Chemical ionization

Chemical ionization scheme

The chemical ionization (CI) is an ionization method that in mass spectrometry is used. It is particularly used to determine the molar mass of easily fragmented substances. Since the sample has to be converted into a gaseous state before ionization, the method can only be used for substances that can be vaporized without decomposition in a high vacuum .

In chemical ionization, new ionized species are created when molecules in the gas phase interact with ions, i.e. that is, it is based on ion – molecule reactions. It takes place through the transfer of an electron, proton or other ions between the reactants. These reactants are the neutral analyte and ions from a reactant gas.

Chemical ionization is very similar to electron impact ionization (EI). As a rule, however, CI spectra are significantly less fragmented than corresponding EI spectra. Since the excess energy transferred to the sample molecule is small, the fragmentation is suppressed. Therefore mainly quasi- molecular ions are produced .

Reactant gases

In addition to the thermally vaporized sample molecules, an excess of reactant gas is admitted into the ion source , which is ionized by electron bombardment (150 eV). The primary ions of the reactant gas formed by the electron bombardment react through a series of collisions with other reactant gas molecules to form the stable ions that actually have an ionizing effect, the CI plasma ions .

The question with which reactant ion, a given analyte can be protonated, can be combined with data for the gas-phase basicity (gas phase basicity, GB) or proton affinity answer (PA proton affinity,). The differences in proton affinity (ΔPA value) determine whether a certain analyte can be protonated by a certain reactant ion and how strongly exothermic this protonation will be. Some fine-tuning of the PICI conditions can be achieved by considering reactant gases other than methane. For this purpose, molecular hydrogen and hydrogen-containing mixtures, isobutane , ammonia , dimethyl ether , diisopropyl ether , acetone , acetaldehyde , benzene and iodomethane come into consideration . Even transition metal ions such as Cu ⁺ and Fe ⁺ can be used as reactant ions to localize double bonds . However, the reactant gas nitrous oxide is more suitable for this .

methane

${\displaystyle \mathrm {CH_{4}+e^{-}\longrightarrow \cdot CH_{4}^{+}+2\ e^{-}\longrightarrow CH_{3}^{+}+H\cdot } }$

An electron knocks an electron out of a methane molecule to form a methane radical cation. A hydrogen radical can split off from this again and a methyl cation remains.

${\displaystyle \mathrm {\cdot CH_{4}^{+}+CH_{4}\longrightarrow CH_{5}^{+}+\cdot CH_{3}} }$

Chain reaction: methane radical cation reacts with methane to form a carbonium ion and a methyl radical.

${\displaystyle \mathrm {\cdot CH_{4}^{+}+CH_{4}\longrightarrow C_{2}H_{5}^{+}+H_{2}+H\cdot } }$

Dimerization reaction

Isobutane

${\displaystyle \mathrm {C_{4}H_{10}+e^{-}\longrightarrow \cdot C_{4}H_{10}^{+}+2\ e^{-}} }$

${\displaystyle \mathrm {\cdot C_{4}H_{10}^{+}+C_{4}H_{10}\longrightarrow C_{4}H_{9}^{+}+\cdot C_{4}H_{9}+H_{2}} }$

ammonia

${\displaystyle \mathrm {NH_{3}+e^{-}\longrightarrow \cdot NH_{3}^{+}+2\ e^{-}} }$

${\displaystyle \mathrm {\cdot NH_{3}^{+}+NH_{3}\longrightarrow NH_{4}^{+}+\cdot NH_{2}} }$

${\displaystyle \mathrm {NH_{4}^{+}+NH_{3}\longrightarrow N_{2}H_{7}^{+}} }$

Formation of molecular ions

If the molecules to be analyzed (M) are introduced, they react to form charged molecular ions.

${\displaystyle \mathrm {M\ +\ CH_{5}^{+}\longrightarrow \ CH_{4}\ +\ [M+H]^{+}} }$( Protonation )

${\displaystyle \mathrm {AH\ +\ CH_{3}^{+}\longrightarrow \ CH_{4}\ +\ A^{+}} }$ (Hydride elimination)

In addition to protonation, analyte ions formed by the addition of reactant gas ions are also observed (e.g. also with NH ₄ ⁺ ).

${\displaystyle \mathrm {M\ +\ CH_{5}^{+}\longrightarrow \ [M+CH_{5}]^{+}} }$( Adduct formation )

${\displaystyle \mathrm {A\ +\ CH_{4}^{+}\longrightarrow \ CH_{4}\ +\ A^{+}} }$( Charge exchange )

variants

Chemical ionization also produces negative ions, which can be detected by reversing the polarity of the voltages in the ion source (negative chemical ionization, NCI). One method in which the high vacuum is dispensed with is chemical ionization at atmospheric pressure .

Chemical ionization processes and terms

polarity	method	acronym	Explanation
positive	Positive ion chemical ionization (or positive chemical ionization)	PICI (or PCI)	Strictly speaking, any CI procedure that delivers positive closed-shell ions of the analyte. Often refers in a narrower sense to the generation of ions through protonation.
	Charge exchange / charge transfer	CE-CI / CT-CI	Positive radical ions are created by exchanging / transferring an electron; the two terms are interchangeable.
negative	Negative Ion Chemical Ionization (or Negative Chemical Ionization)	NICI (or NCI)	Any CI procedure that provides closed-shell negative ions of the analyte.
	Electron capture	EC	No CI procedure in the strict sense; however, the operating mode of the ion source corresponds to that of the NICI.

See also

• List of ionization methods in mass spectrometry

swell

• Entry on chemical ionization (in mass spectrometry) . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.C01026 Version: 2.3.3.
• JH Gross: Mass Spectrometry: A Textbook. 2nd edition, Springer, Berlin 2004, ISBN 978-3-540-40739-3 .
• Manfred Hesse, Herbert Meier, Bernd Zeeh: Spectroscopic methods in organic chemistry. Georg Thieme, Stuttgart 1995, ISBN 3-13-576105-3 , p. 258.

1. ↑ ^{a b c d} Jürgen H. Gross: mass spectrometry - a textbook . Springer-Verlag, 2012, ISBN 978-3-8274-2981-0 , pp. 384 ( limited preview in Google Book Search).