Matrix-Assisted Laser Desorption/Ionization (MALDI) is a soft ionization technique used in mass spectrometry, allowing, among others, to ionise biomolecules like proteins, peptides and sugarchains which tend to be more fragile and quickly lose structure when ionised by more convential ionisation methods.

The ionization is triggered by a laser beam (normally a nitrogen-laser). A matrix is used to protect the biomolecule from being destroyed by direct laserbeam.

The matrix consists of crystallized biomolecules, of which 3 are most common: 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid ), a-cyano-4-hydroxycinnamic acid (alpha-matrix) or 2,5-dihydroxybenzoic acid (DHB). A solution of one of these molecules is made, in a mixture of highly purified water and another organic compound (normally acetonitrille (ACN) or ethanol). Normally some trifluoracetate (TFA) is also added. A good example of a matrix-solution would be 20 mg/mL sinapinic acid in ACN:water:TFA (50:50:0.1).

The matrix-solution is then brought onto the protein-sample which you wish to investigate. The organic compound ACN allows for the hydrophobic proteins in the sample to dissolve into the solution, while the water allows for water-soluable proteins to do the same. The solvents vaporize, leaving only the recrystallized matrix, but now with proteins spread troughout the crystals.

The laser is then fired onto the crystals, which get ionized. They transfer part of their charges to the proteins, thus ionizing the proteins but protecting them from the full force of the laser.

The proteins are then ready to be inserted in a mass spectrometer. A common type of a MALDI spectrometer is the MALDI-TOF (Time-of-flight).

Koichi Tanaka received 1/4 of the 2002 Nobel Prize in Chemistry for Macromolecule Ionization by Laser Irradiation, however, Matrix-Assisted Laser Desorption/Ionization (MALDI) was created by M. Karas and F. Hillenkamp.