Desorption mechanism direct methods

Hemoproteins are a broad class of redox-proteins that act as cofactors, e.g. cytochrome c, or as biocatalysts, e.g. peroxidases. Direct ET between peroxidases such as horseradish peroxidase, lactoperoxidase," or chloropcroxidasc"" and electrode surfaces, mainly carbonaceous materials, were extensively studied. The mechanistic aspects related with the immobilized peroxidases on electrode surfaces and their utilization in developing biosensor devices were reviewed in detail. The direct electrical contact of peroxidases with electrodes was attributed to the location of the heme site at the exterior of the protein that yields close contact with the electrode surface even though the biocatalyst is randomly deposited on the electrode. For example, it was reported " that non-oriented randomly deposited horseradish peroxidase on a graphite electrode resulted in 40-50% of the adsorbed biocatalyst in an electrically contacted configuration. For other hemoproteins such as cytochrome c it was found that the surface modification of the electrodes with promoter units such as pyridine units induced the binding of the hemoproteins in an orientation that facilitated direct electron transfer. By this method, the promoter sites induce a binding-ET process-desorption mechanism at the modified electrode. Alternatively, the site-specific covalent attachment of hemoproteins such as cytochrome c resulted in the orientation of the protein on the electrode surfaces and direct ET communication. ... 

The Rollgen s group [9] reports the thermal desorption of [M + Alkah]" quasi-molecular ions from a electrically heated metal smface (wires or ribbons) for so-dimn alkah salts of carboxylic acids and mixtures of alkali halides with a crown ether, glucose and adenosine. With benzo-15-crown-5 the desorption of [M + Na]" ions takes place even below the threshold temperature for thermionic emission of alkah ions. Alkali ion attachment has also been performed by thermal desorption of thermally labile analytes such as saccharides, pharmaceuticals, peptides, steroids and their mixtirres. Bombick and Alhson [8] describe a desorption/ioniza-tion method where samples are deposited directly on thermionic emission materials (lK20 lAl203 2Si02) and heated within the mass spectrometer s source. Ions representative of the sample are formed. The proposed mechanism involves the gas-phase addition of emitted potassium ions to neutrals desorbed from the surface. 

A new ionization method called desorption electrospray ionization (DESI) was described by Cooks and his co-workers in 2004 [86]. This direct probe exposure method based on ESI can be used on samples under ambient conditions with no preparation. The principle is illustrated in Figure 1.36. An ionized stream of solvent that is produced by an ESI source is sprayed on the surface of the analysed sample. The exact mechanism is not yet established, but it seems that the charged droplets and ions of solvent desorb and extract some sample material and bounce to the inlet capillary of an atmospheric pressure interface of a mass spectrometer. The fact is that samples of peptides or proteins produce multiply charged ions, strongly suggesting dissolution of the analyte in the charged droplet. Furthermore, the solution that is sprayed can be selected to optimize the signal or selectively to ionize particular compounds. 

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