Matrix-assisted Laser Desorption/ionization
Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique used in mass spectrometry, allowing the analysis of biomolecules (biopolymers such as DNA, proteins, peptides and sugars) and large organic molecules (such as polymers, dendrimers and other macromolecules), which tend to be fragile and fragment when ionized by more conventional ionization methods. It is similar in character to electrospray ionization both in relative softness and the ions produced (although it causes many fewer multiply charged ions).
The MALDI is a two step process. First, desorption is triggered by a UV laser beam. Matrix material heavily absorbs UV laser light, leading to the ablation of upper layer (~micron) of the matrix material. A hot plume produced during the ablation contains many species: neutral and ionized matrix molecules, protonated and deprotonated matrix molecules, matrix clusters and nanodroplets. The second step is ionization (more accurately protonation or deprotonation). Protonation (deprotonation) of analyte molecules takes place in the hot plume. Some of the ablated species participate in protonation (deprotonation) of analyte molecules. The mechanism of MALDI is still debated.
Read more about Matrix-assisted Laser Desorption/ionization: Matrix, Laser, Ionization Mechanism, Atmospheric Pressure Matrix-assisted Laser Desorption/ionization, Mass Spectrometer, History, Reproducibility and Performance