Desorption mechanism controversy

An interpretation of the mechanism of analyte retention on the furnace surface has been one of the most controversial problems in ET A AS for years. Since the first studies in this field in the 1970s, two different mechanisms have been suggested. According to the first one, the adsorption/desorption mechanism, it is supposed that the anal de is distributed on the furnace surface in the form of a monolayer of free atoms or molecules, retained on the surface by means of physical or chemical adsorption forces. According to the second one, the condensation/evaporation mechanism, the sample is distributed in a form of solid microparticles retaining all the thermochemical characteristics of the original (bulk) substance. 

The work of Leger, Jura, and Omont (1985) clearly differs from earlier, more pessimistic conclusions concerning desorption. If correct, its implications are that a significant fraction of molecules will continue to exist in the gas phase during the cloud lifetime, and that some molecules formed on grain surfaces can desorb into the gas. Thus, the existing controversy between the proponents of gas phase and grain surface mechanisms for molecule synthesis remains unresolved by this work. 

For hydrogenation, the reaction mechanism is still controversial largely due to discrepancies between theoretical and experimental desorption barriers. Although some alternative mechanisms have been proposed, more studies are needed. 

The mechanism of the catalyzed shift reaction for both copper- and iron-based catalysts remains controversial. Two types of mechanism have been proposed adsorptive and regenerative. In the former, the reactants adsorb on the catalyst surface, where they react to form surface intermediates such as formates, followed by decomposition to products and desorption from the surface. In the regenerative mechanism, on the other hand, the surface undergoes successive oxidation and reduction cycles by water and carbon monoxide, respectively to form the corresponding hydrogen and carbon dioxide products of the WGS reaction. 

Although elucidation of ion production is possibly the most controversial and elusive area of LDI mass spectrometry, the mechanism can be subdivided into absorption, retention, radiation coupling and transfer, desorption, and ionization reactions. Optical properties (TJV-absorption, anti-reflective properties) as well as thermal conductivity are very important features to take into account for the design and synthesis of efficient LDI surfaces displaying high surface area/volume ratios. It was reported that pore size and depth, surface porosity and roughness, could affect LDI efficiencies (Wei etal. 1999 Shen et al. 2001 Lewis etal. 2003 Shenar etal. 2008 Xiao etal. 2009 Piretetal. 2010, 2012 Chen et al. 2011 Dupre et al. 2012). 

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