref name="Duncan2012">{{cite journal|last1=Duncan|first1=Michael A.|title=Invited Review Article: Laser vaporization cluster sources|journal=Review of Scientific Instruments|volume=83|issue=4|pages=041101–041101–19|date=2012|issn=0034-6748|doi=10.1063/1.3697599|pmid=22559508|bibcode=2012RScI...83d1101D}}</ref> The '''Smalley source''' (or '''Smalley cluster source''')<ref>{{cite book|title=Laser Ablation and Desorption|url=https://books.google.com/books?id=iWfddcv0quwC&pg=PA628|date=10 December 1997|publisher=Academic Press|isbn=978-0-08-086020-6|pages=628–}}</ref> was developed by [[Richard Smalley]] at [[Rice University]] in the 1980s and was central to the discovery of [[fullerene]]s in 1985.<ref name="Smalley1997">{{cite journal|last1=Smalley|first1=Richard|title=Discovering the fullerenes|journal=Reviews of Modern Physics|volume=69|issue=3|date=1997|pages=723–730|issn=0034-6861|doi=10.1103/RevModPhys.69.723|bibcode=1997RvMP...69..723S}}</ref><ref name="Johnston2002">{{cite book|author=Roy L. Johnston|title=Atomic and Molecular Clusters|url=https://books.google.com/books?id=pxztbPhmBeIC&pg=PA150|date=25 April 2002|publisher=CRC Press|isbn=978-1-4200-5577-1|pages=150–}}</ref>

====Aerosol ionization====
In [[aerosol mass spectrometry]] with time-of-flight analysis, 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.<ref>{{Cite journal | doi = 10.1016/0021-8502(94)00133-J | title = On-line chemical analysis of aerosols by rapid single-particle mass spectrometry | date = 1995 | author = Carson, P | journal = Journal of Aerosol Science | volume = 26 | pages = 535–545 | last2 = Neubauer | first2 = K | last3 = Johnston | first3 = M | last4 = Wexler | first4 = A | issue = 4 | bibcode = 1995JAerS..26..535C }}</ref><ref>{{Cite journal | doi = 10.1016/S0021-8502(00)90189-7 | title = Real time monitoring of size-resolved single particle chemistry during INDOEX-IFP 99 | date = 2000 | author = Guazzotti, S | journal = Journal of Aerosol Science | volume = 31 | pages = 182–183 | last2 = Coffee | first2 = K | last3 = Prather | first3 = K | bibcode = 2000JAerS..31..182G }}</ref>

==Spray ionization==
[[File:Apci.png|thumb|300px|Atmospheric-pressure chemical ionization source]]
Spray ionization methods involve the formation of aerosol particles from a liquid [[Solution (chemistry)|solution]] and the formation of bare ions after solvent evaporation.<ref name="Dass2007">{{cite book|author=Chhabil Dass|title=Fundamentals of Contemporary Mass Spectrometry|url=https://books.google.com/books?id=CYx9wzBzlIsC|date=11 May 2007|publisher=John Wiley & Sons|isbn=978-0-470-11848-1|pages=45–57}}</ref>

Solvent-assisted ionization (SAI) is a method in which charged droplets are produced by introducing a solution containing analyte into a heated inlet tube of an atmospheric pressure ionization mass spectrometer.  Just as in Electrospray Ionization (ESI), desolvation of the charged droplets produces multiply charged analyte ions. Volatile and nonvolatile compounds are analyzed by SAI, and high voltage is not  required to achieve sensitivity comparable to ESI.<ref name="Pagnotti2011">{{cite journal |vauthors=Pagnotti VS, Chubatyi ND, McEwen CN |journal=Anal. Chem. |title=Solvent Assisted Inlet Ionization: an Ultrasensitive New Liquid Introduction Ionization Method for Mass Spectrometry |volume=83 |issue=11 |pages=3981–3985 |date=2011 |doi=10.1021/ac200556z |pmid=21528896}}</ref> Application of a voltage to the solution entering the hot inlet through a zero dead volume fitting connected to fused silica tubing produces ESI-like mass spectra, but with higher sensitivity.<ref name="Pagnotti2012">{{cite journal |vauthors=Pagnotti VS, Chakrabarty S, Harron AF, McEwen CN |title=Increasing the Sensitivity of Liquid Introduction Mass Spectrometry by Combining Electrospray Ionization and Solvent Assisted Inlet Ionization |journal=Anal. Chem. |volume=84 |issue=15 |pages=6828–6832 |date=2012 |doi=10.1021/ac3014115 |pmid=22742705}}</ref> The inlet tube to the mass spectrometer becomes the ion source.

===Matrix-Assisted Ionization===
Matrix-Assisted Ionization (MAI) is similar to MALDI in sample preparation, but a laser is not required to convert analyte molecules included in a matrix compound into gas-phase ions.  In MAI, analyte ions have charge states similar to electrospray ionization but obtained from a solid matrix rather than a solvent.  No voltage or laser is required, but a laser can be used to obtain spatial resolution for imaging.  Matrix-analyte samples are ionized in the vacuum of a mass spectrometer and can be inserted into the vacuum through an atmospheric pressure inlet. Less volatile matrices such as 2,5-dihydroxybenzoic acid require a hot inlet tube to produce analyte ions by MAI, but more volatile matrices such as 3-nitrobenzonitrile require no heat, voltage, or laser.  Simply introducing the matrix-analyte sample to the inlet aperture of an atmospheric pressure ionization mass spectrometer produces abundant ions. Compounds at least as large as bovine serum albumin [66 kDa] can be ionized with this method.<ref name="Trimpin2013">{{cite journal |vauthors=Trimpin S, Wang B, Lietz CB, Marshall DD, Richards AL, Inutan ED |title=New Ionization Processes and Applications for Use in Mass Spectrometry |journal=Rev. Biochem. Mol. Biol. |year=2013 |volume=48 |issue=5 |pages=409–429|doi=10.3109/10409238.2013.806887 |pmid=23883414 }}</ref> In this method, the inlet to the mass spectrometer can be considered the ion source.

===Atmospheric-pressure chemical ionization===
{{Main|Atmospheric-pressure chemical ionization}}
Atmospheric-pressure chemical ionization uses a solvent spray at atmospheric pressure.<ref name="pmid17405144">{{cite journal |vauthors=Prakash C, Shaffer CL, Nedderman A |title=Analytical strategies for identifying drug metabolites |journal=Mass Spectrometry Reviews |volume=26 |issue=3 |pages=340–69 |date=2007 |pmid=17405144 |doi=10.1002/mas.20128|bibcode = 2007MSRv...26..340P }}</ref> 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 with the evaporated solvent acting as the chemical ionization reagent gas. APCI is not as "soft" (low fragmentation) an ionization technique as ESI.<ref name="pmid16723751">{{cite journal |vauthors=Zaikin VG, Halket JM |title=Derivatization in mass spectrometry--8. Soft ionization mass spectrometry of small molecules |journal=European Journal of Mass Spectrometry |volume=12 |issue=2 |pages=79–115 |date=2006 |pmid=16723751 |doi=10.1255/ejms.798|s2cid=34838846 }}</ref> Note that atmospheric pressure ionization (API) should not be used as a synonym for APCI.<ref>{{Cite book|date=2009|doi=10.1351/goldbook.A00492|chapter=Atmospheric pressure ionization in mass spectrometry|title = IUPAC Compendium of Chemical Terminology|isbn = 978-0-9678550-9-7}}</ref>

===Thermospray ionization===
{{Main|Thermospray ionization}}
Thermospray ionization is a form of atmospheric pressure ionization in [[mass spectrometry]]. It transfers ions from the liquid phase to the gas phase for analysis. It is particularly useful in [[liquid chromatography-mass spectrometry]].<ref>{{cite journal | last1 = Blakley | first1 = C. R. | last2 = Carmody | first2 = J. J. | last3 = Vestal | first3 = M. L. | date = 1980| title = Liquid Chromatograph-Mass Spectrometer for Analysis of Nonvolatile Samples | journal = Analytical Chemistry | volume = 1980 | issue = 52| pages = 1636–1641|doi=10.1021/ac50061a025 }}</ref>

[[File:NanoESIFT.jpg|thumb|300px|Electrospray ion source]]

===Electrospray ionization===
{{Main|Electrospray ionization}}
In electrospray ionization, a liquid is pushed through a very small, charged and usually metal, [[capillary]].<ref>{{cite journal | doi = 10.1002/mas.1280090103 |author1=Fenn, J. B. |author2=Mann, M. |author3=Meng, C. K. |author4=Wong, S. F. |author5=Whitehouse, C. M. | title = Electrospray Ionization-Principles and Practice | journal = [[Mass Spectrometry Reviews]] | date = 1990 | volume = 9 | issue = 1 | pages = 37–70|bibcode = 1990MSRv....9...37F }}</ref> This liquid contains the substance to be studied, the [[analyte]], dissolved in a large amount of [[solvent]], which is usually much more [[Volatility (chemistry)|volatile]] than the analyte. Volatile acids, bases or buffers are often added to this solution as well. The analyte exists as an [[ion]] in solution either in its anion or cation form. Because like charges repel, the liquid pushes itself out of the capillary and forms an aerosol, a mist of small droplets about 10 [[micro-|μm]] across. The aerosol is at least partially produced by a process involving the formation of a [[Taylor cone]] and a jet from the tip of this cone. An uncharged carrier gas such as [[nitrogen]] is sometimes used to help [[nebulizer|nebulize]] the liquid and to help evaporate the neutral solvent in the droplets. As the solvent evaporates, the analyte molecules are forced closer together, repel each other and break up the droplets. This process is called Coulombic fission because it is driven by repulsive [[Coulombic force]]s between charged molecules. The process repeats until the analyte is free of solvent and is a bare ion. The ions observed are created by the addition of a [[proton]] (a hydrogen ion) and denoted {{chem2|[M + H]+}}, or of another [[cation]] such as sodium ion, {{chem2|[M + Na]+}}, or the removal of a proton, {{chem2|[M \s H]-}}. Multiply charged ions such as {{chem2|[M + 2H]^{2+} }} are often observed. For [[macromolecules]], there can be many charge states, occurring with different frequencies; the charge can be as great as {{chem2|[M + 25H]^{25+} }}, for example.{{Citation needed|date=October 2024}}

====Probe electrospray ionization====
{{Main|Probe electrospray ionization}}
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 periodic motion.<ref>{{cite journal |author1=Hiraoka K. |author2=Nishidate K. |author3=Mori K. |author4=Asakawa D. |author5=Suzuki S. | title = Development of probe electrospray using a solid needle | journal = [[Rapid Communications in Mass Spectrometry]] | date = 2007| volume = 21 | pages = 3139–3144 | doi = 10.1002/rcm.3201 | pmid = 17708527 | issue = 18| bibcode = 2007RCMS...21.3139H }}</ref>

===Contactless atmospheric pressure ionization===

Contactless atmospheric pressure ionization is a technique used for analysis of liquid and solid samples by mass spectrometry.<ref name="HsiehChang2011">{{cite journal|last1=Hsieh|first1=Cheng-Huan|last2=Chang|first2=Chia-Hsien|last3=Urban|first3=Pawel L.|last4=Chen|first4=Yu-Chie|title=Capillary Action-Supported Contactless Atmospheric Pressure Ionization for the Combined Sampling and Mass Spectrometric Analysis of Biomolecules|journal=Analytical Chemistry|volume=83|issue=8|date=2011|pages=2866–2869|issn=0003-2700|doi=10.1021/ac200479s|pmid=21446703}}</ref> Contactless API can be operated without an additional electric power supply (supplying voltage to the source emitter), gas supply, or [[syringe pump]]. Thus, the technique provides a facile means for analyzing chemical compounds by mass spectrometry at atmospheric pressure.

===Sonic spray ionization===

Sonic spray ionization is  method for creating ions from a liquid solution, for example, a mixture of methanol and water.<ref name="pmid8779414">{{cite journal |vauthors=Hirabayashi A, Sakairi M, Koizumi H |title=Sonic spray mass spectrometry |journal=Anal. Chem. |volume=67 |issue=17 |pages=2878–82 |date=1995 |pmid=8779414|doi=10.1021/ac00113a023}}</ref> 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. Sonic spray ionization is used to analyze small organic molecules and drugs and can analyze large  molecules when an electric field is applied to the capillary to help increase the charge density and generate multiple charged ions of proteins.<ref>{{Cite journal|last1=Chen|first1=Tsung-Yi|last2=Lin|first2=Jia-Yi|last3=Chen|first3=Jen-Yi|last4=Chen|first4=Yu-Chie|date=2011-11-22|title=Ultrasonication-assisted spray ionization mass spectrometry for the analysis of biomolecules in solution|journal=Journal of the American Society for Mass Spectrometry|language=en|volume=21|issue=9|pages=1547–1553|doi=10.1016/j.jasms.2010.04.021|pmid=20547459|issn=1044-0305|doi-access=free|bibcode=2010JASMS..21.1547C }}</ref>

Sonic spray ionization has been coupled with [[high performance liquid chromatography]] for the analysis of drugs.<ref name="pmid11908800">{{cite journal |vauthors=Arinobu T, Hattori H, Seno H, Ishii A, Suzuki O |title=Comparison of SSI with APCI as an interface of HPLC-mass spectrometry for analysis of a drug and its metabolites |journal=J. Am. Soc. Mass Spectrom. |volume=13 |issue=3 |pages=204–208 |date=2002 |pmid=11908800 |doi=10.1016/S1044-0305(01)00359-2|doi-access=free |bibcode=2002JASMS..13..204A }}</ref><ref name="pmid12141684">{{cite journal |vauthors=Dams R, Benijts T, Günther W, Lambert W, De Leenheer A |title=Sonic spray ionization technology: performance study and application to a LC/MS analysis on a monolithic silica column for heroin impurity profiling |journal=Anal. Chem. |volume=74 |issue=13 |pages=3206–3212 |date=2002 |pmid=12141684 |doi=10.1021/ac0112824}}</ref> Oligonucleotides have been studied with this method.<ref name="pmid11990592">{{cite journal |vauthors=Huang M, Hirabayashi A, Okumura A, Hirabayashi Y |title=Matrix effect on the analysis of oligonucleotides by using a mass spectrometer with a sonic spray ionization source |journal=Anal Sci |volume=17 |issue=10 |pages=1179–1182 |date=2001 |pmid=11990592 |doi=10.2116/analsci.17.1179|doi-access=free }}</ref><ref name="pmid11999509">{{cite journal |vauthors=Huang M, Hirabayashi A |title=Multi-charged oligonucleotide ion formation in sonic spray ionization |journal=Anal Sci |volume=18 |issue=4 |pages=385–390 |date=2002 |pmid=11999509 |doi=10.2116/analsci.18.385|doi-access=free }}</ref> SSI has been used in a manner similar to desorption electrospray ionization<ref name="pmid16941547">{{cite journal |vauthors=Haddad R, Sparrapan R, Eberlin MN |title=Desorption sonic spray ionization for (high) voltage-free ambient mass spectrometry |journal=Rapid Commun. Mass Spectrom. |volume=20 |issue=19 |pages=2901–2905 |date=2006 |pmid=16941547 |doi=10.1002/rcm.2680|bibcode=2006RCMS...20.2901H }}</ref> for [[ambient ionization]] and has been coupled with [[thin-layer chromatography]] in this manner.<ref name="pmid18331004">{{cite journal |vauthors=Haddad R, Milagre HM, Catharino RR, Eberlin MN |title=Easy Ambient Sonic-Spray Ionization Mass Spectrometry Combined with Thin-Layer Chromatography |journal=Anal. Chem. |volume= 80|issue= 8|pages= 2744–2750|date=2008 |pmid=18331004 |doi=10.1021/ac702216q}}</ref>

===Ultrasonication-assisted spray ionization===

Ultrasonication-assisted spray ionization (UASI) is similar to the above techniques but uses an ultrasonic transducer to achieve atomization of the material and generate ions.<ref>{{cite journal|last=Chen|first=Tsung-Yi|title=Ultrasonication-assisted spray ionization mass spectrometry for the analysis of biomolecules in solution|journal=Journal of the American Society for Mass Spectrometry|author2=Lin, Jia-Yi |author3=Chen, Jen-Yi |author4=Chen, Yu-Chie |pages=1547–1553|doi=10.1016/j.jasms.2010.04.021 |pmid=20547459|volume=21|issue=9|year=2010|doi-access=free|bibcode=2010JASMS..21.1547C }}</ref><ref>{{Cite journal|last=Chen|first=Tsung-Yi|title=Ultrasonication-assisted spray ionization mass spectrometry for on-line monitoring of organic reactions|url=https://zenodo.org/record/999715|journal=Chemical Communications|volume=46|issue=44|pages=8347–9|access-date=4 November 2011|author2=Chao, Chin-Sheng |author3=Mong, Kwok-Kong Tony |author4=Chen, Yu-Chie |doi=10.1039/C0CC02629H|pmid=20957254|date=4 November 2010}}</ref>

==Thermal ionization==
{{Main|Thermal ionization}}
Thermal ionization (also known as surface 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 metal|alkali]] atoms (Li, Na, K, Rb, Cs) which have low ionization energies and are easily evaporated.<ref name="Alton1988">{{cite journal|last1=Alton|first1=G. D.|title=Characterization of a cesium surface ionization source with a porous tungsten ionizer. I|journal=Review of Scientific Instruments|volume=59|issue=7|date=1988|page=1039|issn=0034-6748|doi=10.1063/1.1139776|bibcode=1988RScI...59.1039A|url=https://zenodo.org/record/1231832|type=Submitted manuscript}}</ref>

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.<ref>{{Cite web |url=http://www.ornl.gov/~webworks/cpr/v823/pres/110496.pdf |title=A Negative-Surface Ionization for Generation of Halogen Radioactive Ion Beams |access-date=2014-01-20 |archive-url=https://web.archive.org/web/20041218073009/http://www.ornl.gov/~webworks/cpr/v823/pres/110496.pdf |archive-date=2004-12-18 |url-status=dead }}</ref>

==Ambient ionization==
{{Main|Ambient ionization}}
[[File:DART ion source capsule.jpg|thumb|300px|Direct analysis in real time ambient ionization ion source]]
In ambient ionization, ions are formed outside the mass spectrometer without sample preparation or separation.<ref name=Cooks2006>{{Cite journal | last1 = Cooks | first1 = R. Graham | last2 = Ouyang | first2 = Zheng | last3 = Takats | first3 = Zoltan | last4 = Wiseman | first4 = Justin M. | date = 2006 | title = Ambient Mass Spectrometry | journal = Science | volume = 311 | issue = 5767 | pages = 1566–70 | doi = 10.1126/science.1119426 | pmid = 16543450 | bibcode=2006Sci...311.1566C| s2cid = 98131681 }}</ref><ref name="MongeHarris2013">{{cite journal|last1=Monge|first1=María Eugenia|last2=Harris|first2=Glenn A.|last3=Dwivedi|first3=Prabha|last4=Fernández|first4=Facundo M.|title=Mass Spectrometry: Recent Advances in Direct Open Air Surface Sampling/Ionization|journal=Chemical Reviews|volume=113|issue=4|date=2013|pages=2269–2308|issn=0009-2665|doi=10.1021/cr300309q|pmid=23301684}}</ref><ref name="HuangYuan2010">{{cite journal|last1=Huang|first1=Min-Zong|last2=Yuan|first2=Cheng-Hui|last3=Cheng|first3=Sy-Chyi|last4=Cho|first4=Yi-Tzu|last5=Shiea|first5=Jentaie|title=Ambient Ionization Mass Spectrometry|journal=Annual Review of Analytical Chemistry|volume=3|issue=1|date=2010|pages=43–65|issn=1936-1327|doi=10.1146/annurev.anchem.111808.073702|pmid=20636033|bibcode=2010ARAC....3...43H}}</ref> Ions can be formed by extraction into charged electrospray droplets, thermally desorbed and ionized by [[chemical ionization]], or laser [[desorption|desorbed]] or [[Laser ablation|ablated]] and post-ionized before they enter the mass spectrometer.

Solid-liquid extraction based ambient ionization uses a charged spray to create a liquid film on the sample surface.<ref name="MongeHarris2013" /><ref name="Badu-TawiahEberlin2013">{{cite journal|last1=Badu-Tawiah|first1=Abraham K.|last2=Eberlin|first2=Livia S.|last3=Ouyang|first3=Zheng|last4=Cooks|first4=R. Graham|title=Chemical Aspects of the Extractive Methods of Ambient Ionization Mass Spectrometry|journal=Annual Review of Physical Chemistry|volume=64|issue=1|date=2013|pages=481–505|issn=0066-426X|doi=10.1146/annurev-physchem-040412-110026|pmid=23331308|bibcode = 2013ARPC...64..481B }}</ref> Molecules on the surface are extracted into the solvent. The action of the primary droplets hitting the surface produces secondary droplets that are the source of ions for the mass spectrometer. Desorption electrospray ionization (DESI) creates  charged droplets that are directed at a solid sample a few millimeters to a few centimeters away. The charged droplets pick up the sample through interaction with the surface and then form highly charged ions that can be sampled into a mass spectrometer.<ref name="pmid16237663">{{cite journal |vauthors=Takáts Z, Wiseman JM, Cooks RG |title=Ambient mass spectrometry using desorption electrospray ionization (DESI): instrumentation, mechanisms and applications in forensics, chemistry, and biology |journal=Journal of Mass Spectrometry |volume=40 |issue=10 |pages=1261–75 |date=2005 |pmid=16237663 |doi=10.1002/jms.922|bibcode = 2005JMSp...40.1261T |doi-access=free }}</ref>

Plasma-based ambient ionization is based on an electrical discharge in a flowing gas that produces metastable atoms and molecules and reactive ions. Heat is often used to assist in the desorption of volatile species from the sample. Ions are formed by chemical ionization in the gas phase. A [[DART ion source|direct analysis in real time (DART)]] source operates by exposing the sample to a dry gas stream (typically helium or nitrogen) that contains long-lived electronically or vibronically excited neutral atoms or molecules (or [[Metastability in molecules|"metastables"]]).  [[Excited state]]s are typically formed in the DART source by creating a glow discharge in a chamber through which the gas flows. A similar method called atmospheric solids analysis probe (ASAP) uses the heated gas from ESI or APCI probes to vaporize sample placed on a melting point tube inserted into an ESI/APCI source.<ref name="McEwen2005">{{cite journal |vauthors=McEwen CN, McKay RG, Larsen BS |title=Analysis of Solids, Liquids, and Biological Tissues Using Solids Probe Introduction at Atmospheric Pressure on Commercial LC/MS Instruments |journal=Anal. Chem. |volume=77 |issue=23 |pages=7826–7831 |date=2005 |doi=10.1021/ac051470k |pmid=16316194}}</ref>  Ionization is by APCI.

Laser-based ambient ionization is a two-step process in which a pulsed laser is used to desorb or ablate material from a sample and the plume of material interacts with an electrospray or plasma to create ions. Electrospray-assisted laser desorption/ionization (ELDI) uses a 337&nbsp;nm UV laser<ref name="pmid16299699">{{cite journal |vauthors=Shiea J, Huang MZ, Hsu HJ, Lee CY, Yuan CH, Beech I, Sunner J |title=Electrospray-assisted laser desorption/ionization mass spectrometry for direct ambient analysis of solids |journal=Rapid Commun. Mass Spectrom. |volume=19 |issue=24 |pages=3701–4 |date=2005 |pmid=16299699 |doi=10.1002/rcm.2243|bibcode=2005RCMS...19.3701S }}</ref> or 3&nbsp;μm infrared laser<ref name="Peng2010">{{cite journal|last1=Peng|first1=Ivory X.|last2=Ogorzalek Loo|first2=Rachel R.|last3=Margalith|first3=Eli|last4=Little|first4=Mark W.|last5=Loo|first5=Joseph A.|title=Electrospray-assisted laser desorption ionization mass spectrometry (ELDI-MS) with an infrared laser for characterizing peptides and proteins|journal=The Analyst|volume=135|issue=4|date=2010|pages=767–72|issn=0003-2654|doi=10.1039/b923303b|pmid=20349541|bibcode = 2010Ana...135..767P|pmc=3006438}}</ref> to  desorb material into an electrospray source. [[Matrix-assisted laser desorption electrospray ionization]] (MALDESI)<ref>{{cite journal | last1 = Sampson | first1 = JS | last2 = Hawkridge | first2 = AM | last3 = Muddiman | first3 = DC | year = 2006 | title = Generation and detection of multiply charged peptides and proteins by matrix-assisted laser desorption electrospray ionization (MALDESI) Fourier transform ion cyclotron resonance mass spectrometry | journal = J. Am. Soc. Mass Spectrom. | volume = 17 | issue = 12| pages = 1712–6 | doi = 10.1016/j.jasms.2006.08.003 | pmid = 16952462 | doi-access = free | bibcode = 2006JASMS..17.1712S }}</ref> 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 desorbing neutral analyte molecules that are ionized by interaction with electrosprayed solvent droplets generating multiply charged ions. [[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.

==Applications==

===Mass spectrometry===
{{Main|Mass Spectrometry}}
In a mass spectrometer a sample is ionized in an ion source and the resulting ions are separated by their mass-to-charge ratio. The ions are detected and the results are displayed as spectra of the relative abundance of detected ions as a function of the mass-to-charge ratio. The atoms or molecules in the sample can be identified by correlating known masses to the identified masses or through a characteristic fragmentation pattern.

===Particle accelerators===
{{Main|Particle accelerator}}
[[File:CARIBU at ATLAS.jpg|thumb|300px|Surface ionization source at the [[Argonne Tandem Linear Accelerator System]] (ATLAS)]]
[[File:Ion source at used in the Cockcroft-Walton accelerators at Fermilab.jpg|thumb|Ion source used in the [[Cockcroft-Walton generator|Cockcroft-Walton]] pre-accelerator at [[Fermilab]]<ref>{{cite book|title=35 years of H- ions at Fermilab|publisher=Fermilab|pages=12|url=http://www-ad.fnal.gov/proton/PIP/Communicate/Calendar/Repository/2014/35%20years%20of%20H-%20ions%20at%20Fermilab.pdf|access-date=12 August 2015}}</ref>]]
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: [[electron]]s, [[proton]]s, [[Hydride|H<sup>−</sup> ion]] or a [[Heavy ions]].

Electrons are generated with an [[electron gun]], of which there are many varieties.

Protons are generated with a [[plasma (physics)|plasma]]-based device, like a [[duoplasmatron]] or a [[magnetron]].

[[hydride|H<sup>−</sup>]] ions are generated with a [[magnetron]] or a [[Penning ionization|Penning]] source.  A magnetron consists of a central cylindrical cathode surrounded by an anode. The discharge voltage is typically greater than 150 V and the current drain is around 40 A. A [[magnetic field]] of about 0.2 [[tesla (unit)|tesla]] is parallel to the [[cathode]] axis.  Hydrogen gas is introduced by a pulsed gas valve.  [[Caesium]] is often used to lower the [[work function]] of the cathode, enhancing the amount of ions that are produced. Large caesiated sources are also used for [[neutral beam injection|plasma heating]] in nuclear fusion devices.

For a  [[Penning ionization|Penning source]], a strong magnetic field parallel to the electric field of the sheath guides electrons and ions on cyclotron spirals from cathode to cathode. Fast H-minus ions are generated at the cathodes as in the magnetron. They are slowed down due to the charge exchange reaction as they migrate to the plasma aperture. This makes for a beam of ions that is colder than the ions obtained from a magnetron.

Heavy ions can be generated with an [[electron cyclotron resonance]] ion source.  The use of electron cyclotron resonance (ECR) ion sources for the production of intense beams of highly charged ions has immensely grown over the last decade. ECR ion sources are used as injectors into linear accelerators, Van-de-Graaff generators or cyclotrons in nuclear and elementary particle physics. In atomic and surface physics ECR ion sources deliver intense beams of highly charged ions for collision experiments or for the investigation of surfaces. For the highest charge states, however, [[Electron beam ion source]]s (EBIS) are needed. They can generate even bare ions of mid-heavy elements. The [[Electron beam ion trap]] (EBIT), based on the same principle, can produce up to bare uranium ions and can be used as an ion source as well.

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.[[File:Ion implanter schematic.svg|thumb|300px|Ion beam deposition system with mass separator]]

Gas flows through the ion source between the anode and the cathode.  A positive [[voltage]] is applied to the anode.  This voltage, combined with the high magnetic field between the tips of the internal and external cathodes allow a plasma to start. Ions from the plasma are repelled by the anode's electric field.  This creates an ion beam.<ref>{{Cite journal|url=http://www.advanced-energy.com/en/upload/File/Sources/SL-ION-230-02.pdf |title=Ion Beam Sources |journal=Science |volume=311 |issue=5767 |pages=1566–70 |access-date=2006-12-14 |archive-url=https://web.archive.org/web/20061018152802/http://www.advanced-energy.com/en/upload/File/Sources/SL-ION-230-02.pdf |archive-date=2006-10-18 |bibcode=2006Sci...311.1566C |url-status=dead |doi = 10.1126/science.1119426 |pmid=16543450 |year=2006 |last1=Cooks |first1=R. G |last2=Ouyang |first2=Z |last3=Takats |first3=Z |last4=Wiseman |first4=J. M |s2cid=98131681 }}</ref>

===Surface modification===
* Surface cleaning and pretreatment for large area deposition
* [[Thin film]] deposition
* Deposition of thick [[diamond-like carbon]] (DLC) films
* Surface roughening of [[polymers]] for improved [[adhesion]] and/or [[biocompatibility]]<ref>{{cite web|url=http://www.advanced-energy.com/en/Ion.html |title=Ion Beam Source Technology |publisher=Advanced Energy |access-date=2006-12-14 |url-status=dead |archive-url=https://web.archive.org/web/20061018145638/http://www.advanced-energy.com/en/Ion.html |archive-date=October 18, 2006 }}</ref>

==See also==
*[[Ion beam]]
*[[RF antenna ion source]]
*[[On-Line Isotope Mass Separator]]

==References==
{{Reflist|30em}}

{{Mass spectrometry}}
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[[Category:Ion source| ]]
[[Category:Ions]]
[[Category:Accelerator physics]]