Selected ion drift tube

A new development, the selected-ion-drift tube or SIFT technique (Smith and Adams, 1979), enables the isolation of reactions due to a given set of ions, of a selected mfe value, with a neutral. Ions are produced outside the flow cell under high vacuum, and injected into the flow tube through a mass filter. This filter eliminates all ions except the ones selected (according to a specific m/e value), and removes the neutral precursor of the ion in question by differential pumping. 

Fig. 3.16. Temperature dependence of the ternary rate coefficients for the reaction He+ + 2He HeJ + He. The filled circles are data obtained with a 22-pole trap, the open squares have been measured using a selected ion drift tube. The fit fcs = 1.4 x 10 i(300 K/T)° cm /s is used as a thermometer . Note, however, that more precise experiments are needed in order to separate the influence of ion temperature, density of the neutrals and the temperature dependence of fea.

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. 

The published data on the reaction of N" " with O2 is more complicated than the reactions discussed above. Three drift tube studies show flat translational energy dependencies with the rate constant approximately half the collision rate. " In contrast, both HTFA data " and a previous NO A A temperature dependence data found the rate constant to increase with increasing temperature until the rate saturates at approximately the collision limit. The results indicated that rotational energy had a large influence on the reactivity and translational energy did not. This is in contrast to the other results summarized in this chapter. Therefore, we have very recently reexamined the rate constants for this reaction in both the HTFA and a selected ion flow tube (SIFT) in our laboratory. 

The conventional type of drift-tube (DT) experiment, in which ions drift under the influence of a weak electric field, has been widely used in the measurement of ion mobilities, equilibrium constants, and IMR rate constants as a function of ion kinetic energy from 0.05 to about 5 eV [9]. The capabilities of this technique have been extended soon by incorporating an ion drift-tube section into the gas-flowing region of the basic FA device [10]. These FA and DT instruments (namely, flow-tube mass spectrometry) and their many derivative techniques such as the powerful selected ion flow tube (SIFT) method, variable temperature FA, and guided ion beam tandem mass spectrometry have been employed in many laboratories worldwide. 

The techniques for measuring rate constants and prodnct distribntion (branching ratios) for ion-molecule reactions are varied, but the majority of the data have been determined using the FA, drift-tube (DT), selected ion flow tube (SIFT), high-pressure mass spectrometric techniques, or ICR. These methods are detailed in Chap. 4. A number of surveys of all classes of ion-molecule reactions have appeared in the literature Ferguson [16], Sieck et al. [17], Albritton [18], Anicichet al. [19], Ikezoe et al. [20], some of which include termolecular reactions or are limited to selected methods. Anicich listed [21] an index to the hterature for gas phase bimolecular positive ion-molecule reactions as a comprehensive survey of ion-molecule reaction kinetics and product distribution of the reactions. Over 2300 references are cited. This index covers the hterature from 1936 to 2003. It was limited to selected reactions, listed by reactant ion, that were important for chemical modehng ionic processes in planetary atmospheres, cometary comas, and intersteUar clouds. 

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. 

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

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. 

Ions exiting the drift tube are mass analyzed in mass spectrometer MS2, an important feature if reactions are occurring in the drift cell. Ions are generally detected after MS2 by ion counting techniques. The mass spectrometers MSI and MS2 are typically quadrupole mass filters, and either one or the other can be run in RF-only mode for better signal but without mass selection, if desired. 

Fig. 1. Schematic outline of the basic ion mobility-mass spectrometry setup using mass selection before (MSI) and after (MS2) the drift tube...

Similar drift tubes with similar dimensions are used as medium resolution devices employing a VD of several hundred to 1000 V and pressures of several torrs [39]. In this lower pressure, medium resolution setup ions are generally injected into the drift tube from vacuum, often with mass selection prior to the drift tube. 

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