Mass spectrometry, Energy-dependent

Ton-molecule reactions are of great interest and importance in all areas of kinetics where ions are involved in the chemistry of the system. Astrophysics, aeronomy, plasmas, and radiation chemistry are examples of such systems in which ion chemistry plays a dominant role. Mass spectrometry provides the technique of choice for studying ion-neutral reactions, and the phenomena of ion-molecule reactions are of great intrinsic interest to mass spectrometry. However, equal emphasis is deservedly placed on measuring reaction rates for application to other systems. Furthermore, the energy dependence of ion-molecule reaction rates is of fundamental importance in assessing the validity of current theories of ion-molecule reaction rates. Both the practical problem of deducing rate parameters valid for other systems and the desire to provide input to theoretical studies of ion-molecule reactions have served as stimuli for the present work. 

The Use of High Pressure Mass Spectrometry to Determine the Energy Dependence of Ion-Molecule Reaction Rates... 

Crawford, E., Dyson, P., Forest, O., Kwok, S. Mclndoe, J. S. Energy-dependent Electrospray Ionisation Mass Spectrometry of Carbonyl Clusters. 

MS/MS Duty Cycle Typical MS/MS analysis is a serial process, relying on the selection of precursors (peptides) in MS mode, followed by high-energy fragmentation in MS/MS. This process is termed data dependent acquisition (DDA). The duty cycle for the completion of MS and MS/MS cycles (the time necessary for MS/MS spectrum acquisition) is of primary importance. When the separation performance is viewed from the mass spectrometry perspective, the peak capacity can be characterized by the number of MS/MS scans, yielding successful... 

The equilibrium concentration of the ions A- and B- participating in the equlibrium can be directly observed by mass spectrometry. Thus, the free-energy change can be derived from the equilibrium constant, since the concentrations of the neutral species are known in advance. Similarly, by measuring the temperature dependence of the equilibrium constants, the associated enthalpy and entropy can be obtained from van t Hoff plots. By measuring a series of interconnecting equlibria, an appropriate scale can be established. The primary standard in such work has frequently been SO2 whose electron affinity is well established by electron photodetachment36. 

Note Although the discussion of common fragmentation pathways of organic ions is embedded here in the context of EI mass spectrometry, their occurrence is not restricted to this technique. The reactions of isolated gaseous ions do not directly depend on the ionization method, but are almost exclusively governed by intrinsic properties of the respective ion and by its internal energy (Chap. 2). 

Harrison, A.G. Lin, M.S. Stereochemical Applications of Mass Spectrometry. 3. Energy Dependence of the Fragmentation of Stereoisomeric Methylcyclohexanols. Org. Mass Spectrom. 1984,19, 67-71. 

Excitation of acetone with two photons at X = 307 nm delivers 186 kcal/mol of energy, more than enough to break both C—C bonds and give carbon monoxide and two methyl radicals. Following the process with femtosecond mass spectrometry shows that the excited state of acetone of 58 amu rises and falls very quickly, in a spike-like fashion. It is formed and decays with a time constant of 50 fs. For acetone-iig, a similar time dependence is seen The rise and fall of the 64 amu excited species take place in 80 and 80 fs (Fig. 20.5). 

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