The matrix plays a dual role of energy absorption and proton transfer in MALDI: it absorbs the laser to prevent direct photolysis of the analyte and transfers energy to surrounding molecules to promote desorption and ionization. Spraying needs to be uniform and finely crystalline to avoid local over-thickness causing diffusion or masking fine distributions.
Common processes include pneumatic hand/automatic spraying, sublimation deposition, and microdroplet arrays; parameters (matrix type, concentration, solvent, temperature, and humidity) need to be optimized for the tissue and analyte, and improper spraying will introduce background or cause molecular migration.
The matrix itself produces strong background in the low m/z region and may mask small-molecule and metabolite signals, and spraying is an additional sample-preparation step. The matrix-free route avoids this through special sources: LDPI/DPI work in a matrix-free environment through photoionization; matrix-free MALDI reduces matrix through special target surfaces or sublimation processes; DESI itself does not rely on MALDI matrix.
The direct benefit of matrix-free is lower background in the small-molecule region and shorter sample preparation; the cost is that the signal gain mechanism differs for some molecules, requiring verification of coverage for the analyte.
The matrix often produces cluster ions and adduct peaks in the m/z < 700 region, interfering with small-molecule imaging such as drugs and metabolites, i.e., the small-molecule matrix interference problem. Matrix-free or optimized matrices (such as formulations that reduce low-m/z background) are the main countermeasures.
For projects with strong small-molecule imaging needs, prioritize matrix-free sources (LDPI, DPI) or validated low-background matrix schemes to preserve effective signals in the small-molecule region.
If the analyte is mainly proteins, peptides, and lipids and sample preparation is accepted, matrix-spraying MALDI is a mature and stable solution; if focusing on small molecules, metabolites, or requiring the shortest sample preparation, matrix-free sources (LDPI/DPI) or matrix-free MALDI are more suitable.
Regardless of the strategy, sample-preparation consistency determines imaging reproducibility; it is recommended to establish a standard spraying or matrix-free sampling SOP and connect it with the registration workflow of subsequent analysis software (SCiLS Lab, MetaboScape).
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).