Types of Ions Observed in Mass Spectrometry
Ionisation in mass spectrometry can be a complex event with the
resulting mass spectrum being made up of several different types of
ions. In some cases, different ionisation processes compete with each
other. Changing the solution and source conditions (potentials, laser
power, solvents etc.) can sometimes enhance some types of ions and
even lead to totally new ions being observed. This page is a summary
of some of the types of ions formed by the most important ionisation
methods to help with mass spectra interpretation.
Electron Ionisation (EI)
Electron ionisation (EI) occurs by interaction of a beam of high
energy electrons with the vaporised analyte. EI is a high energy
process with considerable energy being retained by the ions after
ionisation leading to spectra with a considerable degree of
fragmentation. Fragmentation can also occur prior to ionisation for
thermally labile analytes which dissociate as they vaporise. For the
analyte molecule [AB], the following ions can be observed:
- [AB]+° - radical molecular ion. Formed by loss of an
electron.
- [AB]+ - molecular cation. Formed by oxidation (for
metal complexes for example).
- [A]+ - cation. Formed be cation/anion separation from
salts.
- [AB]+H+ - protonated molecule. Sometimes termed 'Self
CI'. Often observed for primary amines and alcohols with high
proton affinities. Can also be due to too high analyte pressure.
- A+, B+ - fragment ions. Can be due to
thermal decomposition, spontaneous fragmentation at ionisation or
secondary fragmentation of the energised radical molecular ion.
Chemical Ionisation (CI)
Chemical Ionisation (CI) occurs by interaction of the analyte vapour
with the protonated reagent gas, which in turn is created through EI
of the reagent gas followed by ion/molecule interactions. For the
analyte molecule [AB], the following ions can be observed:
- [AB]+H+ - protonated molecule. Formed by protonation
from the protonated reagent gas to the analyte.
- [AB-N]+ - neutral loss fragment ion. Formed by
elimination of the simple neutral N (e.g. H2O, MeOH,
EtOH, MeCN etc.).
- A+, B+ - fragment ions. Can be due to
thermal decomposition or secondary fragmentation of the protonated
analyte. Fragmentation processes observed are usually lower energy
than with EI.
Electrospray Ionisation (ESI)
Electrospray Ionisation (ESI) is far more complex than most people
think with various types of low energy ionisation events occurring in
competition. Cation transfer from the solvent to the analyte during
solvent evaporation is usually the most abundant. ESI is very low
energy and so very little fragmentation is observed. For the analyte
molecule [AB], the following ions can be observed:
- [AB]+H+ - protonated molecule. Formed by proton
exchange to the analyte.
- [AB]+X+ - cationated molecule. Formed by cation
addition to the analyte when the analyte has a high cation
affinity. Cations can be Na+, K+ NH4+
etc.
- [AB]+° - radical molecular ion. Formed by loss of an
electron for an analyte that has a low ionisation potential and
poor proton/cation affinity. For example highly conjugated
hydrocarbons.
- [AB]+ - molecular cation. Formed by oxidation (for
metal complexes).
- [A]+ - cation. Formed be cation/anion separation from
salts.
- [AB-N]+ - neutral loss fragment ion. Formed by
elimination of the simple neutral N (e.g. H2O, MeOH,
EtOH, MeCN etc.).
- A+, B+ - fragment ions. Can be due to
spontaneous fragmentation of the analyte - usually low intensity.
- [AB]+Xnn+ - multiply charged ion. Formed
by multiple cation addition to the analyte. Cations can be H+,
Na+, K+ NH4+ etc. or a
combination of these.
- [A(n+1)Bn]+ - cation/anion salt
cluster ion.
- [AB]n+X+ - cationated analyte cluster.
Usually indicative of too high analyte concentration.
- [AB-H+X]+H+ - a very specific reaction for certain
acids where the salt is formed in solution (with X cation) proir
to ionisation and then protonated at ionisation.
Of course, negative ions can also formed when you switch the
potentials on the ion source:
- [AB]-H+ - deprotonated molecule. Formed by proton
loss from the analyte.
- [AB]- - molecular anion. Formed by reduction (for
metal complexes).
- [B]- - anion. Formed be cation/anion separation from
salts.
- [AB-N]- - neutral loss fragment ion. Formed by
elimination of the simple neutral N (e.g. H2O, MeOH,
EtOH, MeCN etc.). Less likely in negative mode.
- A-, B- - fragment ions. Can be due to
spontaneous fragmentation of the analyte - usually low intensity.
- [AnB(n+1)]- - cation/anion salt
cluster ion.
Matrix-assisted Laser
Desorption/Ionisation (MALDI)
Matrix-assisted Laser Desorption/Ionisation (MALDI) occurs as a low
energy process through ionisation of a UV active matrix compound by a
laser. The activated matrix then passes on the charge to the analyte
through a series of low energy ion/molecule reactions. For the analyte
[AB], the types of ions observed are similar to those observed in ESI:
- [AB]+H+ - protonated molecule. Formed by proton
exchange to the analyte.
- [AB]+X+ - cationated molecule. Formed by cation
addition to the analyte during volatilisation when the analyte has
a high cation affinity. Cations can be Na+, K+
NH4+ etc.
- [AB]+° - radical molecular ion. Formed by loss of an
electron for an analyte that has a low ionisation potential and
poor proton/cation affinity. For example highly conjugated
molecules.
- [AB]+ - molecular cation. Formed by oxidation (for
metal complexes).
- [A]+ - cation. Formed be cation/anion separation from
salts.
- [AB-N]+ - neutral loss fragment ion. Formed by
elimination of the simple neutral N (e.g. H2O, MeOH,
EtOH, MeCN etc.).
- [AB]+X22+ - doubly charged ion. Formed by
addition of 2 cations to the analyte during volatilisation.
Cations can be H+, Na+, K+ NH4+
etc. Very rarely see higher charge states.
- [A(n+1)Bn]+ - cation/anion salt
cluster ion.
- [AB]n+X+ - cationated analyte (aggregated)
cluster. Usually indicative of too high analyte concentration or
too low matrix concentration.
Negative ions can also be observed in much the same way as with ESI.