Collision/reaction system

Chen, Z., N.I. Khan, G. Owens, et al. 2007. Elimination of chloride interference on arsenic speciation in ion chromatography inductively coupled mass spectrometry using an octopole collision/reaction system. Microchem. J. 87 87-90. 

This equation results from the assumption that the actual reaction step in themial reaction systems can happen only in molecules (or collision pairs) with an energy exceeding some tlireshold energy Eq which is close, in general, to the Arrhenius activation energy defined by equation (A3.13.3). Radiative energization is at the basis of classical photochemistry (see e.g. [4, 3 and 7] and chapter B2.5) and historically has had an interesting sideline in the radiation... 

There is one special class of reaction systems in which a simplification occurs. If collisional energy redistribution of some reactant occurs by collisions with an excess of heat bath atoms or molecules that are considered kinetically structureless, and if fiirthennore the reaction is either unimolecular or occurs again with a reaction partner M having an excess concentration, dien one will have generalized first-order kinetics for populations Pj of the energy levels of the reactant, i.e. with... 

In Table 1 (pp. 251-254), IM rate constants for reaction systems that have been measured at both atmospheric pressure and in the HP or LP range are listed. Also provided are the expected IM collision rate constants calculated from either Langevin or ADO theory. (Note that the rate constants of several IM reactions that have been studied at atmospheric pressure" are not included in Table I because these systems have not been studied in the LP or HP ranges.) In general, it is noted that pressure-related differences in these data sets are not usually large. Where significant differences are noted, the suspected causes have been previously discussed in Section IIB. These include the reactions of Hcj and Ne with NO , for which pressure-enhanced reaction rates have been attributed to the onset of a termolecular collision mechanism at atmospheric pressure and the reactions of Atj with NO and Cl with CHjBr , for which pressure-enhanced rate constants have been attributed to the approach of the high-pressure limit of kinetic behavior for these reaction systems. 

Though combustion is a very fast exothermic chemical reaction compared with other chemical reactions, the reaction time is finite and the combustion products are formed after a large number of molecular collisions, which also produce a large number of intermediate molecules. When the time-averaged numbers of molecules reach a constant level and the temperature becomes constant, the reaction system is said to be in a state of thermal equilibrium.[i 2 l The Gibbs free energy F for one mole of an ideal gas is defined according to... 

In addition to these collision/reaction cell instruments, since 2002 Thermo Fisher Scientific has been selling the XSeries ICP-MS with a hexapole collision cell (developed from ThermoElemental PQExel ICP-MS) as a bench top instrument on the analytical market. A special ion extraction system in Thermo s XSeries", ion optics together with a hexapole collision cell to minimize the interference problem in ICP-MS, provides the lowest background signals for ICP-QMS (< 0.5 cps). 

More recently, the advent of the collision/reaction cell technology has revolutionised commercial quadrupole ICP-MS systems. A gas, such as hydrogen, helium or ammonia, is introduced into the reaction cell (placed inside the mass spectrometer and preceding the analyser quadrupole), where it reacts and dissociates or neutralises the polyatomic species or precursors. Through collision and reaction with appropriate gases in a cell, interferences... 

Dissociation of halomethanes, particularly CF4, by collisions with rare gas active species has also been studied under various experimental conditions. The cross sections of each product ion in the M+/CF4 (M = Ar, Ne, He) reaction system from thermal to 50 eV have been determined using guided ion-beam techniques72. It was found that the energy dependence of the cross sections can be understood by considering the energies needed to access specific electronic states of CF4+ 72. 

M. Iglesias, N. Gilon, E. Poussel, J. M. Mermet, Evaluation of an ICP-collision/ reaction cell-MS system for the sensitive determination of spectrally interfered and non-interfered elements using the same gas conditions, J. Anal. Atom. Spectrom., 17 (2002), 1240D1247. 

The addition of PVMI was expected to enhance the hydroxide-ion catalyzed hydrolysis of the anionic ester, since both the hydroxide ions and the negatively charged ester are attracted to the polycation so that the rate of their mutual collision is increased. For any given polyelectrolyte concentration the increase in the rate of hydrolysis should be independent of pH. And this is what has actually been found at pH values greater than 9. At lower pH values, however, a completely unexpected behavior resulted, i. e. the polycation was found to increase the rate of hydrolysis of NABS by the largest factor in that pH range (pH s 6) in which direct water attack on the ester makes the dominant contribution to the overall reaction rate (Fig. 8). The influence of hydrophobic forces appears to be ruled out in this case since PVMI has no effect on the solvolytic rate of the neutral ester p-nitrophenyl acetate and p-nitrophenyl hexanoate. Thus, the causes of the above-mentioned phenomenon are obscure this very fact adds, in the author s opinion, further interest to the study of reactions in polyelectrolyte solutions. The examination of such factors as the enthalpy and entropy of activation may be of particular relevance for a deeper insight into these complex reaction systems. 

E. E. Nikitin, Pathways of vibrational relaxation of diatoms in collisions with atoms Manifestation of the Ehrenfest adiabatic principle, in Gas Phase Chemical Reaction Systems, Eds. J.Wolfrum et al, Berlin-Heidelberg, Springer, 1996, p 231... 

We consider an ensemble of reactant molecules with quantized energy levels to be immersed in a large excess of (chemically) inert gas which acts as a constant temperature heat bath throughout the reaction. The requirement of a constant temperature T of the heat bath implies that the concentration of reactant molecules is very small compared to the concentration of the heat bath molecules. The reactant molecules are initially in a MaxweD-Boltzmann distribution appropriate to a temperature T0 such that T0 < T. By collision with the heat bath molecules the reactants are excited in a stepwise processs into their higher-energy levels until they reach "level (2V+1) where they are removed irreversibly from the reaction system. The collisional transition probabilities per unit time Wmn which govern the rate of transfer of the reactant molecules between levels with energies En and Em are functions of the quantum numbers n and m and can, in principle, be calculated in terms of the interaction of the reactant molecules with the heat bath. 

As far as we are aware, no a priori calculations, have even been carried out to evaluate Pc from the molecular properties of the collision partners, its order of magnitude has only been found a posteriori for reaction systems. It is, therefore, impossible to make a direct numerical comparison between the frequency factors A found for the process of stepwise activation Eq. VII.36 and for the "all or nothing" kinetic theory activation Eq. VII.41. The following indirect comparison is, however, instructive. Measurements on the rate of activation of J2 at about 300°K in various inert gases such as He, Ne, A, Kr, and N2 have shown22 that the frequency factor A is of the order of about 5x 1016 cm3/mole /sec. Since the collision number Z is only about 1014 cm3/mole/sec at 300°K, even a value of Pc = 1, which corresponds to unit efficiency in direct collisional activation, could not raise the calculated A-value for the standard collision.theory (Eq. VII.41) to the observed one. This is, of course, one of the old and vexing problems in chemical kinetics,17... 

There are many systems that can fluctuate randomly in space and time and cannot be described by deterministic equations. For example. Brownian motion of small particles occurs randomly because of random collisions with molecules of the medium in which the particles are suspended. It is useful to model such systems with what are known as stochastic differential equations. Stochastic differential equations feature noise terms representing the behavior of random elements in the system. Other examples of stochastic behavior arise in chemical reaction systems involving a small number of molecules, such as in a living cell or in the formation of particles in emulsion drops, and so on. A useful reference on stochastic methods is Gardiner (2003). 

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