Swarm Techniques

Mass-Spectrometer Ion Source—Low-Pressure Experiments The early history of ion-molecule studies is largely a chronicle of mass-spectrometer ion-source experiments. Data were easy to obtain, but data which are only dependent on the kinetics of the collision processes were and still are exceedingly difficult to obtain. Moreover, as will be seen, reliable data so obtained have only an indirect relevance to present needs (Section 1.2). Nowadays the technique is not widely used and it might therefore seem appropriate to confine discussion of it to Chapter 2. It will, however, be discussed here for two reasons. The technique is still used and it will be a major purpose of the following remarks to indicate 

The usefulness of these rate parameters, i.e., their relevance to other experimental situations, depends on the validity of the three above assumptions. If they are invalid, the measured rate parameters are restrictively phenomenological, relevant solely to the conditions of the particular measurement. These considerations stress the need to test that validity, as indicated in the discussion of control experiments in the following section. 

If the three assumptions are shown to be valid, it follows rigorously that experimental rate constants may only be compared if Vf is identical in both cases. The same restriction applies to comparisons of experimental phenomenological cross sections. If this is not so, such comparisons can only be made quantitatively if the excitation function has been measured, either by unfolding procedures (Section 2.2) or by direct measurement. 

If k v) has been shown to be an insensitive function of v, this restriction may be relaxed for qualitative comparisons and predictions. It may never be relaxed for any kind of comparison of phenomenological cross sections in that case, the excitation function must be known. As noted in Section 2, it is for this reason that the rate constant rather than the phenomenological cross section should be reported for such measurements. Moreover, the exit energy Ef must always be reported, not the field strength, since it is the former which defines the upper limit of the averaging according to Eq. (15). 

Granted that rate constant data have seldom been unfolded to give the excitation function, to what kind of situation can such rate constants, i.e., k(VfX be applied Rigorously, the answer is none, except in the situation of a mass-spectrometer ion source, since this particular rate constant relates to a very odd effective velocity distribution. Comparison with Eq. (2) shows that the analytical form of this normalized velocity distribution is the following  

---

Momentum-transfer cross sections are normally determined by the electron swarm technique. A detailed discussion of the drift and diffusion of electrons in gases under the influence of electric and magnetic fields is beyond the scope of this book and only a brief summary will be given. The book by Huxley and Crompton (1974) should be consulted for a full description of the experimental methods and analysis procedures. 

The transport coefficients depend on a balance between the rates of acquiring energy from acceleration in the field and losing it in collisions. Since this balance can be made very close the swarm technique is particularly suited for providing cross-section data at low electron energies. 

PTR-MS combines the concept of Cl with the swarm technique of the flow tube and flow-drift-tube mentioned above. In a PTR-MS instrument, we apply a Cl system which is based on proton-transfer reactions, and preferentially use HsO" " as the primary reactant ion. As discussed earlier, HsO" " is a most suitable primary reactant ion when air samples containing a wide variety of trace gases or VOCs are to be analyzed. HsO" " ions do not react with any of the natural components of air, as these have proton affinities lower than that of H2O molecules this is illustrated in Table 1. This table also shows that common VOCs containing a polar functional group or unsaturated bonds (e.g. alkenes, arenes) have proton affinities larger than that of H2O and therefore proton transfer occurs between H30" and any of these compounds (see Equation 4). The measured thermal rate constants for proton transfer to VOCs are nearly identical to calculated thermal, collisional limiting values (Table 1), illustrating that proton transfer occurs on every collision. 

The electron attachment rate constant for SFg in nitrogen at ambient temperature and pressure showed a smooth decline with increasing E/N over the range of 0.39-0.78 Td [56]. As shown in Figure 13.7, the results obtained by IMS agree closely with those obtained by the well-established high-pressure swarm technique [57]. A further series of experiments with E/N from 0.05 to 0.9 Td confirmed this excellent agreement between the two methods [55]. 

Mayhew, C.A. Critchley, A.D.J. Howse, D.C. Mikhailov, V. Parkes, M.A. Measurements of thermal electron attachment rate coefficients to molecules using an electron swarm technique. Eur. Phys. J. D 2005,35, 307-312. 

Smith D and Spanel P (1995) Swarm techniques. In Dunning FB and Hulet RG (eds) Experimental Methods in the Physical Sciences Atomic, Molecular, and Optical Physics Charged Particles, pp 273-298. New York Academic Press. 

PTR-MS has its origins in the development of the flowing afterglow (FA) method for the study of ion-molecule reaction kinetics. This so-called ion-swarm technique was introduced in the 1960s by Ferguson and co-workers and it revolutionized the study of ion-molecule reaction kinetics and thermodynamics [9,10]. 

---