Chemical Ionization

This technique uses an ion source similar to that of EI except that it is designed to work at higher ion source pressures (0.5-1 Torr). This is achieved by introducing a reaction gas directly into the ion source volume in addition to the sample, and by using a stronger turbo molecular vacuum pump on the ion source housing to maintain adequate vacuum in the mass analyzer. The cation radicals formed after the collision of neutral reagent-gas molecules with high energy electrons do not survive for a long time and react further with neutral reagent gas to produce a set of ions that are nonreactive with the reagent gas itself but can undergo ion-molecule reactions with a sample.

Figure. Chemical ionization (closed) and electron ionization (open) ion volumes

Examples of processes that usually occur in the CI plasma.

Formation of reagent ions:

R + e → R+• + 2e

R+• + R → RH+ + (R-H)•,        or (R-H)+ + RH•

Possible reactions of reagent ions with the molecules of the sample:

RH+ + M → MH+ + R (protonation)

(R-H)+ + M → MH+ + (R-2H)  (protonation)

(R-H)+ + M → (M-H)+ + R     (hydride abstraction)

RH+ + M → M-H+-R,    or (R-H)+ + M → M-(R-H)+  (adduct formation)

R+• + M → M+• + R      (charge exchange)

   These reactions can be partially controlled by the source temperature, the pressure of the reagent gas, and the proton affinities of the reagent gas and sample. Most gaseous or volatile compounds may be used as reagent gases for CI mass spectrometry (inert gases, N2, CO2, CS2, benzene as charge exchange reagents, acetone, acetonitrile, methanol, water, methane, isobutane, ammonia, CH2Cl2, etc. for protonation or adduct formation). For practical reasons, methane, isobutane, and ammonia are the most commonly used reagent gases for confirming the molecular weight of compounds.

   The dominant reagent ions in the plasma of methane are CH5+ and C2H5+:

2CH4 + 2e- → CH4•+ + CH3+ + H• + 4e-

CH4•+ + CH4 → CH5+ + CH3•

CH3+ + CH4 → C2H5+ + H2

CH5+ + CH4 → no products

C2H5+ + CH4 → no products

   The proton affinity (PA) of methane is 129.9 kcal/mol, lower than that of most other organic compounds, therefore CH5+ is a highly reactive ion, and CI-methane mass spectra usually have fragmented character.

   The dominant reactant ions in high pressure of isobutane are t-C4H9+ and i-C3H7+ at m/z 57 and 43 respectively. The t-C4H9+ is a weaker Bronsted acid than CH5+ or C2H5+ (PA of isobutene is 191.7 kcal/mol) and also a weaker Lewis acid than C2H5+; consequently, i-C4H10 CI mass spectra contain fewer fragment ions than CH4 CI mass spectra. If the CI-isobutane mass spectra of the compounds contain only MH+ ions and there is no other information about the class of compound involved then additional information can be obtained from the MS/MS fragmentation spectrum of MH+ ion.   Methane/ammonia mixtures give NH4+ as the dominant reagent ion that will react only with basic or polar samples to give MH+ or MNH4+ ions and almost no fragmentation.

Thermochemical data of some reagent gasses and organic compounds (taken from NIST):