Flow drift tube

Figure A3.5.7. Schematic diagram of a selected ion flow drift tube with supersonic expansion ion source.

In obtaining experimental information about the isomeric forms of ions, a variety of techniques have been used. These include ion cyclotron resonance (ICR),31 flow tube techniques, notably the selected ion flow tube (SIFT),32 and the selected ion flow drift tube (SIFDT)32 (and its simpler variant33), collision induced dissociation (CID),10,11 and the decomposition of metastable ions in mass spectrometers.13 All of these techniques are mentioned in the text of Section in whore they have provided data relevant to the present review. 

Reactant ions produced by Penning ionization of 02 by He(23S) rate constant measured in a flow-drift tube for the relative kinetic energies in the range 0.04 to 2 eV. 

Figure 6. Schematic diagram of flow-drift tube apparatus.46...

Recently,464 the flow-drift-tube method has been employed to determine the dependence on translational energy of the rate coefficients for various charge-transfer reactions of 02+ (a4TTu). The results for one such reaction are shown in Fig. 17, and the other processes studied exhibit similar behavior. 

The temperature dependence of the kinetic isotope effect for the gas-phase S 2 reaction of CL with MeBr was examined in a variable-temperature selected ion flow drift tube. The results were then interpreted by a molecular modeling study250. 

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. 

To reach a high sensitivity requires a high ion count rate i(RH" ) per unit density [R] in the gas to be analyzed. This obviously can be achieved by keeping the [H30" ] density high, and by not diluting the gas to be analyzed in an additional buffer gas like helium as is done in conventional Flow-Drift-Tube experiments, but by using the air itself (which contains the trace constituents to be analyzed) as the buffer gas. This can especially be done when H30" ions are used as the ionic reactant species because, as discussed above, these ions do not react with the major components of air. 

Figure 6. Rate constants for the reaction of with NO as a function of average translational plus rotational energy. The HTFA, CRESU, ° flow drift tube, and static drift tube data are shown as squares, circles, triangles and inverted triangles, respectively.

The study of internal energy effects in the HTFA is a continuation of work that was started at lower temperatures using the variable temperature selected ion flow drift tube (VT-SIFDT). That data has been summarized previously and several trends were noted. The HTFA comparisons taken as a function of translational energy allow us to verify those trends and look for new ones which become apparent due to the extended energy range. Data from other experiments that probe internal energy effects, sometimes with quantum state resolution, are now available and can be included in the comparison. 

The effects of energy on ion-molecule rate processes have been investigated by a variety of methods. The influence of reactant translational energy, as studied by SIFDT (selected ion-flow drift-tube) techniques in swarm experiments, by ICR, and by beam and other single-collision techniques, is reviewed in other parts of this chapter or of this book. In this section, we will concentrate specifically on the influence of reactant internal energy on ion-molecule reactions. There are basically two sources of data that address this problem ... 

FIGURE 28.5 Schematic drawing of the PTR-TOF-MS system with a hollow cathode ion source. SD, source drift tube FDT, flow drift tube TO, transfer optics MCP, microchannel plate. Reprinted with permission from Reference [15]. Copyright 2005 Elsevier. 

An important adaptation of the FA technique comprises the implementation of ion separation methods, which allows for more advanced flow drift tubes and selected ion flow tubes (SIFT) [105, 106] (see below). More recently, flow drift tubes (see Sect. 4.6) and flowing atmospheric pressure afterglow (FAPA) devices [107] have been developed. 

The degree of sophistication of flow tube, drift tube and flow drift tube techniques, especially following the development of selected ion injection sources, is such that now it is possible to study a very wide variety of ion-neutral reactions, including the reactions of state selected ions, doubly-charged ions, weakly-bound (cluster) ions, etc. 

Figure 2 Rate coefficients for the reaction of Ol with methane measured as a function of average kinetic energy at several temperatures in a variable-temperature selected-ion flow drift tube (VT-SIFDT). Also shown are data taken with a high-temperature flowing afterglow (HFTA) and a drift tube. Reproduced with permission from Viggiano AA and Morris MA (1996) Journal of Physical Chemistry 100 19227-19240.

Fairly, D. A., Milligan, D. B., Freeman, C. G. et al. (1999) Competitive association and charge transfer in the reactions of NO+ with some ketones a selected ion flow drift tube study. Int. J. Mass Spectrom. 193,35. 

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