Editing Chemical ionization (section)

== Instrumentation ==
The CI source design for a mass spectrometer is very similar to that of the EI source. To facilitate the reactions between the ions and molecules, the chamber is kept relatively gas tight at a pressure of about 1 torr.<ref name=":1">{{cite book|last1=Dass|first1=Chhabil|title=Fundamentals of contemporary mass spectrometry|date=2007|publisher=Wiley-Interscience|location=Hoboken, N.J.|isbn=9780470118498|edition=[Online-Ausg.].}}</ref> Electrons are produced externally to the source volume (at a lower pressure of 10<sup>−4</sup> torr<ref name=":1" /> or below) by heating a metal filament which is made of [[tungsten]], [[rhenium]], or [[iridium]].<ref name=":0" /> The electrons are introduced through a small aperture in the source wall at energies 200–1000 eV<ref name=":1" /><ref name=":6">{{Cite journal |last=Vestal |first=Marvin L. |date=2000 |title=Methods of Ion Generation |url=http://www.igg.cas.cn/jgsz/zcxt/sygcxt/ggsys/djsdlztzp/201010/P020140310408095177115.pdf |journal=Chemical Reviews |volume=101 |issue=2 |pages=361–375|doi=10.1021/cr990104w |pmid=11712251 }}</ref> so that they penetrate to at least the centre of the box.<ref name=":6" /> In contrast to EI, the magnet and the electron trap are not needed for CI, since the electrons do not travel to the end of the chamber. Many modern sources are dual or combination EI/CI sources and can be switched from EI mode to CI mode and back in seconds.<ref>{{Cite book |last=Gross |first=J. H. |title=Mass Spectrometry |publisher=Springer |year=2004 |isbn=978-3-642-07388-5 |location=Berlin, Heidelberg |pages=331–354}}</ref>