Selected-ion flow tubes

Figure A3.5.5. Rate constants for the reaction of Ar with O2 as a fiinction of temperature. CRESU stands for the French translation of reaction kinetics at supersonic conditions, SIFT is selected ion flow tube, FA is flowing afterglow and HTFA is high temperature flowing afterglow.

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. 

Description of the SIFT (selected ion flow tube) technique, which allows detection of gas traces in real time. 

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... 

Anions (9), (10a) and (11) derived from cycloctatetraene have been studied using the selected ion flow tube technique and MO calculations, as a continuation of previous work on the structures and energetics of eight-membered-ring species, CgH and... 

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. 

Note The acronyms used here are OSPED (optical spectroscopy in a pulsed electrical discharge), FAMS (flowing afterglow mass spectrometry), SIFT (selected ion flow tube), TRAPI (time-resolved atmospheric pressure ionization mass spectrometry), PHPMS (pulsed high-pressure ionization mass spectrometry), ICRMS (ion cyclotron resonance mass spectrometry), and ADO (averaged dipole orientation collision rate theory). 

Smith D, Spanel P (2005) Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis. Mass Spectrom Rev 24(5) 661-700... 

The technique used is the Flowing Afterglow Selected Ion Flow Tube (FA-SIFT), a short description of which is given elsewhere337,338 it allows the preparation of ions in a first flow tube, their separation from complex reaction mixtures with a quadrupole, the... 

The field of ion-molecule reactions has profited immensely from the introduction of new techniques in mass spectrometry such as Fourier Transform Ion-Cyclotron Resonance Mass Spectrometry (FT-ICR) and Selected-Ion Flow Tube methods (SIFT). Several accounts of the progress in the field of ion-molecule reactions involving silicon-containing molecules have been published. For review articles the reader is referred elsewhere1,3,4,40-51 A detailed compilation of the kinetic data for bimolecular ion-molecule reactions of positive silicon ions has been published by Anicich52. 

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. 

In 1988, Damrauer and cowerkers found the gas-phase acidity of H2C=Si to he between that of CH3SH and acetophenone, making H2C=Si a stronger acid than acetylene421. It was necessary for these workers to assume the HCSi- structure for the ion that they studied in tandem flowing afterglow selected ion flow tube proton-transfer experiments, as well as the H2C=Si structure for the product. 

A selected ion flow tube has been used to study the reaction of C2H70+, generated by the collision of H3 0+ with C2H4. Reactions of the latter species with EtOCHO, Pr OH, AcOH, toluene, CH3CH=0, allene, MeOH, cyclopropane, H2O and C2H6 were studied to obtain information on the isomeric forms of the C2H70+ ions and the data were... 

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. 

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