Editing Chemical ionization (section)

==Variants==<!--NGI, ECNCI redirects here-->

===Negative chemical ionization===
Chemical ionization for gas phase analysis is either positive or negative.<ref name="pmid7025931">{{cite journal |author=Dougherty R.C. |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.

In order to see a response by negative chemical ionization (NCI, also NICI), 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"/>{{rp|23}}Moreover, negative chemical ionization is more selective and demonstrates a higher sensitivity toward oxidizing agents and alkylating agents.<ref name=":4">{{cite journal|last1=Dougherty|first1=Ralph C.|title=Negative chemical ionization mass spectrometry|journal=Analytical Chemistry|volume=53|issue=4|pages=625–636|doi=10.1021/ac00227a003|language=en|year=2002}}</ref>

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=":4" /> [[pesticides]], and [[fire retardant]]s.<ref name=":4" /> 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, environmental toxic substances such as halogenated species, oxidizing and alkylating agents<ref name="pmid7025931" /> are frequently analyzed using an [[electron capture detector]] coupled to a [[gas chromatograph]].

Negative ions are formed by resonance capture of a near-thermal energy electron, dissociative capture of a low energy electron and via ion-molecular interactions such as proton transfer, charge transfer and hydride transfer.<ref name="pmid7025931" /> Compared to the other methods involving negative ion techniques, NCI is quite advantageous, as the reactivity of anions can be monitored in the absence of a solvent. Electron affinities and energies of low-lying valencies can be determined by this technique as well.<ref name="pmid7025931" />

===Charge-exchange chemical ionization===
This is also similar to CI and the difference lies in the production of a radical cation with an odd number of electrons. The reagent gas molecules are bombarded with high energy electrons and the product reagent gas ions abstract electrons from the analyte to form radical cations. The common reagent gases used for this technique are toluene, benzene, NO, Xe, Ar and He.

Careful control over the selection of reagent gases and the consideration toward the difference between the resonance energy of the reagent gas radical cation and the ionization energy of the analyte can be used to control fragmentation.<ref name=":1" /> The reactions for charge-exchange chemical ionization are as follows.

:<chem> He{} + e^- -> He^{+\bullet}{} + 2e^- </chem>
:<chem> He^{+\bullet}{} + M -> M^{+\bullet} </chem>
[[File:Apci.png|thumb|Atmospheric pressure chemical ionization source]]

===Atmospheric-pressure chemical ionization===
Chemical ionization in an atmospheric pressure electric discharge is called [[atmospheric pressure chemical ionization]] (APCI), which usually uses water as the reagent gas. An APCI source is composed of a [[Liquid chromatography–mass spectrometry|liquid chromatography]] outlet, nebulizing the eluent, a heated vaporizer tube, a corona discharge needle and a pinhole entrance to 10<sup>−3</sup> torr vacuum.<ref name=":3" /> 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]] or [[helium]] and the aerosol spray is subjected to a corona discharge to create ions. It is applicable to relatively less polar and thermally less stable compounds. The difference between APCI and CI is that APCI functions under atmospheric pressure, where the frequency of collisions is higher. This enables the improvement in sensitivity and ionization efficiency.<ref name=":1" />