Mass spectrometry, and ion—molecule reactions

Knewstubb, P.F., Mass Spectrometry and Ion-Molecule Reactions, Cambridge University Press, London, 1969. Laeter, J.R. di. Applications of Inorganic Chemistry, Wiley, New York, 2001. 

Organophosphorus Chemistry series regularly lists new mass spectra in the Physical Methods chapter . With this in mind, an approach which considers fundamental aspects of organophosphorus ions (i.e. structure and reactivity) in the gas phase has been adopted. The gas-phase structure and reactivity of ions can be probed via several different techniques, including thermochemical measurements, kinetic energy release of metastable ions, collisional activation mass spectrometry, neutralization reionization mass spectrometry and ion-molecule reactions. An example is the molecule HCP (Table 1) its ionization potentiaP, proton affinity and the IR and rotational spectroscopy of the HCP ion " have all been determined in the gas phase. Another important tool for understanding the structure and reactivity of gas phase ions is ab initio molecular orbital theory. With advances in computational hardware and software, it is now possible to carry out high-level ab initio calculations on smaller systems. Indeed, the interplay between experiment and theory has fuelled many studies ... 

M. N. Eberlin, Triple-stage pentaquadrupole (QqQqQ) mass spectrometry and ion/molecule reactions. Mass Spectrom. Rev. 16, 113-144 (1997). 

Wang, Y.J., Han, H.Y., Shen, C.Y, Li, J.Q., Wang, H.M., Chu,Y.N. (2009) Control of solvent use in medical devices by proton transfer reaction mass spectrometry and ion molecule reaction mass spectrometry. Journal of Pharmaceutical and Biomedical Analysis, 50, 252-256. 

P. F. Knewstubb, Mass Spectrometry and Ion-Molecule Reactions (Cambridge University Press, 1969). 

Xu Y. and Herman J. A., Detection of nitrotoluene isomers by ion cyclotron resonance mass spectrometry using ion/molecule reactions with NO+ as reagent. Rapid. Commun. Mass. Spectrom., 6(7), 425-428, 1992. 

In the end, mass spectrometry and ion techniques will continue to be powerful tools for the investigation of the structure, bonding, energetics, and reactivity of unusual organic molecules. New sophisticated techniques will continue to be developed and applied to interesting problems in physical organic chemistry. These studies, along with the continued improvements in computational methods (Chapter 9), provide means to obtain very detailed and accurate descriptions of chemical reactions. 

Many reaction types can be studied in the mass spectrometer e.g. flash photolysis, shock tube, combustion, explosions, electric discharge and complex gas reactions. Mass spectrometry is ideal for ion-ion and ion-molecule reactions, isotopic analysis and kinetic isotope effect studies. 

FT-ICR, first developed more than a decade ago (Comisarow and Marshall, 1974a,b), has become very popular in recent years for both analytical and ion/molecule reaction studies. In the literature this method is also frequently termed Fourier transform mass spectrometry (FTMS). The term FT-ICR, however, indicates the physical principles of the method more precisely and is less confusing the mathematical operation of Fourier transformation can also be applied to some other forms of mass spectrometry such as time-of-flight mass spectrometry as has been demonstrated recently (Knorr et al., 1986). 

Techniques based on cyclotron resonance are also interesting because they allow the observation of ions over long time spans. This allows the study of slow fragmentations that are not observable in classical mass spectrometry, and also equilibria between ionic species and ion-molecule reactions. 

A major problem in mass spectrometry of free radicals is the extraction of the gas sample from the reacting system and its transport into the ionization chamber without materially changing the composition of the sample and, in particular, avoiding loss of the free radicals in the process. In some cases, spurious free radical signals may be produced in the mass spectrometer by ion-molecule reactions and by reactions of incoming molecules with the heated filament. The mass spectrometer used in the studies reported here employs a collision-free molecular... 

Jennings and coworkers " were the first to report on a mass spectrometric method of locating double bonds by use of gas-phase ion/molecule reactions. Ionized alkyl vinyl ethers proved to be highly suitable reagent ions, and the principle of this gas-phase method for double-bond localization is illustrated for prototype dienes 76-78 in Scheme 22. At the same time. Hunt and coworkers introduced nitric oxide (NO) as a reagent gas in Cl mass spectrometry and studied the reactions of the NO+ and [NONO]+ ions with alkenes and also with several acyclic dienes and 1,3,5-cycloheptatriene. Thereafter, several groups have contributed to the development and only some classical work concerning simple alkenes may be mentioned here. 

Electron impact ionization of the parent molecule is only one of several important ion formation processes in nonthermal plasmas. Secondary processes such as electron impact ionization of neutral fragments produced by dissociation of the parent molecule and ion-molecule reactions are other mechanisms contributing to the formation of plasma ions. It is interesting to compare ion abundances in a realistic plasma with the ion abundances predicted from electron impact ionization cross sections measured under single-collision conditions. Although mass spectrometry of plasma ions is a known and well-developed diagnostic method (Osher, 1965 Drawln, 1968 Schmidt et al., 1999), its application to plasmas for thin-film deposition is not very common. The main reasons are deleterious effects of insulating deposits on the ion collection orifice (which connects the mass spectrometer to the plasma) and on the ion transfer optics, which render it... 

Other Mass Spectrometers. Spectrometers such as ion traps and ion cyclotron resonance have also been used in atomic mass spectrometry, but have not yet been incorporated into a commercial instrument. The former instrument has the advantage of ion storage and ion-molecule reaction capability, while the latter instrument is capable of extremely high resolution. 

Kebarle P, Yamdagni R, Hiroaka JK and McMahon TB (1976) Ion molecule reactions at high pressure recent proton affinities, gas phase acidities and hydrocarbon clustering results. International Journal of Mass Spectrometry and Ion Physics 19 71-87. 

Witt, M. Fuchser, J. Baykut, G. In-source H/D exchange and ion-molecule reactions using matrix assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry with pulsed collision and reaction gases. ]. Am. Soc. Mass Spectrom. 2002, 13, 308-317. 

Olesik, J. W, Hensman, C., Rabb, S., and Rago, D. (2001) Sample introduction, plasma -sample interactions, ion transport and ion-molecule reactions fundamental understanding and practical improvements in ICP-MS. In Plasma Source Mass Spectrometry (eds G. Holland and S.D. Tanner), Royal Society of Chemistry, Cambrigde, pp. 3-16. 

Dolnikowski, G.G., Kristo, M.J., Enke, C.G., and Watson, J.T. (1988) Ion-trapping technique for ion/molecule reaction studies in the center quadrupole of a triple quadrupole mass spectrometer. International Journal of Mass Spectrometry and Ion Processes, 82,1-15. 

Freiser B S 1988 Fourier transform mass spectrometry Techniques for the Study of Ion-Molecule Reactions ed J M Farrar and W H Saunders (New York Wiley-Interscience)... 

However, because the mass spectrometer remains the most useful and for many systems the only possible tool for studying ion-molecule reactions, most of the papers describe mass spectrometric investigations. Present day mass spectrometry has acquired great versatility through the... 

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. 

One of the chief reasons for the recent extensive work in this field has been the recognition that ion-molecule reactions are highly relevant to radiation chemistry. The possibility that certain simple reactions, such as the formation of H3+, participate in the mechanism of product formation was appreciated much earlier 14), but wider applicability of this concept required that the generality of such reactions be demonstrated by an independent, unequivocal method. Mass spectrometry has been the predominant means of investigating ion-molecule reactions. The direct identification of reactant and product ions is appealing, at least in part, because of the conceptual simplicity of this approach. However, the neutral products of ion-molecule reactions cannot be determined directly and must be inferred. Gross chemical measurements can serve as an auxiliary technique since they allow identification of un-... 

Ion-Molecule Reactions. The list of ion-molecule reactions observed by mass spectrometry is highly impressive at this time we can easily count several hundred. Most of these were observed at relatively low pressures and in the presence of a draw-out or pusher field in the ionization chamber. Using this information to account for the radiation chemical product distribution requires one to recognize its restrictions to this use. 

Cl is an efficient, and relatively mild, method of ionization which takes place at a relatively high pressure, when compared to other methods of ionization used in mass spectrometry. The kinetics of the ion-molecule reactions involved would suggest that ultimate sensitivity should be obtained when ionization takes place at atmospheric pressure. It is not possible, however, to use the conventional source of electrons, a heated metallic filament, to effect the initial ionization of a reagent gas at such pressures, and an alternative, such as Ni, a emitter, or a corona discharge, must be employed. The corona discharge is used in commercially available APCI systems as it gives greater sensitivity and is less hazardous than the alternative. 

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