A counter ion source such as trifluoroacetic acid (TFA) is usually added to generate the ions. A solution of one of these molecules is made, often in a mixture of highly purified water and an organic solvent such as acetonitrile (ACN) or ethanol. The matrix consists of crystallized molecules, of which the three most commonly used are sinapinic acid, α-cyano-4-hydroxycinnamic acid (α-CHCA, alpha-cyano or alpha-matrix) and 2,5-dihydroxybenzoic acid (DHB). Peptides, nucleotides, oligonucleotides, oligosaccharidesģ,5-dimethoxy-4-hydroxycinnamic acid Ĥ-hydroxy-3-methoxycinnamic acid Matrix UV MALDI matrix list CompoundĢ,5-dihydroxy benzoic acid (gentisic acid) Īcetonitrile, water, methanol, acetone, chloroform Today, mostly organic matrices are used for MALDI mass spectrometry. The availability of small and relatively inexpensive nitrogen lasers operating at 337 nm wavelength and the first commercial instruments introduced in the early 1990s brought MALDI to an increasing number of researchers. Further improvements were realized through the use of a 355 nm laser and the cinnamic acid derivatives ferulic acid, caffeic acid and sinapinic acid as the matrix. Karas and Hillenkamp were subsequently able to ionize the 67 kDa protein albumin using a nicotinic acid matrix and a 266 nm laser. Tanaka received one-quarter of the 2002 Nobel Prize in Chemistry for demonstrating that, with the proper combination of laser wavelength and matrix, a protein can be ionized. Using this laser and matrix combination, Tanaka was able to ionize biomolecules as large as the 34,472 Da protein carboxypeptidase-A.
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The breakthrough for large molecule laser desorption ionization came in 1987 when Koichi Tanaka of Shimadzu Corporation and his co-workers used what they called the "ultra fine metal plus liquid matrix method" that combined 30 nm cobalt particles in glycerol with a 337 nm nitrogen laser for ionization. Peptides up to the 2843 Da peptide melittin could be ionized when mixed with this kind of "matrix". The tryptophan was absorbing the laser energy and helping to ionize the non-absorbing alanine. These researchers found that the amino acid alanine could be ionized more easily if it was mixed with the amino acid tryptophan and irradiated with a pulsed 266 nm laser.
The term matrix-assisted laser desorption ionization (MALDI) was coined in 1985 by Franz Hillenkamp, Michael Karas and their colleagues.
Main article: History of mass spectrometry Finally, the analyte molecules are ionized by being protonated or deprotonated in the hot plume of ablated gases, and then they can be accelerated into whichever mass spectrometer is used to analyse them. Second, a pulsed laser irradiates the sample, triggering ablation and desorption of the sample and matrix material. First, the sample is mixed with a suitable matrix material and applied to a metal plate. MALDI methodology is a three-step process. It is similar in character to electrospray ionization (ESI) in that both techniques are relatively soft (low fragmentation) ways of obtaining ions of large molecules in the gas phase, though MALDI typically produces far fewer multi-charged ions. It has been applied to the analysis of biomolecules ( biopolymers such as DNA, proteins, peptides and carbohydrates) and various organic molecules (such as polymers, dendrimers and other macromolecules), which tend to be fragile and fragment when ionized by more conventional ionization methods. In mass spectrometry, matrix-assisted laser desorption/ionization ( MALDI) is an ionization technique that uses a laser energy absorbing matrix to create ions from large molecules with minimal fragmentation.
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