CHEMICAL IONIZATION

Chemical Ionization (CI) is especially useful technique when molecular ion is not observed in EI mass spectrum, and also in the case of confirming the mass to charge ratio of the molecular ion.

Chemical ionization involves ion-molecular chemical interactions between the sample molecules and a reagent gas like CH₄ , NH₃ , i-C₄H₁₀.  The pressure of the chamber is maintained at 0.1 – 1 torr.

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, A reagent gas is ionized by electron impact ionization in the source with energy up to 200-500 eV to give ionized reagent gas molecules.

CH₄ + e^- -----> CH₄^+. + 2e^- ------> CH₃⁺ + H^.   (electron impact)

i-C₄H₁₀ + e^- -----> i-C₄H₁₀^+. + 2e^-            (electron impact)

NH₃ + e^- -----> NH₃^+. + 2e^-               (electron impact)

Primary reagent ionization is followed by second order process in which the primary ion reacts with additional reagent gas molecules to produce a stabilized reagent ion.

Methane:

CH₄^+. + CH₄ -----> CH₅⁺ +CH₃^.       (secondary ions)

CH₄^+. + CH₄ -----> C₂H₅⁺ + H₂ + H^.     (secondary ions)

Isobutane:

i-C₄H₁₀^+. + i-C₄H₁₀ ------> i-C₄H₉⁺ + C₄H₉ +H₂   (secondary ions)

Ammonia:

NH₃^+. + NH₃ ------> NH₄⁺ + NH₂^.    (secondary ions)

NH₄⁺ + NH₃ ---------> N₂H₇⁺     (secondary ions)

 

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

i.e.,

Methane:

CH₅⁺  + M    ------à  [M+H]⁺ +CH₄

CH₅⁺  + M    ------à  [M+ CH₅]⁺

C₂H₅⁺ + M    ------à  [M+ C₂H₅]⁺

Isobutane:

i-C₄H₉⁺ + M ----à [M+H]⁺ + C₄H₈

Ammonia:

NH₄⁺ + M ---à [M+H]⁺ +NH₃

NH₄⁺ + M ---à [M+NH₄]⁺

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.

Choice of reagent gas:

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

  Proton affinity PA

  Energy transfer
 

 Choice of reagent gas affect the extend of fragmentation of the

quasi-molecular ion.

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

In isobutane 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]⁺.

  Thus molecular information is obtained from protonation of sample molecules and the observed m/z values is one unit greater that that of the molecular ion is known as quasi molecular ion.

The internal energy of MH⁺ produced from CH₅⁺ , i-C₄H₉⁺ , NH₄⁺ is in the order

CH₅⁺ > i-C₄H₉⁺ > NH₄⁺

Therefore 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.