Ion source: the ‘first line of defence’ in mass spectrometry, determining how samples are ionised
2026-07-10 72
An ion source is a device that generates atomic and molecular ions. In mass spectrometry, the sample is ionised within the ion source; the resulting ions are then separated according to their mass-to-charge ratio (m/z) and detected, ultimately producing a spectrum showing the relative abundance of ions as a function of their mass-to-charge ratio (Wikipedia: Ion source). Put simply, the ion source determines ‘how the sample molecules are ionised’ and is the first stage connecting the sample to the mass analyser.
What does the ion source do in mass spectrometry?
The general workflow of mass spectrometry is: sample introduction → ionisation → mass analysis → detection. The ion source is located at the ‘ionisation’ stage, converting neutral atoms or molecules into positively or negatively charged ions, which can then be separated and identified by the mass analyser. Different ionisation methods yield different types of ions (molecular ions, fragment ions, adduct ions, etc.), which directly influence the information that can be obtained. The IUPAC definition of ‘photoionisation’ also notes that neither electrons nor photons ‘collide’ with the molecule; rather, ionisation is triggered through interaction. Consequently, the use of terms such as ‘electron impact’ or ‘photon impact’ is not recommended in mass spectrometry terminology (IUPAC Gold Book: photoionisation, P04620).
Main types of ion sources
Based on the physical mechanism of ionisation, ion sources can be broadly categorised as follows (Wikipedia: Ion source):
Electron ionisation (EI): This involves bombarding gaseous molecules with an electron beam. It was the earliest ionisation method used in organic mass spectrometry, producing a rich spectrum of fragment ions and is suitable for spectral library searches.
Chemical ionisation (CI): Ionisation is achieved through ion–molecule reactions; the energy involved is lower than that of EI, and it typically yields quasi-molecular ions with fewer fragments.
Gas discharge types: These include inductively coupled plasma (ICP), glow discharge and microwave-induced plasma, amongst others, and are commonly used for elemental analysis.
Photoionisation (PI): Photons interact with atoms or molecules to cause them to lose electrons; this includes single-photon ionisation, multi-photon ionisation (MPI), resonance-enhanced multi-photon ionisation (REMPI) and atmospheric pressure photoionisation (APPI), amongst others.
Desorption ionisation: Includes matrix-assisted laser desorption/ionisation (MALDI), laser desorption/ionisation (LDI), secondary ion mass spectrometry (SIMS) and fast atomic bombardment (FAB), amongst others, and is commonly used for macromolecular or surface/imaging analysis.
Spray ionisation: Includes electrospray ionisation (ESI) and atmospheric pressure chemical ionisation (APCI), amongst others, and is suitable for liquid-phase coupling.
Thermal ionisation (TI): Also known as surface ionisation, this involves evaporating neutral atoms onto a hot surface and re-evaporating them in ionic form; it is commonly used for isotope analysis.
Ambient ionisation: Generates ions in an open environment at atmospheric pressure without the need for sample pretreatment; typical examples include DESI and DART.
Ion sources related to mass spectrometry imaging
In mass spectrometry imaging, common ionisation methods are concentrated in the following categories: DESI, MALDI, LDI, SIMS and photoionisation. Neo Source INSTRUMENT’s two imaging ion sources, the MSI DPI (DESI+PI) and MSI LDPI (LDI+PI), are based on the “electrospray desorption + photoionisation” and “laser desorption + photoionisation” approaches respectively, and can be fitted to mass spectrometers from Agilent, AB SCIEX and Thermo to enable mass spectrometry imaging.
References
Wikipedia: Ion source. https://en.wikipedia.org/wiki/Ion_source
IUPAC Gold Book: photoionisation. https://goldbook.iupac.org/terms/view/P04620