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.

Negative mode

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.