MALDI mass spectrometry neutral matrix

Various molecular and quasi-molecular ions can be formed under MALDI conditions. The formation of protonated analyte (A) molecules, [A -F H]+, is generally most important at least for samples containing slightly basic centres, such as the peptides and proteins, MALDI mass spectrometry of which is known to be most facile and reproducible. Therefore, proton transfer from the electronically excited, neutral or ionized, or protonated matrix species is considered to be crucial in the overall MALDI process . Notably, proton transfer can occur already in the condensed phase, followed by desorption of the preformed ions . However, the generation of the [A -F H]+ ions is believed to take place preferably in the so-called plume , that is, in the energized, short-hved and relatively dense vapour phase generated above the solid matrix upon excitation by the laser pulse. The actual proton donor species (be it one or several) in a given case is still a matter of... 

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.

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. 

Suzuki,H.,Yamagaki,T.,Tachibana,K.(2005) Optimization of matrix and amount of ammonium chloride additive for effective ionization of neutral oligosaccharides as chloride ion adducts in negative-mode MALDI-TOF mass spectrometry. Journal of the Mass Spectrometry Society of Japan, 53,227-229. 

Laser desorption (ionisation) - mass spectrometry (LDMS or one-step LDI-MS) experiments may be manipulated by varying (i) sample presentation (ii) power density, wavelength and pulse duration of the laser desorption technique to ablate the sample and (Hi) mass analyser type combined with a suitable ionisation mode. At variance to MALDI experiments in LDI-MS no matrix is required and no special sample preparation of any kind is necessary. The sample can be subjected to the pulsed desorption laser in a variety of ways. Most commonly, the sample is presented as a solid layer, deposited from solution, on a substrate, which absorbs at the laser wavelength. There is a well-defined threshold, above which successful desorption of neutrals can... 

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