What is ionisation?

Electron Ionisation (EI)

Gas phase molecules are bombarded with a stream of electrons to produce a radical cation. This is a harsh ionisation technique.
M (g) + e -> M⁺. (g) + 2e
The molecular ion (MI) can undergo fragmentation
M⁺. -> F⁺ + R.
M⁺. -> F⁺. + nm
The Fragment ions can also undergo fragmentation
F⁺. -> F₁⁺ + R.
F⁺ -> F₁⁺ + nm

Chemical Ionisation (CI)

A reagent gas is used and is bombarded with a stream of electrons to produce a radical cation.
RG (g) + e -> RG⁺. (g) + 2e
The reagent gas MI, e.g. ammonia, undergoes collision with neutral reagent gas
NH₃⁺. + NH₃ -> NH₄⁺ + NH₂.
Proton transfer takes place between the protonated ammonia (in excess) and the analyte M
NH₄⁺ + M -> MH + NH₃
With ammonia the adduct ions can also be formed
NH₄⁺ + M -> MNH₄⁺

CI is a softer ionisation technique than EI, some fragmentation may occur.

Note:( MNH₄⁺ - H₂O) has the same nominal mass as M.
Rarely do you see radical cations in CI spectra; unless there is a problem with the gas supply or ion source. Only use CI if you do not observe the MI by EI.

Electrospray Ionisation (ESI)

This is a very mild, solution based ionisation process. It relies on the analyte having either a basic site or acidic site for ionisation. The eluent is sprayed at a high electropotential.
M (sol) -> (M + nH)_n⁺
In the case of singly charged ions signals are observe for the quasimolecular ion(s)(M + X)⁺ where X = H, NH₄, Na, K

For larger molecules then multiple charging can take place. Note that within the multiple charge envelope the charge states will be incremental, i.e. vary by 1 charge. The true mass of the molecule can be obtained by solving the simultaneous equations or by using one of the many deconvolution packages available with the instruments.

Atmospheric Pressure Chemical Ionisation (APCI)

Very similar to ES, except that the spray is neutral and passes through a corona discharge. Singly charged protonated species are produced. Ionisation is not so dependent on the functionality of the analyte, though some functionality is required.

Matrix Assisted Laser Desorption Ionisation (MALDI)

The sample is mixed with a UV absorbing matrix, the latter in great excess. Singly charged QMIs are produced, hence the need for a large mass range analyser. Good for involatile small molecules, peptides, biopolymers and polymers. Used in conjunction with TOFs to give large mass range. Uses pmols of material.