The primary step (laser desorption, laser ablation, or electrospray desorption) sends molecules into the gas phase, but many exist as neutrals; post-photoionization uses suitable photon energy to directly photoionize or resonantly ionize these neutral molecules, converting them into detectable ions. The two steps are separated in time and space, allowing separate optimization.
Photon-energy matching is key: too low fails to ionize, too high introduces fragments. Thus post-photoionization often needs wavelength and flux chosen for molecule classes, balancing signal gain against fragmentation.
Laser-induced post-ionization is the concrete form of post-photoionization implemented with a second laser, the core gain mechanism of MALDI-2 and the principle basis of LDPI/DPI matrix-free photochemical ionization. The two are collectively called the post-photoionization family.
The difference is application context: MALDI-2 is stacked on the matrix-desorption system; LDPI/DPI use photochemical steps in matrix-free ambient conditions to raise ionization, skipping matrix spraying.
The most direct value of post-photoionization is expanding detectable molecule types and raising signal-to-noise of low-abundance species, especially significant in lipid and metabolite imaging; it does not change pixel resolution and can be stacked on high-resolution imaging sources.
The cost is extra optical parameters and method-development expense; improper settings amplify background. Post-photoionization is generally a complementary acquisition mode.
For existing MALDI platforms, a MALDI-2 module expands weak-signal coverage at low retrofit cost; for building a new matrix-free ambient imaging line, LDPI/DPI directly apply the post-photoionization idea and skip matrix. The two divide labor by platform and preparation acceptance.
From the standpoint of molecular applicability, neutral, low-polarity, or hard-to-ionize molecules benefit most; molecules that are strongly polar and readily ionized gain limited benefit.
To obtain detailed specifications, compatible models, or a quotation for the MSI LDPI / DPI full series imaging ion sources, visit the Neo-Source official website, or contact the official team for compatibility advice tailored to your mass spectrometer (Agilent / SCIEX / Thermo and other mainstream MS).