Vacuum ultraviolet photoionisation and post-ionisation/secondary photoionisation: providing desorbed neutral molecules with a ‘second chance at ionisation’
2026-07-10 113
Photoionisation is a physical process in which a photon interacts with an atom or molecule, causing the latter to lose an electron and form an ion. Photoionisation only occurs when the energy of the photon exceeds the ionisation energy of the substance; for photons below the ionisation threshold, the photoionisation cross-section (the probability of ionisation, which can be understood as the ‘effective cross-section’) is close to zero. Vacuum ultraviolet (VUV) photons have an energy of approximately 10 eV, which is sufficient to ionise the vast majority of small organic molecules; they are therefore commonly used as a ‘universal’ ionisation method.
What is post-photoionisation / secondary photoionisation?
In mass spectrometry imaging, although primary ionisation (such as DESI or LDI) offers high desorption efficiency, the proportion of molecules that are actually ionised is extremely low. The research group led by Pan Yang at the University of Science and Technology of China has pointed out that in conventional mass spectrometry imaging, the ionisation efficiency of the sample is less than 1/1000, meaning that the vast majority of desorbed molecules remain in neutral form; simultaneously, the high-salt environment on the surface of biological tissues can also cause ion suppression.
Post-photoionisation (also commonly referred to as secondary photoionisation) involves the following procedure: following primary desorption and ionisation, the desorbed neutral molecules are passed through a beam of vacuum ultraviolet photons, thereby ionising those neutral molecules that were not ionised during the primary stage. As this step involves no direct contact with the sample itself or the salt matrix, it circumvents ion suppression and significantly recovers the signal that would otherwise have been lost.
Authoritative research data
The research group led by Pan Yang and Liu Chengyuan at the University of Science and Technology of China, in collaboration with Professor Tian Changlin, has developed an atmospheric-pressure transmission laser desorption/post-photoionisation (t-AP-LDI/PI) method, which was published as a cover article in *Analytical Chemistry* on 25 March 2024 (DOI: 10.1021/acs.analchem.3c05605). This work achieves a spatial resolution of 4 μm, and post-photoionisation enhances the detection sensitivity of histamine, adenosine, choline, adenine, vitamin D3, cholesterol, glutamic acid and various lipids by 2–3 orders of magnitude. This approach builds upon the research group’s previous work on DESI/PI (Analytical Chemistry 2022, 94, 15108; 2019, 91, 6616).
Relation to Neo Source products
Neo Source INSTRUMENT’s two imaging ion sources—MSI DPI (DESI+PI) and MSI LDPI (LDI+PI)—both incorporate a post-photoionisation step following primary desorption and ionisation to enhance the detection sensitivity of fully polar molecules. The underlying technical principles are derived from the aforementioned work by the University of Science and Technology of China.
References
Wikipedia: Photoionisation. https://en.wikipedia.org/wiki/Photoionization
IUPAC Gold Book: photoionisation. https://goldbook.iupac.org/terms/view/P04620
University of Science and Technology of China: New progress in the development of a compact photoionisation mass spectrometry imaging source. https://kyb.ustc.edu.cn/_t1002/2024/0401/c6076a634490/page.htm
Qi K, et al. Development of Transmission Ambient Pressure Laser Desorption Ionisation/Post-photoionisation Mass Spectrometry Imaging. Analytical Chemistry. 2024. DOI:10.1021/acs.analchem.3c05605