Laser Desorption (LDPI) and 355 nm Laser Ionisation

Laser desorption is a process in which a pulsed laser is directed at the surface of a sample, causing surface molecules to absorb energy, detach from the matrix and enter the gas phase. If the laser energy is precisely calibrated for ‘desorption’ rather than ‘fragmentation’, the molecules are ejected in a neutral state and subsequently converted into ions during a subsequent ionisation step; this is known as the ‘desorption + ionisation’ stepwise route. MSI LDPI employs ‘transmission laser desorption + post-photochemical ionisation’: a 355 nm pulsed laser (frequency 1000 Hz) illuminates tissue sections from the rear, causing neutral molecules such as lipids and metabolites at the cellular scale to desorb, after which they are captured by photochemical ionisation in the ambient pressure region.

Unlike MALDI, LDPI does not rely on matrix spraying; consequently, there is no masking of signals in the low-mass region (e.g. m/z < 200) by matrix peaks. The minimum detectable mass-to-charge ratio is as low as approximately 70 (covering molecular weights of approximately 70–1000), and the sample preparation step involving matrix spraying is eliminated. Its high spatial resolution (2–3 μm, optimised to 4 μm in hippocampal tissue) stems from the fine focusing of the laser spot, enabling the visualisation of molecular distribution at the single-cell scale. This approach is described on the official website as a “world first”, advancing mass spectrometry imaging from the tissue scale to the cellular scale.

Why is a ‘step-by-step’ approach significant? MALDI combines ‘desorption’ and ‘ionisation’ within the matrix crystal in a single step; whilst the matrix is necessary, it introduces background peaks and a low-mass blind spot; LDPI physically separates desorption (laser) from ionisation (photochemical). Desorption is solely responsible for ‘extracting’ molecules from the sample, whilst ionisation is solely responsible for ‘activating’ neutral molecules into ions; the two processes do not interfere with one another, thereby preserving the low-mass region whilst maintaining non-polar discrimination. Subsequent photochemical ionisation acts on both polar and non-polar molecules, which is precisely why LDPI can simultaneously detect histamine, adenosine, choline, adenine, vitamin D₃, cholesterol, glutamic acid, threonine, guanine, ceramides and various lipids (fatty acids, PE, PA, MAG, DG, HexCer, GalCer, etc.) at a resolution of 2–3 μm.

MSI DPI and MSI LDPI are compatible with Agilent, AB SCIEX and Thermo mass spectrometers, enabling the upgrade of existing mass spectrometers into imaging platforms capable of cell-scale resolution.