Selected Ion Flow Tube SIFT

These days, remarkably high-resolution spectra are obtained for positive and negative ions using coaxial-beam spectrometers and various microwave and IR absorption teclmiques as described earlier. Infonnation on molecular bond strengths, isomeric fonus and energetics may also be obtained from the teclmiques discussed earlier. The kinetics of cluster-ion fonuation, as studied in a selected-ion flow tube (SIFT) or by high-pressure... 

Adams N G and Smith D 1976 The selected ion flow tube (SIFT) a technique for studying ion-neutral reactions Int J. Mass Spectrom. Ion Phys. 21 349... 

Smith D and Adams N G 1988 The selected ion flow tube (SIFT) studies of ion-neutral reactions Adv. At Mol. Phys. 24 1-49... 

Figure Bl.7.12. A schematic diagram of a typical selected-ion flow (SIFT) apparatus. (Smith D and Adams N G 1988 The selected ion flow tube (SIFT) studies of ion-neutral reactions Advances in Atomic and Molecular Physics vol 24, ed D Bates and B Bederson p 4. Copyright Academic Press, Inc. Reproduced with pennission.)...

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. 

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. 

The reactions of the species H3O+, NO+, and O2+ with a range of aldehydes and ketones have been studied by the selected ion flow tube (SIFT) method. H3O+ protonates ketones and aldehydes, with the latter eliminating water under the conditions of measurement. Similarly, NO+ associates with ketones, but this is followed by hydride transfer for the aldehydes. O2+ reactions typically produce several ionic products. 

The HO + CH2F2 reaction has been studied by selected ion flow tube (SIFT) experiments and ab initio calculations. SIFT experiments at 300 K showed that a bimolecular process [leading to CHF2 (86%), F (11%), and HF2 (3%)] competes with a three-body association leading to HO. CH2F2. The bimolecular rate coefflcient... 

To overcome this, instrumental techniques such as pulsed high-pressure mass spectrometry (PHPMS), the flowing afterglow (FA) and allied techniques like the selected-ion flow tube (SIFT), and ion cyclotron resonance (ICR) spectrometry and its modem variant, Fourier transform mass spectrometry (FTMS), have been developed. These extend either the reaction time (ICR) or the concentration of species (PHPMS, FA), so that bimolecular chemistry occurs. The difference in the effect of increasing the pressure versus increasing the time, in order to achieve bimolecular reactivity, results in some variation in the chemistry observed with the techniques, and these will be addressed in this review as needed. 

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. 

An important development for ion/molecule reaction studies by FA is the extension of the method using so-called selected ion flow tube (SIFT) facilities (Adams and Smith, 1976). In the latter configuration ions are generated in an external ion source, extracted and separated by a quadruple mass filter, after which ionic species of a single mass-to-charge ratio are injected into the flow tube. This set-up permits the ion/molecule reactions of mass selected ions to be studied in the absence of ions of other masses (similar to studies of mass selected ions in FT-ICR after application of so-called ion ejection techniques see above) and neutral precursors, while a wide choice of neutral substrates is possible. 

The reactions of atomic silicon cations with a large variety of neutral molecules have been reported and summarized in reviews focussed on the kinetic, energetic and the mechanistic study of these reactions by selected-ion flow tube (SIFT)10 and guided ion beam techniques11. 

The gas phase acid/base properties of molecules have been subject to equilibrium or bracketing measurements employing mass spectrometric techniques like ion cyclotron resonance (ICR) [4], Fourier transform ion cyclotron resonance (FT-ICR) [5,6], Flowing afterglow (FA) and Selected ion flow tube (SIFT) [7], and high pressure mass spectrometry (HPMS) [8]. Proton transfer between neutral molecules are then investigated by measurements of reactions... 

Fig. 3. Schematic diagram of the Selected-Ion Flow Tube (SIFT) tandem mass spectrometer. Ions from the ion source are selected with a quadrupole mass filter before injection into the flow tube. A second quadrupole mass filter is used downstream to separate reactant and product ions in the sampled reaction mixture. The He pressure in the flow tube is ca. 0.4 Torr...

Fig. 10. Schematic view of the Inductively-Coupled Plasma (ICP) Selected-Ion Flow Tube (SIFT) tandem mass spectrometer. The ICP torch produces metal ions M+ that are mass-selected and injected into the flow tube. The M+ ions may be ligated with molecules L before they enter the reaction region further downstream...

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. 

Reactions of toluene, ethylbenzene, and w-propylbenzene with ions having recombination energies ranging from 9.26 eV (NO ) to 15.58 eV (NJ) were studied at 300 K in a selected ion flow tube (SIFT) yielding information regarding the role of electronic energy in these reactions. These... 

Several methods exist that allow the determination of the standard enthalpies of formation of the ionic species. The reader is referred to two recent rigorous and detailed chapters by Lias and Bartmess and Ervin. The vast majority of the experimental data reported here are obtained by means of Fourier transform ion cyclotron resonance spectroscopy (FT ICR), high-pressure mass spectrometry (HPMS), selected ion flow tube (SIFT), and pulsed-field ionization (PFI) techniques, particularly pulsed-field ionization photoelectron photoion coincidence (PFI-PEPICO). All these experimental techniques have been examined quite recently, respectively, by Marshall, Kebarle, B6hme," ° Ng" and Baer. These chapters appear in a single (remarkable) issue of the International Journal of Mass Spectrometry. An excellent independent discussion of the thermochemical data of ions, with a careful survey of these and other experimental methods, is given in Ref. 37. 

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