Matrix assisted laser desorption spectrometry

Until about the 1990s, visible light played little intrinsic part in the development of mainstream mass spectrometry for analysis, but, more recently, lasers have become very important as ionization and ablation sources, particularly for polar organic substances (matrix-assisted laser desorption ionization, MALDI) and intractable solids (isotope analysis), respectively. 

The three techniques — laser desorption ionization, laser ablation with secondary ionization, and matrix-assisted laser desorption — are all used for mass spectrometry of a wide variety of substances from rock, ceramics, and bone to proteins, peptides, and oligonucleotides. 

Laser-desorption mass spectrometry (LDMS) or matrix-assisted laser desorption ionization (MALDI) coupled to a time-of-flight analyzer produces protonated or deprotonated molecular ion clusters for peptides and proteins up to masses of several thousand. 

MALDI = matrix assisted laser desorption, ftms = Fourier transform mass spectrometry TOF = time of flight. 

With the identities and amounts of amino acids known, the peptide is sequenced to find out in what order the amino acids are linked together. Much peptide sequencing is now done by mass spectrometry, using either electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) linked to a time-of-flight (TOF) mass analyzer, as described in Section 12.4. Also in common use is a chemical method of peptide sequencing called the Edman degradation. 

MALDI (Section 12.4) Matrix-assisted laser desorption ionization a mild method for ionizing a molecule so that fragmentation is minimized during mass spectrometry. 

Matrix-assisted Laser Desorption/lonization Mass Spectrometry... 

Matrix-assisted laser desorption mass spectrometry (MALDI-MS) is, after electrospray ionization (ESI), the second most commonly used method for ionization of biomolecules in mass spectrometry. Samples are mixed with a UV-absorbing matrix substance and are air-dried on a metal target. Ionization and desorption of intact molecular ions are performed using a UV laser pulse. 

Tandem mass spectrometry (MS/MS) is a method for obtaining sequence and structural information by measurement of the mass-to-charge ratios of ionized molecules before and after dissociation reactions within a mass spectrometer which consists essentially of two mass spectrometers in tandem. In the first step, precursor ions are selected for further fragmentation by energy impact and interaction with a collision gas. The generated product ions can be analyzed by a second scan step. MS/MS measurements of peptides can be performed using electrospray or matrix-assisted laser desorption/ionization in combination with triple quadruple, ion trap, quadrupole-TOF (time-of-flight), TOF-TOF or ion cyclotron resonance MS. Tandem... 

Two relatively new techniques, matrix assisted laser desorption ionization-lime of flight mass spectrometry (MALDI-TOF) and electrospray ionization (FS1), offer new possibilities for analysis of polymers with molecular weights in the tens of thousands. PS molecular weights as high as 1.5 million have been determined by MALDI-TOF. Recent reviews on the application of these techniques to synthetic polymers include those by Ilantoif54 and Nielen.555 The methods have been much used to provide evidence for initiation and termination mechanisms in various forms of living and controlled radical polymerization.550 Some examples of the application of MALDI-TOF and ESI in end group determination are provided in Table 3.12. The table is not intended to be a comprehensive survey. 

The molecular weights and molecular weight distributions (MWD) of phenolic oligomers have been evaluated using gel permeation chromatography (GPC),23,24 NMR spectroscopy,25 vapor pressure osmometry (VPO),26 intrinsic viscosity,27 and more recently matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS).28... 

Maleamic acid, cyclization of, 293 Maleic anhydride, 59 Maleimido azine, 307 Manganese diacetate catalysts, 71 Mark-Houwink-Sakurada equation, 57 Material safety data sheets (MSDSs), 246 Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), 385, 388 McGrath, J. E., 327 MDI isomers, 210 MDIs. See Methylene diphenyl diisocyanates (MDIs)... 

Matrix-assisted laser desorption ionization post-source-decay mass spectrometry... 

The unseparated digest mixture was studied directly by mass spectrometry using matrix-assisted laser desorption ionization (MALDI) and this showed six of the polypeptides detected by LC-MS and three of the expected polypeptides that had not been detected by LC-MS. In contrast, MALDI did not show three polypeptides observed by LC-MS. 

Anke, G. H., Thomas, M., Frank, G., Reiner, S., Helge, B., Markus, K., and Monika, K. (2008). Challenging the problem of clostridial identification with matrix-assisted laser desorption and ionization-time-of-fight mass spectrometry (MALDl-TOF MS). Anaerobe 14, 242-249. 

Liquid chromatography/mass spectrometry Lower limit of detection Limit of detection Limit of quantitation Florseshoe crab hemocyanin Liquid scintillation counting Matrix-assisted laser desorption/ ionization mass spectrometry m -Maleimidobenzoy 1-A -Hydroxysuccinimide 1 -Cyclohexyl-3-(2-Morptiolino-ethyl)carbodiimide rnetlio-/ -Toluenesulfonate (same as CDI)... 

Whittal, R.M., Russon, L.M., and Li, L., Development of liquid chromatogra-phy-mass spectrometry using continuous-flow matrix-assisted laser desorption ionization time-of-flight mass spectrometry, /. Chromatogr. A, 794, 367, 1998. 

Choudhary, G., Chakel, J., Hancock, W., Torres-Duarte, A., McMahon, G., and Wainer, I., Investigation of the potential of capillary electrophoresis with offline matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for clinical analysis examination of a glycoprotein factor associated with cancer cachexia, Anal. Chem. 71, 855, 1999. 

Fast atom bombardment (FAB) Plasma desorption (PD) Liquid secondary-ion mass spectrometry (LSIMS) Thermospray (TSP)/plasmaspray (PSP) Electrohydrodynamic ionisation (EHI) Multiphoton ionisation (MPI) Atmospheric pressure chemical ionisation (APCI) Electrospray ionisation (ESI) Ion spray (ISP) Matrix-assisted laser desorption/ionisation (MALDI) Atmospheric pressure photoionisation (APPI) Triple quadrupole (QQQ) Four sector (EBEB) Hybrid (EBQQ) Hybrid (EB-ToF, Q-ToF) Tandem ToF-ToF Photomultiplier... 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

Wigge, P. A., Jensen, O. N., Holmes, S., Soues, S., Mann, M., and Kilmartin, J. V. (1998). Analysis of the Saccharomyces spindle pole by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. J. Cell Biol. 141, 967-977. 

---