Chemical ionization

Chemical ionization (CI) is an ionization technique used in mass spectrometry.

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.

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.

Primary Ion Formation:

<math>CH_4 + e^- \to CH_4^+ + 2e^-</math>

Secondary Reagent Ions:

<math>CH_4 + CH_4^+ \to CH_5^+ + CH_3</math>

<math>CH_4 + CH_3^+ \to C_2H_5^+ + H_2</math>

Product Ion Formation:

<math>M + CH_5^+ \to CH_4 + [M + H]^+</math> (protonation)

<math>AH + CH_3^+ \to CH_4 + A^+</math> (<math>H^-</math> abstraction)

<math>M + CH_5^+ \to [M+ CH_5]^+</math> (adduct formation)

<math>A + CH_4^+ \to CH_4 + A^+</math> (charge exchange)

Variations

Gas Phase Chemical Ionization

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

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.

Positive Chemical Ionization

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

Liquid Phase Chemical Ionization

• Chemical ionization in an atmospheric pressure electric discharge is called atmospheric pressure chemical ionization.

External links

• American Society for Mass Spectrometry: What other techniques are used to produce ions?
• Chemical Ionization Tutorial
• IUPAC Gold Book Definition (PDF)

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