Chemical Ionization

For organic chemists, Chemical Ionization (CI) is especially useful technique when no molecular ion is observed in EI mass spectrum, and also in the case of confirming the mass to charge ratio of the molecular ion. Chemical ionization technique uses virtually the same ion source device as in electron impact, except, CI uses tight ion source, and reagent gas. Reagent gas (e.g. ammonia) is first subjected to electron impact. Sample ions are formed by the interaction of reagent gas ions and sample molecules. This phenomenon is called ion-molecule reactions. Reagent gas molecules are present in the ratio of about 100:1 with respect to sample molecules. Positive ions and negative ions are formed in the CI process. Depending on the setup of the instrument (source voltages, detector, etc...) only positive ions or only negative ions are recorded.

In CI, ion molecule reactions occur between ionized reagent gas molecules (G) and volatile analyte neutral molecules (M) to produce analyte ions. Pseudo-molecular ion MH+ (positive ion mode) or [M-H]- (negative ion mode) are often observed. Unlike molecular ions obtained in EI method, MH+ and [M-H]- detection occurs in high yield and less fragment ions are observed.

Positive ion mode:

GH+ + M ------> MH+ + G

Negative ion mode:

[G-H]- + M ------> [M-H]- + G

These simple proton transfer reactions are true gas-phase Acid-Base processes in the Bronsted-lowrey sense.

A"tight" ion source (pressure=0.1-2 torr) is used to maximize collisions which results in increasing sensitivity. To take place these ion molecule reactions must be exothermic.

Proton transfer is one of the simple processes observed in positive CI:

RH+ + M -----> MH+ + R

One of the decisive parameter in this reaction is the proton affinity. For the reaction to occur, the proton affinity of the molecule M must be higher that the one of the gas molecule.

The main reagent gases used in CI are: Ammonia, Methane, and Isobutane. Choice of reagent gas affect the extend of fragmentation of the quasi-molecular ion.

Positive chemical ionization:

Methane:

CH4 + e -----> CH4+. + 2e ------> CH3+ + H.

CH4+. + CH4 -----> CH5+ CH3.

CH4+. + CH4 -----> C2H5+ + H2 + H.

Isobutane:

i-C4H10 + e -----> i-C4H10+. + 2e

i-C4H10+. + i-C4H10 ------> i-C4H9+ + C4H9 +H2

Ammonia:

NH3 + e -----> NH3+. + 2e

NH3+. + NH3 ------> NH4+ + NH2.

NH4+ + NH3 --------->N2H7+

In methane positive ion mode CI the relevant peak observed are MH+, [M+CH5]+, and [M+C2H5]+; but mainly MH+

In isbutane positive ion mode CI the main peak observed is MH+.

In ammonia positive ion mode CI the main peaks observed are MH+ and [M+NH4]+.

Choice of reagent gas:

Two factors determine the choice of the gas to be used:

Proton affinity PA

Energy transfer

NH3 (ammonia) is the most used reagent gas in CI because of the low energy transfer of NH4+ compare to CH5+ for example. With NH3 as reagent gas, usually MH+ and MNH4+ (17 mass units difference) are observed.

Negative Ion Chemical Ionization:

Three mechanisms can be underlined:

1- Electron capture reaction due to attainment of slow moving, low energy "thermalized" electrons which may be transfered more efficiently to sample molecules.

2- Electron transfer from ionized reagent gas (e.g. NH2- may transfer an electron to a molecule having a greater electron affinity than NH2).

2- Reagent gas ions participate in true CI reactions (e.g. proton abstraction, according to relative acidities).

Molecular ions observed in negative ion chemical ionization mass spectra are usually M- or [M-H]-

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