Desorption Electrospray Ionisation (DESI): a method of ionisation in which the sample surface is ‘scoured’ by charged droplets

Desorption Electrospray Ionisation (DESI) is an atmospheric-pressure open-system ionisation technique. Its operation can be broken down into three steps: first, a high-speed stream of charged solvent droplets is ejected from the spray needle and strikes the sample surface at a certain angle; the impact ‘wets’ the surface molecules, causing them to desorb into secondary microdroplets; these microdroplets, carrying the analytes, continuously evaporate and shrink as they travel towards the mass spectrometer inlet, resulting in an increase in surface charge density; ultimately, the analytes are converted into gaseous ions via charge transfer and drawn into the mass spectrometer. The entire process takes place at atmospheric pressure in an open environment, with virtually no sample preparation required.

Precisely because of its ‘open-system, atmospheric pressure and no pre-treatment’ characteristics, DESI is particularly well-suited to the direct analysis of solid surfaces: tissue sections, traditional Chinese medicinal materials, plant leaves, fingerprints and even ink marks on documents can all be placed directly on the sample stage for scanning. The applications listed on the Neo Source INSTRUMENT website include imaging of mouse brain and melanocytic nevus tissues, metabolic imaging of plants such as sage, ginkgo, tea leaves and tobacco leaves, as well as chronological analysis of vermilion and ink and fingerprint imaging in forensic science—samples that would often be difficult to analyse directly using ionisation methods requiring vacuum conditions or complex sample preparation.

However, conventional DESI has a fundamental limitation: it relies on charge transfer from charged droplets for ionisation, favouring highly polar and easily charged components whilst exhibiting low ionisation efficiency for weakly polar and non-polar components, resulting in ‘polarity discrimination’; in complex matrices, co-existing components may compete for charge, causing significant ion suppression that suppresses the response of target compounds, often necessitating sample purification beforehand. This means that when using DESI alone, many components—such as lipids, terpenes, flavonoids and glycosides—cannot be fully detected.

New Source’s MSI DPI has been specifically designed to address this shortcoming: by adding a photoionisation system and high-efficiency ion transfer channels downstream of the conventional DESI spray assembly, it utilises vacuum ultraviolet photons to perform photochemical secondary ionisation on neutral molecules that have been desorbed but not subjected to primary ionisation. By switching the photoionisation source, a single instrument can perform both conventional DESI and cover the full spectrum of polar, weakly polar and non-polar molecules. Overall metabolite signals are enhanced by 1–3 orders of magnitude, ion suppression is significantly reduced, and the tissue sections remain intact after imaging, allowing for subsequent H&E histological examination. MSI DPI offers models compatible with three brands (MSI DPI-25A for Agilent, MSI DPI-25S for AB SCIEX, and MSI DPI-25T for Thermo), which are ready to use straight out of the box.