Matrix-assisted laser desorption/ionization crystallization

Matrix-assisted laser desorption ionization (MALDI) A method used for the ionization of high-molecular-weight compounds. In this approach, the analyte is crystallized with a solid matrix and then bombarded with a laser of a frequency which is absorbed by the matrix material. 

Figure 2.9. Schematic of a matrix-assisted laser desorption/ionization (MALDI) event. The SEM micrograph depicts sinapinic acid-equine myoglobin crystal from a sample prepared according to the dried drop sample preparation method. In the desorption event neutral matrix molecules (M), positive matrix ions (M+), negative matrix ions (M-), neutral analyte molecules (N), positive analyte ions (+), and negative analyte ions (-) are created and/or transferred to the gas phase. Reprinted from A. Westman-Brinkmalm and G. Brinkmalm (2002). In Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, J. Silberring and R. Ekman (eds.) New York John Wiley Sons, 47-105. With permission of John Wiley Sons, Inc.

S. Berkenkamp, M. Karas, and F. Hillenkamp. Ice as a Matrix for IR-Matrix-Assisted Laser Desorption/Ionization Mass Spectra from a Protein Single Crystal. Proc. Natl. Acad. Sci. USA, 93(1996) 7003-7007. 

Major methods for introducing proteins and other macromolecules into mass spectrometers are electrospray and matrix-assisted laser desorption/ionization (MALDI).18-27 Most often, MALDI is used with a time-of-flight mass spectrometer, which can measure mlz up to 106. Typically, 1 p,L of a 10 jxM solution of analyte is mixed with 1 p,L of a 1-100 mM solution of an ultraviolet-absorbing compound such as 2,5-dihydroxybenzoic acid (the matrix) directly on a probe that fits into the source of the spectrometer. Evaporation of the liquid leaves an intimate mixture of fine crystals of matrix plus analyte. 

Matrix-assisted laser desorption/ionization (MALDI) On applying a high-energy laser beam to a biomolecule (co-crystallized in a matrix), it rapidly turns into a gas and ionizes. 

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.

With a larger orifice (3.75 x 5.64 A) on the surface of the Cgo derivative 31, quantitative encapsulation of H2 molecule was accomplished (at 200°C, 800 atm) (Fig. 13) (54). The encapsulated H2 molecule exhibited a sharp singlet at —7.25 ppm in the NMR spectrum (o-dichlorobenzene-d4). It was also detected by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry (MS). The activation energy for the H2 escape of 34kcal mol was experimentally obtained. Spectacularly, a single H2 molecule encapsulated inside 31 was recently observed by a single-crystal X-ray diffraction analysis (55). 

Moreover, the typical tools of supramolecular chemistry, such as NMR spectrometry, require concentrations usually in excess of 10 " mol/1. and other favorite methods such as mass spectroscopy [fast-atom bombardment (FAB), electrospray ionization (ESI), matrix-assisted laser desorption ionization (MALDI)], and vapor-pressure os-moinetry do not directly provide information about supramolecular behavior in solution. The most favorite method, x-ray analysis, suffers from the limitation posed by the ultimate requirement of being able to grow single crystals. While this is. in numerous instances, possible in the case of pure molecular entities, supramolecules, being mixed molecular objects by nature, are usually difficult to grow in the form of a single crystal. 

A variation of laser-based ionization that merits separate discussion is matrix-assisted laser desorption ionization (MALDI). The sample is placed on a crystal matrix. 

Unlike FAB/LSIMS, matrix-assisted laser desorption ionization (MALDI) uses a crystalline, rather than liquid, matrix, and a beam of photons, rather than atoms or ions. The net result is a dramatic increase in both sensitivity and mass range of compounds that may be analyzed. The sample is dissolved in a matrix and is allowed to crystallize on a stainless-steel target. The target is then inserted into the mass spectrometer and the surface bombarded with a pulsed laser beam. Molecules are desorbed from the surface and ionize, usually by protonation or deprotonation. Any fragment or multiply charged ions are generally of low abundance in this ionization mode. The pulsed nature of the laser excitation renders this technique compatible with TOF, and the combined technique enjoys an almost limitless mass range. 

Homeffer, V. Dreisewerd, K. Lude-mann, H.-C. Hillenkamp, F. Lage, M. Stmpat, K. Is the Incorporation of Analytes into Matrix Crystals a Prerequisite for Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry A Study of Hve Positional Isomers of Di-hydroxyhenzoic Acid. Int. J. Mass Spectrom. 1999,185/186/187,859-870. 

The matrix-assisted effect refers to enhanced ionization efficiency of molecules that is observed when they are desorbed with another molecule in the mixture that acts as a primary chromophore as compared to their direct laser desorption ionization efficiency as a pure compound. Thus, in a matrix-assisted laser desorption/ionization (MALDI) experiment, one must always add the matrix to enhance the ionization efficiency of the analytes. Good matrices generally have high absorption coefficients for the laser wavelength of interest and are usually acidic to be able to donate a proton (positive ion mode) in the plume. It is also often desirable for a matrix to form large, flat crystals upon evaporation of solvent. This latter requirement is particularly true for time-of-flight mass analyzers because the resolving power is dependent on the flatness of the sample surface. 

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