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Matrix-Assisted Laser Desorption Ionization or MALDI is a soft ionisation technique used in mass spectrometry, allowing, amongst others, to ionise biomolecules (biopolymers like proteins, peptides, sugarchains,...) who tend to be more fragile and quickly lose structure when ionised by more convential ionisation methods.
The principle is simple. The ionization is triggered by a laserbeam (normally a nitrogen-laser), the problem is, really, that a direct laserbeam on the biomolecule (for instance, a protein) would destroy it. That is where the matrix comes in play.
The matrix consists out 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 also some trifluoracetate (TFA) is added.
A good example of a matrix-solution would be: 20mg/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 vaporizes, leaving only the recrystallized matrix, but now with proteins spread troughout the crystals.
The laser is then fired onto the crystals. These will take the full blast themselves, getting ionized, they will however transfer part of their charges to the proteins, thus ionizing the proteins, but still 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 his development of the foundations of the method.
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