Editing Ion source (section)

==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.