Maldi

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  • Topic: Mass spectrometry, Matrix-assisted laser desorption/ionization, Ion source
  • Pages : 7 (2263 words )
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  • Published : January 3, 2013
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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, peptidesand 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. Contents  [hide]  * 1 Matrix * 2 Laser * 3 Ionization mechanism * 4 Atmospheric pressure matrix-assisted laser desorption/ionization * 5 Mass spectrometer * 6 History * 7 Use * 7.1 Biochemistry * 7.2 Organic chemistry * 7.3 Polymer chemistry * 7.4 Microbiology * 8 Reproducibility and performance * 9 See also * 10 References * 11 Bibliography * 12 External links| -------------------------------------------------

[edit]Matrix
UV MALDI Matrix List|
Compound| Other Names| Solvent| Wavelength (nm)| Applications| 2,5-dihydroxy benzoic acid[1]| DHB,Gentisic acid| acetonitrile,water,methanol,acetone,chloroform| 337, 355, 266| peptides,nucleotides,oligonucleotides,oligosaccharides| 3,5-dimethoxy-4-hydroxycinnamic acid[2][3]| sinapic acid;sinapinic acid; SA| acetonitrile, water, acetone, chloroform| 337, 355, 266| peptides, proteins, lipids| 4-hydroxy-3-methoxycinnamic acid[2][3]| ferulic acid| acetonitrile, water,propanol| 337, 355, 266| proteins| α-cyano-4-hydroxycinnamic acid[4]| CHCA| acetonitrile, water,ethanol, acetone| 337, 355| peptides, lipids, nucleotides| Picolinic acid[5]| PA| Ethanol| 266| oligonucleotides| 3-hydroxy picolinic acid[6]| HPA| Ethanol| 337, 355| oligonucleotides| The matrix consists of crystallized molecules, of which the three most commonly used are 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid), α-cyano-4-hydroxycinnamic acid (alpha-cyano or alpha-matrix) and2,5-dihydroxybenzoic acid (DHB). A solution of one of these molecules is made, often in a mixture of highly purified water and an organic solvent(normally acetonitrile (ACN) or ethanol). Trifluoroacetic acid (TFA) may also be added. A good example of a matrix-solution would be 20 mg/mLsinapinic acid in ACN:water:TFA (50:50:0.1).

Notation for cinnamic acid substitutions.
The identification of suitable matrix compounds is determined to some extent by trial and error, but they are based on some specific molecular design considerations: * They are of a fairly low molecular weight (to allow facile vaporization), but are large enough (with a low enough vapor pressure) not to evaporate during sample preparation or while standing in the spectrometer. * They are often acidic, therefore act as a proton source to encourage ionization of the analyte. Basic matrices have also been reported.[7] * They have a strong optical absorption in either the UV or IR range,[8]so that they rapidly and efficiently absorb the laser irradiation. This efficiency is commonly associated with chemical structures incorporating several conjugated double bonds, as seen in the structure of cinnamic acid. * They are functionalized with polar groups, allowing their use in aqueous...
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