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| '''Chemical ionization''' ('''CI''') is an [[ionization]] technique used in [[mass spectrometry]].<ref>Munson, M.S.B.; Field, F.H. ''J. Am. Chem. Soc.'' '''1966''', ''88'', 2621-2630. [http://dx.doi.org/10.1021/ja00964a001 Chemical Ionization Mass Spectrometry. I. General Introduction].</ref><ref name="pmid4569234">{{cite journal |author=Fales HM, Milne GW, Pisano JJ, Brewer HB, Blum MS, MacConnell JG, Brand J, Law N |title=Biological applications of electron ionization and chemical ionization mass spectrometry |journal=Recent Prog. Horm. Res. |volume=28 |issue= |pages=591–626 |year=1972 |pmid=4569234 |doi=}}</ref><ref>{{cite journal|title=chemical ionization in mass spectrometry|year=2009|doi=10.1351/goldbook.C01026}}</ref> Chemical ionization is a lower energy process than [[electron ionization]]. The lower energy yields less fragmentation, and usually a simpler [[spectrum]]. A typical CI spectra has an easily identifiable intact molecular species (e.g. a protonated molecule).<ref name="MSPA">{{cite book|last=de Hoffmann|first=Edmond|coauthors=Vincent Stroobant|title=Mass Spectrometry: Principles and Applications|publisher=John Wiley & Sons, Ltd.|location=Toronto|year=2003|edition=Second| isbn = 0-471-48566-7|page=14 }}</ref>
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| ==Mechanism==
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| In a CI experiment, ions are produced through the collision of the analyte with ions of a reagent gas that are present 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.<ref name="MSPA" />
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| === Primary ion formation ===
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| :<math>CH_4 + e^- \to CH_4^+ + 2e^-</math>
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| === Secondary reagent ions ===
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| :<math>CH_4 + CH_4^+ \to CH_5^+ + CH_3</math>
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| :<math>CH_4 + CH_3^+ \to C_2H_5^+ + H_2</math>
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| === Product ion formation ===
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| :<math>M + CH_5^+ \to CH_4 + [M + H]^+</math> (protonation)
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| :<math>AH + CH_3^+ \to CH_4 + A^+</math> (<math>H^-</math> abstraction)
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| :<math>M + C_2H_5^+ \to [M+ C_2H_5]^+</math> (adduct formation)
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| :<math>A + CH_4^+ \to CH_4 + A^+</math> ([[Charge-exchange ionization|charge exchange]])
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| Self chemical ionization occurs when the reagent ion is an ionized form of the analyte.<ref>{{Cite journal | doi = 10.1021/ac00226a011 | title = Chemical ionization in Fourier transform mass spectrometry | year = 1981 | author = Sahba. Ghaderi; P. S. Kulkarni; Edward B. Ledford; Charles L. Wilkins; Michael L. Gross | journal = Analytical Chemistry | volume = 53 | pages = 428–437 | issue = 3}}</ref>
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| ==Variations==<!--NGI, ECNCI redirects here-->
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| ===Negative chemical ionization (NCI)===
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| Chemical ionization for gas phase analysis is either positive or negative.<ref name="pmid7025931">{{cite journal |author=Dougherty RC |title=Negative chemical ionization mass spectrometry: applications in environmental analytical chemistry |journal=Biomed. Mass Spectrom. |volume=8 |issue=7 |pages=283–292 |year=1981 |pmid=7025931 |doi=10.1002/bms.1200080702}}</ref> Almost all neutral analytes can form positive ions through the reactions described above.
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| In order to see a response by negative chemical ionization, the analyte must be capable of producing a negative ion (stabilize a negative charge) for example by [[electron capture ionization]]. Because not all analytes can do this, using NCI provides a certain degree of selectivity that is not available with other, more universal ionization techniques (EI, PCI). NCI can be used for the analysis of compounds containing acidic groups or electronegative elements (especially halogens).<ref name="MSPA">{{cite book|last=de Hoffmann|first=Edmond|coauthors=Vincent Stroobant|title=Mass Spectrometry: Principles and Applications|publisher=John Wiley & Sons, Ltd.|location=Toronto|year=2003|edition=Second| isbn = 0-471-48566-7|page=23 }}</ref>
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| Because of the high electronegativity of [[halogen]] atoms, NCI is a common choice for their analysis. This includes many groups of compounds, such as [[polychlorinated biphenyls|PCBs]],<ref name="J3">{{cite journal|last=Kontsas|first=Helena|coauthors=Kaija Pekari|date=2003-07-05|title=Determination of polychlorinated biphenyls in serum using gas chromatography–mass spectrometry with negative chemical ionization for exposure estimation|journal=Journal of Chromatography B|publisher=Elsevier, Ltd.|volume=791|issue=1–2|pages=117–125|url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6X0P-48B5FR8-D&_user=1069125&_coverDate=07%2F05%2F2003&_alid=759548054&_rdoc=3&_fmt=high&_orig=search&_cdi=7220&_sort=d&_docanchor=&view=c&_ct=8&_acct=C000051259&_version=1&_urlVersion=0&_userid=1069125&md5=a1aa7b8733e3fe4a2e0cb1bc49509fc0|accessdate=2008-06-26|doi=10.1016/S1570-0232(03)00216-2}}</ref> [[pesticides]],<ref name="J2">{{cite journal|last=Rivera-Rodríguez|first=Laura B. |coauthors=Ricardo Rodríguez-Estrella, James Jackson Ellington and John J. Evans|date=July 2007|title=Quantification of low levels of organochlorine pesticidesnext term using small volumes (≤100 μl) of plasma of wild birds through gas chromatography negative chemical ionization mass spectrometry|journal=Environmental Pollution|publisher=Elsevier, Ltd.|volume=148|issue=2|pages=654–662|url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VB5-4MVN09G-8&_user=1069125&_coverDate=07%2F31%2F2007&_alid=759547122&_rdoc=2&_fmt=high&_orig=search&_cdi=5917&_sort=d&_docanchor=&view=c&_ct=16&_acct=C000051259&_version=1&_urlVersion=0&_userid=1069125&md5=d3b5083a60609546bfb2a7f3e5bdb87e|accessdate=2008-06-26|doi=10.1016/j.envpol.2006.11.018|pmid=17240024}}</ref> and fire retardants.<ref name="J1">{{cite journal|last=Lacorte|first=Sylvia|coauthors=Míriam Guillamon|title=Validation of a pressurized solvent extraction and GC–NCI–MSnext term method for the low level determination of 40 polybrominated diphenyl ethers in mothers' milk|journal=Chemosphere|publisher=Elsevier, Ltd.|doi=10.1016/j.chemosphere.2008.05.021|year=2008|volume=73|pages=70–75|pmid=18582915|issue=1}}</ref> Most of these compounds are environmental contaminants, thus much of the NCI analysis that takes place is done under the auspices of environmental analysis. In cases where very low limits of detection are needed, halogenated species are frequently analyzed using an [[electron capture detector]] coupled to a [[gas chromatograph]].
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| ===Atmospheric pressure chemical ionization (APCI)===
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| Chemical ionization in an atmospheric pressure electric discharge is called [[atmospheric pressure chemical ionization]]. The analyte is a gas or liquid spray and ionization is accomplished using an atmospheric pressure corona discharge. This ionization method is often coupled with [[high performance liquid chromatography]] where the mobile phase containing eluting analyte sprayed with high flow rates of nitrogen and the aerosol spray is subjected to a corona discharge to create ions.
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| == References ==
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| {{reflist}}
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| == External links ==
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| *[http://littlemsandsailing.wordpress.com/2011/05/01/chemical-ionization-gas-selection-and-manifold-construction/ Using Amines as Chemical Ionization Reagents and Building Custom Manifold]
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| {{Mass spectrometry}}
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| {{States of matter}}
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| {{DEFAULTSORT:Chemical Ionization}}
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| [[Category:Ion source]]
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| [[Category:Mass spectrometry]]
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| [[Category:Scientific techniques]]
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