Chemical ionization

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Chemical ionization (CI) is an ionization technique used in mass spectrometry.[1][2][3]

With chemical ionization, the ions produced have little excess energy which results in less fragmentation than electron ionization. With CI, the spectra generated show relatively little fragmentation, and an easily identifiable molecular ion.

Contents

[edit] How it works

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. Positive and negative ions of the analyte are formed by reactions with this plasma.[4]

[edit] Primary Ion Formation:

CH_4 + e^- \to CH_4^+ + 2e^-

[edit] Secondary Reagent Ions:

CH_4 + CH_4^+ \to CH_5^+ + CH_3
CH_4 + CH_3^+ \to C_2H_5^+ + H_2

[edit] Product Ion Formation:

M + CH_5^+ \to CH_4 + [M + H]^+ (protonation)
AH + CH_3^+ \to CH_4 + A^+ (H − abstraction)
M + CH_5^+ \to [M+ CH_5]^+ (adduct formation)
A + CH_4^+ \to CH_4 + A^+ (charge exchange)

[edit] Variations

[edit] Gas Phase Chemical Ionization

Chemical ionization for gas phase analysis is either positive or negative.

[edit] Negative Chemical Ionization

Because not all analytes are capable of producing negative ions, a certain degree of selectivity can be gained when analyzing compounds with acidic groups or electronegative elements. This is especially useful for the analysis of mixtures.

[edit] Positive Chemical Ionization

Almost all neutral analytes can form positive ions through the reactions described above.

[edit] Liquid Phase Chemical Ionization

[edit] References

  1. ^ Munson, M.S.B.; Field, F.H. J. Am. Chem. Soc. 1966, 88, 2621-2630. Chemical Ionization Mass Spectrometry. I. General Introduction.
  2. ^ Fales HM, Milne GW, Pisano JJ, Brewer HB, Blum MS, MacConnell JG, Brand J, Law N (1972). "Biological applications of electron ionization and chemical ionization mass spectrometry". Recent Prog. Horm. Res. 28: 591-626. PMID 4569234. 
  3. ^ Dougherty RC (1981). "Negative chemical ionization mass spectrometry: applications in environmental analytical chemistry". Biomed. Mass Spectrom. 8 (7): 283-92. doi:10.1002/bms.1200080702. PMID 7025931. 
  4. ^ deHoffman, Edmond; Stroobant, Vincent (2003). Mass Spectrometry: Principles and Applications. Toronto: Wiley, 14. ISBN 0471485667. 

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