Mass spectrometer - unimolecular

In the FFR of the sector mass spectrometer, the unimolecular decomposition fragments, and B, of tire mass selected metastable ion AB will, by the conservation of energy and momentum, have lower translational kinetic energy, T, than their precursor ... 

The quasi-equilibrium theory (QET) of mass spectra is a theoretical approach to describe the unimolecular decompositions of ions and hence their mass spectra. [12-14,14] QET has been developed as an adaptation of Rice-Ramsperger-Marcus-Kassel (RRKM) theory to fit the conditions of mass spectrometry and it represents a landmark in the theory of mass spectra. [11] In the mass spectrometer almost all processes occur under high vacuum conditions, i.e., in the highly diluted gas phase, and one has to become aware of the differences to chemical reactions in the condensed phase as they are usually carried out in the laboratory. [15,16] Consequently, bimolecular reactions are rare and the chemistry in a mass spectrometer is rather the chemistry of isolated ions in the gas phase. Isolated ions are not in thermal equilibrium with their surroundings as assumed by RRKM theory. Instead, to be isolated in the gas phase means for an ion that it may only internally redistribute energy and that it may only undergo unimolecular reactions such as isomerization or dissociation. This is why the theory of unimolecular reactions plays an important role in mass spectrometry. 

Since the ionic states formed by high-energy radiation seem to be the chemically important ones, let us consider their reactions. The reactions between ions and neutral molecules in the gas phase can be studied directly in a mass spectrometer. Under ordinary operating conditions the pressure in the ionizing chamber of the mass spectrometer is about 10 6 mm. and the ions formed have little chance to collide with a molecule during their brief lifetime (10-5 sec.) before collection. Therefore, mainly unimolecular decomposition reactions occur and it is the products of these that are detected. The intensity of these primary ions increases with the first power of the pressure in the ionization chamber. However, when the pressure becomes great enough so that ion molecule collisions can occur readily, additional secondary ions which are the products of these ion molecule Collisions appear. The intensity of these secondary product ions depends on the concentrations of both the molecules and the primary ions, and thus on the square of the pressure. 

Metastable ions is a term commonly used for ions which decompose unimolecularly in the field-free regions between the analysers, between the source and analyser, or between analyser and detector of ordinary mass spectrometers [186, 421]. These ions typically have lifetimes of the order of microseconds. The lifetimes are determined by the times of flight through the mass spectrometer. The term is used in this review in a broader sense to mean any ions selected according to lifetimes when those lifetimes are of the order of microseconds [i.e. in terms of eqn. (9), i h /us]. 

In this chapter we will discuss the basis for the relationship between reactivity in a mass spectrometer and reactivity under thermal or photochemical activation. We will present an empirical guide which may be useful in predicting relationships between the three types of unimolecular reactivity. In the hght of this analysis, we will review the cases where the reactivity relationships appear to have broken down and finally, we will review the successful examples of the relationship between mass spectrometry and thermochemistry, and mass spectrometry and photochemistry. 

The gas phase pjnrolysis of alkyl hahdes has been extensively reviewed 58>, and in general the unimolecular gas phase reactions of alkyl halides parallel their reactivity in a mass spectrometer. For example, ethylchloride yields ethylene and HCl on thermolysis 5 >, and the ethylene ion in the mass spectrum of ethyl chloride is significantly more intense than the molecule ion. 1,2-dichloroethane also eliminated HCl thermolytically and the corresponding ion is the base peak in its mass spectrum. Elimination of HCl is also common to the mass spectra and thermochemistry of chloroprene dimers.Although in this case the major ion at mje 91 had no definite analog in the thermochemistry. This is probably due to the fact that mje 91 was a tropylium ion which would not be stabihzed as a neutral. 

The unimolecular reactions of ions in a mass spectrometer axe remarkably well correlated with both photochemical and thermochemical reactions of the corresponding neutrals. The reactivity correlations are seldom quantitative and exceptions to the correlation should be expected when heteroatoms or delocalization effects convey unusual stabiUty to the ions in the mass spectrometer as compared to their neutral analogs. In spite of the special effects on ionic stability and the relatively large amount of energy that is available in 70 eV electron impact, it appears that mass spectra will be an increasingly useful guide to new photochemical and thermochemical reactions. 

The unimolecular decomposition of C6H5Cr has been studied by analysing the distorted C5H5 peak shape observed in a TOP mass spectrometer following MPI of jet-cooled chlorobenzene. The analysis provided values for the specific reaction rate constants, k(E), which agreed well with the results of previous studies, validating the MPI/TOF m.s. technique as a means for investigating ion decomposition rates. 

The unimolecular and bimolecular reactions of gas-phase ions that occur both during the ionization process and while the ions are inside the mass spectrometer are controlled by both thermochemistry [4] and kinetics [5]. Some key thermochemical quantities associated with cations and anions are presented below. 

Theoretical studies of unimolecular reaction kinetics of polyatomic molecules in a mass spectrometer requires the knowledge of the number of states of a molecule with internal energy E, fV(E). It has been shown by Rosenstock et... 

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