DESI (Desorption Electrospray Ionization)

Working Principle

High-voltage electrospray forms charged droplets at the capillary tip, which impact the sample surface at a certain angle at high speed. The droplets spread on the surface, desorb, and carry surface molecules to form second-generation charged droplets (the analyte inside the droplets is ionized), which are sent into the mass analyzer through an atmospheric pressure interface. The entire process occurs at ambient pressure, with the sample placed on a movable ambient platform for point-by-point imaging.

Since ionization occurs at the sample surface rather than internally, DESI is highly tolerant of sample morphology: tissue sections, whole tissues, and even living surfaces can be analyzed directly. This also makes it naturally compatible with the 'in-situ, real-time, no sample preparation' analytical philosophy, and it is an important representative of ambient ionization techniques.

Advantages of Ambient Matrix-Free Operation

The most significant advantage of DESI is that it requires no matrix spraying and no vacuum. This brings threefold benefits: extremely simple sample preparation, significantly reducing labor and batch variation; no matrix peak interference, with a clean background in the low-mass region, favorable for small-molecule drug and metabolite imaging; and the sample can remain near its natural state, supporting in-vivo/in-situ analysis. These characteristics make it quite attractive in clinical rapid pathology, in-vivo analysis, traditional Chinese medicine, and whole-animal distribution studies.

Compared with MALDI, DESI sacrifices some resolution (mostly tens to hundreds of micrometers) and coverage of certain large molecules, but gains simplified workflow and ambient-friendliness in return. The two are not a replacement relationship, but complement each other according to problem scale and molecular type.

Applicable Molecules and Scenarios

DESI responds well to polar/medium-polarity molecules, and is widely used for tissue distribution imaging of metabolites, lipids, drugs, and their metabolites. In drug development, it can quickly present the spatial distribution of candidate drugs within organs and tumors; in metabolomics, it naturally fits the spatial metabolomics workflow.

Limitations and Improvements

The main limitations of DESI include: polarity bias, with weak response to weakly polar/non-polar molecules; ion suppression effects, where signals are easily suppressed by co-eluting species in complex matrices, affecting broad coverage and quantification; and resolution limited by spray spot and moving stage, making it difficult to reach the cellular scale. In response, academia has developed improvements such as nano-DESI (nano-electrospray, near non-destructive, low background), AFADESI (air-flow assisted to improve transmission efficiency and throughput), and post-ionization enhancement (such as the DPI approach).

Neo-Source MSI DPI introduces an electrospray/photochemical ionization dual process on top of DESI, claiming no polarity bias, low ion suppression, and sensitivity 1–4 orders of magnitude higher than traditional DESI, making it an upgrade option for the DESI route in high sensitivity and wide polarity coverage.