desorptionionisation

Desorption ionisation, or desorption/ionisation, is a mass spectrometry process in which analyte molecules are desorbed from a surface and converted into gas-phase ions for detection. The desorption and ionisation steps are often coupled, occurring in a single operation in many techniques.

The best-known example is matrix-assisted laser desorption/ionisation (MALDI). In MALDI, a light-absorbing matrix co-crystallizes with the analyte and transfers laser energy to the sample, enabling gentle desorption and ionisation with minimal fragmentation. This approach is particularly suited to large biomolecules such as proteins, peptides, and complex lipids.

Desorption electrospray ionisation (DESI) is another prominent method. DESI directs a spray of charged solvent droplets at a surface; impact promotes desorption and ionisation in the spray plume under ambient conditions, enabling rapid analysis with minimal sample preparation.

Other variants include laser desorption/ionisation without a matrix (LDI) and surface-assisted laser desorption/ionisation (SALDI). Ionisation in these methods often yields intact molecular ions or adducts such as [M+H]+ or [M+Na]+, supporting analysis of a broad range of compounds, including polymers and small molecules. Instrumentation typically involves mass analysers such as time-of-flight, Orbitrap, or FT-ICR, which affect resolution, mass accuracy, and sensitivity.

Applications of desorption ionisation span proteomics, metabolomics, imaging mass spectrometry of tissues, surface analysis, and materials characterization. Limitations can include matrix-related background signals in MALDI, ion suppression effects, and trade-offs between spatial resolution and sensitivity in imaging techniques. The development of MALDI in the 1980s and DESI in the early 2000s has significantly shaped modern soft ionisation mass spectrometry.