Irradiation-matrix-assisted laser desorption ionization

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. 

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). 

In addition to gel electrophoresis, proteins can also be characterized using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Developed by Karas etal. (1987), this system uses a laser to irradiate intact cells, which ionizes surface cell proteins that are then characterized. The patterns of proteins yield a profile unique to the microorganism and are therefore used for identification purposes. MALDI-TOF-MS offers the advantage of rapid results with minimal sample preparation and reagents. However, the cost of equipment makes these methods impractical for wineries. 

Matrix-assisted laser desorption ionization (MALDI) In MALDI, the polymer is mixed intensively (usually from a solution) with a matrix, and possibly with other additives such as a salt. It is then deposited as a small spot on a target (MALDI plate). The deposited mixture is irradiated by a laser pulse. The matrix is selected such that it strongly adsorbs the laser light, heats up very rapidly, and induces the transfer of a single charge (usually an adduct ion) to the polymer molecule. The... 

Also of interest are models from closely related fields, such as Matrix Assisted Laser Desorption/Ionization (MALDI) and Electro Spray Ionization Mass Spectrometry (ESI-MS). MALDI in particular attracts attention as this technique records the large molecular ion emissions resulting from the localized deposition of energy occurring on laser irradiation of the solid s surface. Indeed, there appear to be many similarities in the recorded emissions from SIMS and MALDI. Examples of areas of interest include processes described within the Cluster-based mechanisms and the pooling mechanism. These along with their applicability to SIMS are discussed in Section 3.3.4.2. 

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).

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. 

Matrix-Assisted Laser Desorption/lonization (MALDI) is one of two soft ionization techniques used to introduce biological samples into a mass spectrometer. In MALDI, the analyte is co-crystaUized 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 MALDI (along with the developer of ESI) were awarded the 2002 Nobel Prize in Chemistry. 

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