MALDI, Matrix assisted laser desorption/ionization spectrometry

Other MS based analytical approaches have occasionally been applied to ancient resin samples, in particular for paint varnishes. Such techniques include FABS (fast atom bombardment mass spectrometry) [35], MALDI (matrix assisted laser desorption-ionization mass spectrometry) and GALDI (graphite assisted laser desorption-ionization mass spectrometry) [36 38]. 

In 1974, Comarisov and Marshall60 developed Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). This technique allows mass spectrometric measurements at ultrahigh mass resolution (R = 100000-1000000), which is higher than that of any other type of mass spectrometer and has the highest mass accuracy at attomole detection limits. FTICR-MS is applied today together with soft ionization techniques, such as nano ESI (electrospray ionization) or MALDI (matrix assisted laser/desorption ionization) sources. 

The only mass spectrometric methods available during the era of the first cascade synthesis in 1978 [30] were electron impact (El) and field desorption (FD) mass spectrometry [31]. Fast atom bombardment (FAB) mass spectrometry is limited to fairly low mass ranges and not very suitable for compounds of low polarity. It was not until the development of new and gentle ionisation methods such as MALDI (matrix-assisted laser desorption ionization) [32] and ESI (electrospray ionization) [33] that the conditions were fulfilled for the start of intense research in the field of dendrimer chemistry. The following section will present the special features of these mass-spectrometric methods and their importance in dendrimer analysis. 

Bis- -diketones have been demonstrated to be powerful ligands in the engineering of supramolecular architectures. Pikramenou and coworkers [53a] used two bis-P-diketones of l,3-bis(3-phenyl-3-oxopropanoyl)benzene (H2L ) and l,3-bis(3-phenyl-3-oxopropanoyl) 5-ethoxy-benzene (H2L ) to synthesize neutral homodimetallic complexes [Ln2(L )3] (Ln = Eu, Nd, Sm, Y, Gd) and [Ln2(L )3] (Ln = Eu, Nd) and also an anionic dinuclear lanthanide complex [Eu2(L )4] . The detailed studies by NMR, electrospray, and MALDI (matrix assisted laser desorption ionization) mass spectrometry, by addition of chiral Pirkle s reagent, revealed that [Ln2(L" )3] and [Eu2(L )4] (see Eigure 2.38) are chiral, and have triple- and quadruple-stranded supramolecular structures, respectively, with the latter being more strongly emissive than the former. 

Mass spectrometry is one physical technique that does not (at least directly) involve electromagnetic radiation. However, some sample desorption and ionization processes do use high intensity pulses of laser light in techniques such as MALDI (Matrix-Assisted Laser Desorption Ionization) that have proved very useful in mass analysis of proteins and other biologic macromolecules. High resolution mass spectrometry derives from atomic/molecular beam studies in which the trajectories of ionized particles in a vacuum can be manipulated by static... 

Then, within a short period, both MALDI (matrix-assisted laser desorption ionization) and electrospray ionization were introduced and found widespread use in biomolecular mass spectrometry. As noted earlier, Koichi Tanaka (MALDI) and John Fenn (electrospray ionization) were awarded the 2002 Nobel Prize in Chemistry for these innovations. Both MALDI and electrospray ionization are considered to be soft ionization techniques that induce little, if any, molecular fragmentation. 

Figure 5. Methods employed for identifiying and characterizing proteins separated by 2DE. Abbreviations used 2DE, two-dimensional electrophoresis ESI, electrospray ionization HPLC, high performance liquid chromatography IR, infrared M, relative molecular mass MALDI, matrix-assisted laser desorption ionization MS, mass spectrometry MS/MS tandem mass spectrometry p/, isoelectric point PSD, post-source decay.

MALDI (matrix-assisted laser desorption ionization) a method for introducing huge molecules in their ionic form into the gas phase prior to mass spectrometry... 

The proof of Equations (5.49) and (5.50) can be found in the references [54,56]. Note, incidentally, that other techniques for measuring the MMD, such as MALDI (matrix-assisted laser desorption ionization mass spectrometry) will have a different relation between the signal and the MMD. Note also that the assumption that the GPC signal is proportitHial to the mass of polymer depends on what sort of detection is used for example, this will not hold if the detector is sensitive just to the end-group of a polymer chain. 

APCI, atmospheric pressure chemical ionization Cl, chemical ionization El, electron impact ESI, electrospray ionization FAB, fast atom bombardment GC, gas chromatography LC, liquid chromatography MALDI, matrix-assisted laser desorption ionization MS, mass spectrometry MS/MS, tandem mass spectrometry PAHs, polycyclic aromatic hydrocarbons PEG, polyethylene glycol PPG, polypropylene glycol. 

MALDI Matrix-assisted laser desorption/ionization mass spectrometry... 

Recently, on-resin mass spectrometry (MS) via the MALDI (matrix-assisted laser desorption/ionization) technique has been developed. This method can be used as a semi-on-line monitoring technique during SPPS (141). An especially elegant approach for on-resin MS is to use the photolabile a-methylphenacyl ester linker (Table 5, Q -bromophenacyl entry), which is cleaved directly upon laser photolysis and MALDI ionization (142,143). 

Mass spectrometry (MS) is combined to TLC through an ESI (electrospray ionization) interface or a MALDI (matrix-assisted laser desorption ionization) interface. In the first case, E SI-M S is carried out using the extracts of the eluted compounds. The spots/zones need to be scraped oflF from the plate before the compounds are extracted. MALDI can be carried out directly on the plate after adding an appropriate MALDI matrix. 

Solid samples can be analyzed by mass spectrometry with techniques providing ionization by desorption (13) such as MALDI (matrix assisted laser desorption ionization) (14) and S-SIMS (static-secondary ion mass spectrometry) (15). Ions are produced by energy deposition on the sample surface. The analysis can be performed at the bead level. Most of all, chemical images can be produced to localize specific compounds on the studied surfaces. 

The use of MALDI (Matrix-Assisted Laser Desorption Ionization) equipped with a time of flight (TOP) spectrometer offers an elegant and effective method for the precise and absolute determination of polymer molar masses. Initially designed for the characterization of biomolecules, MALDI-TOF spectrometry is now extensively applied to synthetic polymers in this technique the polymer to be studied is dispersed in an organic matrix that is volatilized under the effect of a laser radiation whose wavelength is in the absorption range of the matrix. Owing to the specific molecular interactions between polymer and matrix—which plays an important role in the chain desorption—each polymer requires a suitable matrix. For example, 1,8,9-trihydroxyanthracene is well suited to poly(methyl methacrylate). 

Mass Spectrometry MALDI (Matrix-Assisted Laser Desorption/ Ionization) and ESI (Electrospray Ionization)... 

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. 

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. 

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 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. 

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)... 

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. 

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