SIFT technique

Adams N G and Smith D 1988 Flowing afterglow and SIFT Techniques for the Study of Ion-Molecule Reactions ed J M Farrar and W FI Saunders Jr (New York Wiley)... 

Figure 3 demonstrates the simplifications in the spectrum of an optimized laser pulse that can be achieved through the application of the sifting technique [see Fq. (7)]. The excitation efficiency of the pulse is only minimally reduced due to the additional restrictions imposed in the sifting procedure. The example used in this case is for a vibrational-rotational excitation process, H2(v = 0,7 = 0) H2(v =1,/ = 2). 

Using a selected ion flow tube (SIFT) technique, DePuy and coworkers studied such rates of deuterium-hydrogen exchange for a series of neutral carbon acids10. Table 1 contains some selected rates of exchange with DO- from DePuy s work these rates are approximate measures of relative acidity in the gas phase. 

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. 

The amount of literature which has accumulated over the last few years concerning the subject of ion-molecule reactions of silicon cations with neutral molecules is tremendous. In a review of the various reactions of Si+ with neutral molecules published in 1990, Bohme44 lists almost 100 reactions studied by selected-ion flow tube (SIFT) techniques alone84,85. In the meantime, this list will have grown to even larger dimensions and we can therefore only confine ourselves to the description of a few exemplary cases45 50. 

Until recently, little reliable data was available on the temperature dependence of ternary association reactions. Good181 has reviewed the data available up to 1975. With the inception of the SIFT technique accurate temperature dependencies have been obtained for several ternary association reactions which indicates that the variation of the ternary rate coefficients with temperature closely conforms to a simple power law behaviour (k a T-n) as predicted by statistical theory, but with n much smaller than predicted131-133. Such data is contributing to agrowing understanding of the mechanistic aspects of ion-molecule association reactions134,13S. ... 

A number of research groups have used SIFT instruments for measurements directed toward IS chemistry. The Birmingham group of Adams and Smith, the inventors of the SIFT technique [16], was particularly active in this regard and a major focus of their SIFT measurements was the systematic study of reactions of hydrogenated ions, e.g. CH,, CjH,, NH , HnS+, HnCO+ etc., with numerous molecular species [18]. Further contributions by this group include detailed studies of isotope exchange in ion-neutral reactions, studies for which the SIFT is eminently suited, since the ion source gas and the reactant gas are not mixed. From these studies and detailed kinetic models of interstellar ionic reactions, it is now understood that the observed enhancement of the rare isotopes (e.g. D, 13C) in some IS molecules is due to the process of isotope fractionation in ion-neutral reactions [19]. 

The application of the ICP/SIFT technique was first reported in 2000 and since then has been employed in measurements of reactions of metal ions with 02 [113,114] and N20 [114] and the trapping of metal ions by benzene vapour [111]. As a convenient and highly adaptable source of atomic ions, the ICP clearly has great potential in the exploration of IS metal-ion chemistry. 

Some polyatomic ions, such as CH, were found to form very long-lived complexes (t > 10 s) even at room temperature. Three-body association rate constants for CH with a variety for diatomic and triatomic molecules, as measured by the SIFT technique (Adams and Smith, 1981), are given in Fig. 

In swarm experiments, an important step forward was the development of a new method in which the SIFT techniques were combined with laser-induced-fluorescence (LIF) detection for monitoring the ion vibrational states (Kato et al., 1993). In this way, both the vibrational states of the reactant ion and the vibrational states of some reaction products could be detected, and the influence of vibrational energy on reaction rates of thermal ions (where the translational-to-vibrational energy transfer is negligible) could be studied. The method was primarily used to study reactions of Nj(t) = 0 to 4) with Ar (which will be discussed separately), N2 and O2 (Kato et al., 1993), Kr (Kato et al., 1996), H2 (de Grouw et al., 1995), and HCl (Krishnamurthy et al., 1997). In the reaction... 

Measurements were made In the Ton Chemistry Laboratory of York University with the Selected-Ion Flow Tube (SIFT) technique (5,6). With this technique ground-state atomic silicon ions are selected from a suitable source, and injected into flowing helium gas to which is added the desired neutral reactant (7). Moderate pressures (ca. 0.35 Torr) of helium buffer gas were used. The reactant and product ions are monitored with a mass spectrometer as a function of the addition of the neutral reagent. The observations provide a measurement of the specific rate and products of the chosen reactions and Information about the secondary and higher order ion chemistry at 296 +/- 2 K. 

Adams NG, Smith D (1976) Selected ion flow tube (sift) - technique for studying ion-neutral reactions. Int J Mass Spectrom 21 349-359... 

The hydrogen exchange between amide and ammonia in the gas phase at 300it2 K was investigated by the selected-ion flow tube (SIFT) technique. The following reactions and rate constants of amide consumption were found [1] ... 

The ion-molecule reactions of collisionally relaxed NH2 at 297 to 300 K in the gas phase with inorganic compounds are summarized in Table 20 and those with organoelement compounds in Table 21. The reactions were analyzed with a mass spectrometer [1, 2] in earlier experiments and later by Fourier transform (ion cyclotron resonance) mass spectrometry [3 to 7]. In other, more recent investigations, the flowing afterglow technique was applied and occasionally the newer selected-ion flow tube (SIFT) technique [8, 9], where the NHJ formed is separated from the other ions before the reaction. The methods allow the mass spectrometric identification of the anions only the other products have to be deduced from the mass balance. 

The selected ion flow tube, SIFT, technique is a fast-flow tube/ion-swarm method for the study of the reactions of ions (positive or negative) with atoms and molecules under truly thermalized conditions over a wide range of temperature. It has been extensively used to study ion-molecule kinetics. Its application to atmospheric and interstellar ion chemistry by several eminent groups over a 20-year period has been crucial to the advancement and understanding of these interesting topics. Recently it has been developed as a very sensitive analytical technique for the detection and quantification of trace gases in air and in human breath down to the ppb level and in real time. 

The SIFT technique was developed because the original fast-flow-tube method, the eminently... 

It is a simple matter to determine the forward (exothermic) and reverse (endothermic) rate coefficients, and for such reactions using the SIFT technique (ideally suited for such studies because separate isotopic ions can be injected and the reaction temperature can be accurately controlled even down to 80 K). For reactions [16] it is observed that increasingly exceeds as the temperature is reduced. This is because the reverse reaction is endothermic by 39.8 meV (3.84 kj mol ), by virtue of the zero-point-energy difference between H2 and HD, and the ionization energies of H and D differ by 4 meV (0.38 kJ mol" ). Hence in cold ISC the reverse reaction is effectively stopped whilst the forward reaction proceeds at the gas kinetic rate. This effectively ensures that much of the deuterium in dense interstellar... 

Spanel P, Cocker J, Rajan B and Smith D (1997) Validation of the SIFT technique for trace gas analysis of breath using the syringe injection technique. Annals of Occupational Hygiene 41 373-382. 

Spanel P and Smith D (1996) Application of ion chemistry and the SIFT technique to the quantitative analysis of trace gases in air and on breath. International Review of Physical Chemistry 15 231-271. 

In an ongoing project in the Ion Chemistry Laboratory at York University, numerous ion-molecule reaction rate coefficients have been determined using a selected-ion flow tube (SIFT) tandem mass spectrometer modified to accept ions, generated by a plasma ion source, through an atmosphere-vacuum interface. The SIFT technique is a powerful and versatile means for... 

The development of PTR-MS owes much to its precursors, the FA and SIFT techniques. However, PTR-MS is intended as an analytical technique rather than a means of measuring kinetics and it achieves this by essentially inverting the original aims of the FA and SIFT techniques. Instead of measuring the rate coefficient for a specific reaction, it is now presumed known from earlier kinetic studies or it can be deduced from calculations, as described in Section 2.23.2, or it may be omitted completely if gas calibration is employed (as described in Chapter 4). This information can then be used to determine the concentration of a compound, as shown later in Section 1.4.3. 

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