Ionization, metal-assisted

Unlike the insertion of 2-monosubstituted alkenyl carbenoids (69, 70, and 73), the reaction of 2,2-disubstituted alkenyl carbenoids with alkenyl zirconocene chlorides afforded the expected products as a mixture of stereoisomers. Thus, when 77, derived from the deprotonation of the stereodefined E-l-chloro-2-methyl-l-octene 76, was reacted with -l-hexenylzirconocene chloride 78 at low temperature, a partial inversion of configuration at the alkenyl carbenoid center occurred before or during the rearrangement to afford the expected metalated diene 79 with an E Z isomeric ratio of 58 42 after hydrolysis (see 80, Scheme 27) [53]. The poor stereocontrol was attributed to the metal-assisted ionization [58-60], which promotes the interconversion of the E- to the Z-alkenyl carbenoids 77. The latter occurs at a rate comparable with that of the insertion into organozirconocene chloride, and hence this is responsible for the loss of stereochemistry. 

Walborsky created the term metal-assisted ionization (MAI) for reactions where a carbenoid acts as an electrophile. An example appears in Scheme 1 <1993JOC546>. The r-butyl-substituted product is formed with inversion of configuration, supporting the proposed mechanism. While the presence of a three-membered ring in the intermediate is hypothetical, the need to replace the Cl with a Li dictates an interaction between the two. 

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 variations of conventional SIMS have evolved. In one, known as matrix-enhanced SIMS, the MALDI sample preparation protocol is used to coat the sample surface with an organic acid (e.g., 2,5-dihydroxybenzoic acid) or other suitable matrix material to improve ionization efficiency [24,25], This approach has been used to image brain tissue samples from freshwater snails (Lymnaea stagnalis). Another variation employs a coating of a thin layer of gold or other metals to enhance analytical signals [26], This method, known as metal-assisted SIMS, provides images with improved spatial and chemical resolution. 

Chen WY, Huang JT, Cheng YC, Chien CC, Tsao CW (2011) Fabrication of nanostructured silicon by metal-assisted etching and its elTects on matrix-free laser desorption/ionization mass spectrometry. Anal Chim Acta 687 97-104... 

There are some variants that have emerged in the wake of DESI. By replacing the electrospray emitter by a metal needle and allowing solvent vapor into the coaxial gas flow desorption APCI (DAPCI) can be performed [106], Other versions are atmospheric-pressure solids analysis probe (ASAP) where a heated gas jet desorbs the analyte, which is subsequently ionized by a corona discharge [107], and electrospray-assisted laser desorption/ionization (ELDI) where a laser ablates the analyte and charged droplets from an electrospray postionizes the desorbed neutrals [108],... 

Matrix-assisted laser desorption/ionization (MALDI) was developed by Karas, Hillenkamp, and coworkers in the late 1980s [161-163]. At the same time, a related technique was introduced by Tanaka et al. and involved mixing of the analyte with a very finely ground metal powder [164],... 

Mass spectrometry is also used for nanoparticles investigations. Two ionization techniques often used with liquid and solid biological samples include electrospray ionization (ESI) and matrix-assisted laser desorption/ioiuzation (MALDI). Inductively coupled plasma (ICP) sources are mainly used for metal analysis. In general,... 

The development of matrix-assisted laser desorption ionization (MAEDI) has advanced the entire field of mass spectrometry. To use this ionization method, the sample is mixed into a matrix that absorbs the laser wavelength extremely well (approximately 10,000 1 matrix analyte) and the mixture is placed on a solid substrate. Absorption of the laser causes the matrix to explode, ejecting the intact, nonvolatile molecules of interest into the gas phase. Proton exchange or alkali metal attachment occurs in the gas plume and the ionized species can be detected. 

Keki, S., Deak, G., and Zsuga, M., Fragmentation study of rutin, a naturally occurring flavone glycoside cationized with different alkali metal ions, using post-source decay matrix-assisted laser desorption/ionization mass spectrometry, J. Mass Spectrom., 36, 1312, 2001. 

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