Mass spectrometer - unimolecular reactions

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. 

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... 

The ions observed as a result of unimolecular dissociations (metastable ions) in the mass spectrometer correspond to reactions in the low- Xs time frame ". Due to the duality of the reaction rate and the available energy, only processes with rather low-energy requirements are observed ", which is nicely reflected in the metastable ion spectrum of nitromethane (Figure 1) . Two dominant processes, i.e. the loss of OH (m/z 44) and CH3O (m/z 30), respectively, are observed leading directly to the conclusion that these reactions have critical energies within a few hundredths of meV. Recent literature reviews offer excellent discussions on the metastable ion dissociations. Nevertheless, it appears reasonable to summarize the more important aspects. 

The dissociation of molecules is one of the basic processes in chemistry the study of the kinetics of these reactions is therefore of considerable theoretical and practical interest, A simple method of obtaining information about dissociation reactions is to heat the gas to a sufficiently high temperature and then look for thermal decomposition. However for rich mixtures bimolecular reactions may well contribute to the reaction their influence must be separated out so that the unimolecular dissociation can be isolated. The rate of the primary dissociation is determined by elementary physical processes including both energy transfer between particles and internal energy flow. Dissociation reactions, isomerisation processes, photolytic reactions, dissociation of ions (e.g. in a mass spectrometer) and chemical activation experiments are closely related processes. 

The reactions that take place in a mass spectrometer are unimolecular, that is, they do not involve collisions between molecules or ions. This is true because the pressure is kept so low (10 torr) that reactions involving bimolecular collisions do not occur. 

If one wishes to carry ont gas-phase experiments, that is, to manipulate mass-selected ions inside the mass spectrometer, ion-trap analyzers offer the broadest arsenal of experiments including unimolecular fragmentations as well as bimolecular reactions with sufficiently volatile neutral reagents. Consequently, the choice of analyzer is also an important point. Mass analyzers use static or dynamic electric or magnetic fields to separate the ions either in time or in space. Sector-field mass analyzers use magnetic (B) and electrostatic (E) sectors to separate the ions... 

---