Irradiation-matrix-assisted laser

Figure 6 The SELDI technology. This type of proteomic analytical tool is a class of mass spectroscopy instrument that is useful in high-throughput proteomic fingerprinting of serum. Using a robotic sample dispenser, 1 p,L of serum is applied to the surface of a protein-binding chip. A subset of the proteins in the sample binds to the surface of the chip. The bound proteins are treated with a matrix-assisted laser desorption and ionization matrix and are washed and dried. The chip, which contains multiple patient samples, is inserted into a vacuum chamber where it is irradiated with a laser. The laser desorbs the adherent proteins and causes them to be launched as ions. The TOF of the ion before detection by an electrode is a measure of the mass-to-charge (m/z) value of the ion. The ion spectra can be analyzed by computer-assisted tools that classify a subset of the spectra by characteristic patterns of relative intensity (adapted from www.evmsdoctors.com).

Another example of a Pc-based 1-D polymer is that reported by Armstrong and co-workers [158], They prepared a Pc with eight styrene-type polymerizable sites at the end of the peripheral substituents. This molecule forms highly ordered, rod-like aggregates at the air-water interface that can be transferred onto solid supports. Irradiation of the thin films affords polymerization between the olefin moieties of adjacent molecules by photostimulated [2 + 2] cycloaddition. The rod-like Pc macromolecules were conveniently studied by matrix-assisted laser desorp-tion/ionization (MALDI-TOF) spectrometry and atomic force microscopy (ATM), the latter showing rods with lengths up to 290 nm. 

These direct ion sources exist under two types liquid-phase ion sources and solid-state ion sources. In liquid-phase ion sources the analyte is in solution. This solution is introduced, by nebulization, as droplets into the source where ions are produced at atmospheric pressure and focused into the mass spectrometer through some vacuum pumping stages. Electrospray, atmospheric pressure chemical ionization and atmospheric pressure photoionization sources correspond to this type. In solid-state ion sources, the analyte is in an involatile deposit. It is obtained by various preparation methods which frequently involve the introduction of a matrix that can be either a solid or a viscous fluid. This deposit is then irradiated by energetic particles or photons that desorb ions near the surface of the deposit. These ions can be extracted by an electric field and focused towards the analyser. Matrix-assisted laser desorption, secondary ion mass spectrometry, plasma desorption and field desorption sources all use this strategy to produce ions. Fast atom bombardment uses an involatile liquid matrix. 

Hettich, R.L. and Stemmier, E.A. (1996) Investigation of oligonucleotide fragmentation with matrix-assisted laser desorption ionization Fourier-transform mass spectrometry and sustained off-resonance irradiation. Rapid Commun. Mass Spectrom., 10 (3), 321-7. 

In 1988, Karas and HiUenkamp embedded proteins in a large molar excess of a UV-absorbing crystal matrix and irradiated the sample with a laser beam at suitable wavelengths (Karas and HiUenkamp, 1988). In this process of matrix-assisted laser desorphon/ionization (MALDI), large proteins can be transferred into the gas phase as intact molecules and become, for the most part, singly protonated. Since MALDI rehes on the use ofa pulsed laser, mass analysis is usuaUy performed... 

Figure 7-6 A generic view of the process of matrix-assisted laser desorption Ionization. Co-crystallized matrix and analyte molecules are irradiated with a UV laser.The laser vaporizes the matrix, producing a plume of matrix ions, analyte ions, and neutrals. Gas-phase ions are directed into a mass analyzer.

Matrix-assisted laser desorption ionization has simultaneously been developed by Karas and Hillenkamp16 in Germany and by Tanaka et al.11 in Japan in 1985. With this technique molecules are ionized via laser irradiation of the sample and with the help of other small organic molecules, called the matrix. The matrix strongly absorbs the light of the laser and transfers it together with a charge, mostly a proton, to the analytes. Thereby, analytes reach the gas phase as ions that are ultimately analyzed by the mass spectrometer. 

In MALDl (matrix-assisted laser desorption/ionization), the ionization process occurs in two steps an initial primary ionization followed by a secondary reaction [28]. During primary ionization, the ions are formed after the sample has absorbed the energy from the laser beam, and then, upon continuing laser beam irradiation, the analyte undergoes secondary neutralization reactions with free electrons until they become singly charged. Meanwhile, neutral analyte molecules evaporate and are charged by secondary protonation reaction. In this way, they can be detected. 

An absolute method for molecular weight determination is matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) (Kona et al., 2005 Creel, 1993 Nielsen, 1999 Cho et al., 2001). The sample is dispersed in a UV-absorbing matrix (e.g., trans -cinnamic acid or 2,5-dihydroxybennzoic acid). Irradiation with a UV laser induces evaporation of ionized polymer chains, which are then detected using TOF. The technique requires relatively narrow MWD samples. Alternative ionization methods have been employed, such as electrospray ionization mass spectrometry (ESI-MS), which may have advantages for certain polymer end groups (Vana et al., 2002). IFFF and MALDI-TOF can be coupled to analyze polydisperse samples and polymer mixtures (Kassalainen and Williams, 2003). 

The mass spectrometer (MS) is an analytical tool that provides information about sample composition based on the mass-to-charge ratio (m/z). In order to analyze biological samples by MS, the relevant analytes must be driven into the gas phase and ionized (charged). Two techniques are currently the most popular for the ionization of biological analytes, ESI (electrospray ionization) and MALDl (matrix-assisted laser desorp-tion/ionization). In ESI, an electric field is applied to a solution of analyte to form a spray of charged droplets. Subsequent solvent evaporation and ion release enables analysis by MS. In MALDl, the analyte is co-crystallized with a solid matrix to form a dry spot on a surface. Under vacuum, the sample is then irradiated with a laser, which desorbs the sample from the surface and ionizes it. The developers of these... 

Laser desorption generates high mass ions by irradiating a solid with a pulsed laser beam. In the case of matrix-assisted laser desorption, a solid mixture of an analyte is dissolved in a suitable matrix compound and the laser pulse desorbs and indirectly ionizes the analyte molecules. The selection of appropriate MALDI matrix, cationization salt, sample concentration, and sample preparation technique are critical success factors for obtaining a reliable mass spectrum that influence the polymer distribution. An alternative technique used to eliminate matrix and cationization salt effects is direct LDI, which is generally used for polymer samples with relatively low molecular weights. 

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