Matrix-assisted laser desorption/ionization
From Freepedia
Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique used in mass spectrometry, allowing, among others, to ionize biomolecules (biopolymers like proteins, peptides and sugars) which tend to be more fragile and quickly lose structure when ionized by more conventional ionization methods. It is most similar in character to electrospray ionization both in relative softness and ions produced.
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 laser beam.
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Matrix
The matrix consists of crystallized molecules, of which the three most commonly used are 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid), <math>\alpha</math>-cyano-4-hydroxycinnamic acid (alpha-cyano or alpha-matrix) and 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 acetonitrile (ACN) or ethanol). Normally some trifluoroacetate (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 mixed with the analyte molecule (e.g. 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-soluble (hydrophilic) proteins to do the same. This solution is spotted onto a MALDI plate (usually a metal plate designed for this purpose). The solvents vaporize, leaving only the recrystallized matrix, but now with proteins spread throughout the crystals. The matrix and the analyte are said to be co-crystalized in a MALDI spot.
Laser
The laser is fired at the crystals in the MALDI spot. The spot absorbs the laser energy and it is thought that primarily the matrix is ionized by this event. The matrix is then thought to transfer part of their charge to the analyte (e.g. a protein), thus ionizing them while still protecting them from the disruptive energy of the laser. Ions observed after this process are quasimolecular ions that are ionized by the addition of a proton to [M+H]+, or other cation such as sodium ion [M+Na]+, or the removal of a proton [M-H]- for example. MALDI generally produces singly-charged ions, but doubly-charged ions such as [M+2H]2+ have been observed as well. Note that these are all even-electron species. Ion signals of radical cations can be observed eg. in case of matrix molecules and other stable molecules.
Mass spectrometer
The proteins are then ready to be extracted into a mass spectrometer. A common type of a MALDI spectrometer is the MALDI-TOF, with TOF standing for Time-of-flight. Usually the ionized proteins in MALDI are not measured by deflection, but by the amount of time it takes for them to traverse a certain distance. The ionized particles are accelerated using a voltage which translates into to a certain kinetic energy (and can even be subjected to a velocity discriminator). Then they traverse a tube and the amount of time from acceleration to detection is recorded. This can then be used to calculate velocity, and thus mass to charge ratio of ions.
In Time-of-flight mass spectrometry the mass accuracy can be enhanced by increasing the length of the tube for a given detection system. Common TOF-MALDI instruments are equipped with a "reflectron" which acts as a "ion mirror", deflecting molecular ions within an electric field at the end of the tube, thus nearly doubling the traveling distance and increasing precision.
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 invented by M. Karas and F. Hillenkamp.
