This page provides information on the types of ionisation methods available for mass spectrometery, along with information to help select the appropriate technique.

Ionisation methods for mass spectrometry are broadly classified into soft and hard methods. Soft ionisation methods usually result in entire molecules being ionised and detected, which provides mass information for the entire molecule of interest. Hard ionisation methods result in molecular fragmentation.

Electrospray ionisation (ESI)

Electrospray ionisation is the most commonly used method for mass spectrometry analysis in the department. It is quick and easy to run, with positive and negative modes available.

Positive mode usually results in the molecule of interest being ionised as a protonated (M+H⁺), sodiated (M+Na⁺) or potassiated (M+K⁺) species. Peaks in the resulting spectrum therefore correspond to the molecular mass plus 1 for H, 23 for Na or 39 for K.

ESI positive is suitable for molecules which can be readily ionised by these methods, which means molecules containing heteroatoms such as O, N or S which are easily ionised.

Negative mode...

ESI negative is suitable for molecules which

For both modes the molecular ion adducts will show isotope patterns which can be useful for identification. However the intensity of observed peaks relates to their ease of ionisation, and does not allow for any quantification of components and is not a measure of purity. ESI can only confirm the presence of a particular mass formula.

Atmospheric Pressure Chemical Ionisation (APCI)

APCI is an alternative ionisation technique which is routinely available and may be more suitable for some samples.

Liquid injection field desorption/ionization (LIFDI)

LIFDI

Matrix-Assisted Laser Desorption/Ionization (MALDI)

MALDI is used to analyze large, fragile biomolecules such as proteins and peptides or for polymers.

Field Ionization (FI)

FI

Electron ionisation (EI)

Electron ionisation involves the sample being ionised into the gas phase and the molecules colliding with electrons which results in the formation of positive ions through the fragmentation of the molecule, with these fragments being detected.

Please note: EI samples are recorded using a GC-EI instrument. This limits the recording of EI mass spectrometry to samples which are sufficiently volatile for Gas Chromatography to be viable. Samples should still be submitted using the Mass Spec only option on the form, unless the chromatography is also required (see information for tandem methods).

The choice of ionisation method depends on the properties of the sample being analysed and the information which is desired about a compound. 

Confirmation of identity for a pure compound

Where confirmation of the expected identity for a purified compound is sought, electrospray ionisation (ESI) should generally be the first choice of technique. ESI will result in a molecular ion peak, including isotope patterns, along with a high resolution mass. The high resolution mass result will be compared to the submitted formula and will indicate if this matches. ESI positive is suitable for a very wide array of molecules, except for organic compounds without heteroatoms, eg alkanes, in which case ### may be more suitable.

Analysis of pure compounds of unknown identity

Analysis of mixtures