14 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Chemical ionization | 1/2 | https://en.wikipedia.org/wiki/Chemical_ionization | reference | science, encyclopedia | 2026-05-05T10:03:57.240704+00:00 | kb-cron |
Chemical ionization (CI) is a soft ionization technique used in mass spectrometry. This was first introduced by Burnaby Munson and Frank H. Field in 1966. This technique is a branch of gaseous ion-molecule chemistry. Reagent gas molecules (often methane or ammonia) are ionized by electron ionization to form reagent ions, which subsequently react with analyte molecules in the gas phase to create analyte ions for analysis by mass spectrometry. Negative chemical ionization (NCI), charge-exchange chemical ionization, atmospheric-pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) are some of the common variants of the technique. CI mass spectrometry finds general application in the identification, structure elucidation and quantitation of organic compounds as well as some utility in biochemical analysis. Samples to be analyzed must be in vapour form, or else (in the case of liquids or solids), must be vapourized before introduction into the source.
== Principles of operation == The chemical ionization process generally imparts less energy to an analyte molecule than does electron impact (EI) ionization, resulting in less fragmentation and usually a simpler spectrum. The amount of fragmentation, and therefore the amount of structural information produced by the process can be controlled to some degree by selection of the reagent ion. In addition to some characteristic fragment ion peaks, a CI spectrum usually has an identifiable protonated molecular ion peak [M+1]+, allowing determination of the molecular mass. CI is thus useful as an alternative technique in cases where EI produces excessive fragmentation of the analyte, causing the molecular-ion peak to be weak or completely absent.
== 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. Electrons are produced externally to the source volume (at a lower pressure of 10−4 torr or below) by heating a metal filament which is made of tungsten, rhenium, or iridium. The electrons are introduced through a small aperture in the source wall at energies 200–1000 eV so that they penetrate to at least the centre of the box. 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.
== Mechanism == A CI experiment involves the use of gas phase acid-base reactions in the chamber. Some common reagent gases include: methane, ammonia, water and isobutane. Inside the ion source, the reagent gas is present in large excess compared to the analyte. Electrons entering the source will mainly ionize the reagent gas because it is in large excess compared to the analyte. The primary reagent ions then undergo secondary ion/molecule reactions (as below) to produce more stable reagent ions which ultimately collide and react with the lower concentration analyte molecules to form product ions. The collisions between reagent ions and analyte molecules occur at close to thermal energies, so that the energy available to fragment the analyte ions is limited to the exothermicity of the ion-molecule reaction. For a proton transfer reaction, this is just the difference in proton affinity between the neutral reagent molecule and the neutral analyte molecule. This results in significantly less fragmentation than does 70 eV electron ionization (EI). The following reactions are possible with methane as the reagent gas.
=== Primary ion formation ===
CH
4
+
e
−
⟶
CH
4
+
∙
+
2
e
−
{\displaystyle {\ce {CH4{}+e^{-}->CH4^{+\bullet }{}+2e^{-}}}}
=== Secondary reagent ions ===
CH
4
+
CH
4
+
∙
⟶
CH
5
+
+
CH
3
∙
{\displaystyle {\ce {CH4{}+CH4^{+\bullet }->CH5+{}+CH3^{\bullet }}}}
CH
4
+
CH
3
+
⟶
C
2
H
5
+
+
H
2
{\displaystyle {\ce {CH4 + CH3^+ -> C2H5+ + H2}}}
=== Product ion formation ===
M
+
CH
5
+
⟶
CH
4
+
[
M
+
H
]
+
{\displaystyle {\ce {M + CH5+ -> CH4 + [M + H]+}}}
(protonation)
AH
+
CH
3
+
⟶
CH
4
+
A
+
{\displaystyle {\ce {AH + CH3+ -> CH4 + A+}}}
(
H
−
{\displaystyle {\ce {H^-}}}
abstraction)
M
+
C
2
H
5
+
⟶
[
M
+
C
2
H
5
]
+
{\displaystyle {\ce {M + C2H5+ -> [M + C2H5]+}}}
(adduct formation)
A
+
CH
4
+
⟶
CH
4
+
A
+
{\displaystyle {\ce {A + CH4+ -> CH4 + A+}}}
(charge exchange)
If ammonia is the reagent gas,
NH
3
+
e
−
⟶
NH
3
+
∙
+
2
e
−
{\displaystyle {\ce {NH3{}+e^{-}->NH3^{+\bullet }{}+2e^{-}}}}
NH
3
+
NH
3
+
∙
⟶
NH
4
+
+
NH
2
{\displaystyle {\ce {NH3{}+NH3^{+\bullet }->NH4+{}+NH2}}}
M
+
NH
4
+
⟶
MH
+
+
NH
3
{\displaystyle {\ce {M + NH4^+ -> MH+ + NH3}}}
For isobutane as the reagent gas,
C
4
H
10
+
e
−
⟶
C
4
H
10
+
∙
+
2
e
−
(
+
C
3
H
7
+
and other ions
)
{\displaystyle {\ce {C4H10{}+e^{-}->C4H10^{+\bullet }{}+2e^{-}}}({\ce {+C3H7+}}{\text{and other ions}})}
C
3
H
7
+
+
C
4
H
10
+
∙
⟶
C
4
H
9
+
+
C
3
H
8
{\displaystyle {\ce {C3H7^{+}{}+C4H10^{+\bullet }->C4H9^{+}{}+C3H8}}}
M
+
C
4
H
9
+
⟶
MH
+
+
C
4
H
8
{\displaystyle {\ce {M + C4H9^+ -> MH^+ + C4H8}}}
Self chemical ionization is possible if the reagent ion is an ionized form of the analyte.