Ionization methods, mass unimolecular

The progress of the mass spectrometry of organozinc compounds has paralleled the development of new MS techniques. The first complexes studied by MS should have been volatile. However, the development of ionization methods such as fast atom bombardment, electrospray ionization and laser ionization has allowed the analysis of non-volatile, high molecular weight and ionic complexes. The evolution of methods of studying unimolecular... 

Mass spectrometry concerns the dynamics of unimolecular ionic reactions. Given that an ion has no memory of its mode of formation, the method of ionization is incidental and the ion s reactivity depends upon its own energy state. Experimental conditions are such as to minimise the occurrence of ion—molecule reactions [497] and their effects can usually be neglected. Mass spectrometry is a molecular beam experiment in the sense that each ion is an isolated system. The assembly of ions is not at a temperature, although in limited circumstances it may be possible to speak of their rotational temperature, translational temperature and perhaps even vibrational temperature. The familiar mass spectrum identifies the reaction products, but provides little other information about the reaction dynamics. This purist s view of mass spectrometry colours this article. 

The basic NR mass spectrum contains information on the fraction of undissociated (survivor) ions and also allows one to identify dissociation products that are formed by purely unimolecular reactions. NRMS thus provides information on the intrinsic properties of isolated transient molecules that are not affected by interactions with solvent, matrix, surfaces, trace impurities, radical quenchers, etc. However, because collisional ionization is accompanied by ion excitation and dissociation, the products of neutral and post-reionization dissociations overlap in the NR mass spectra. Several methods have been developed to distinguish neutral and ion dissociations and to characterize further short lived neutral intermediates in the fast beam. Moreover, collisionally activated dissociation (CAD) spectra have been used to characterize the ions produced by collisional reionization of transient neutral intermediates [51]. This NR-CAD analysis adds another dimension to the characterization of neutral intermediates, because it allows one to uncover isomerizations that do not result in a change of mass and thus are not apparent from NR mass spectra alone. 

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