Selected ion flow tube technique

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

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... 

It would appear today, however, that the acid-base equilibrium of diazomethane with its anion is better known in the gas-phase than in solution The same group (DePuy et al., 1989) investigated reactions of the diazomethyl anion with CS2, COS, CO2, SO2, and with a series of a,)ff-unsaturated aldehydes and ketones — all in the gas phase using a flowing afterglow apparatus (DePuy and Bierbaum, 1981) and selected ion flow tube techniques (Van Doren et al., 1987). 

N3 +H2. The rate constant k 2 x 10 cm molecule" s for the reaction N3 + H2 - N2H + NH was determined using a selected-ion flow tube technique. Only branching into the N2H product channel was observed [2]. 

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... 

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. 

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 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. 

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 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... 

However, the desire to reduce or even eliminate the required sample preparation has led to the development of newer techniques, such as proton-transfer-reaction mass spectrometry (PTR-MS) and selected ion flow tube mass spectrometry coupled with ion mobility spectrometry (MS-SIFT IMS). These techniques allow real-time measurements within a single second. This enables analysis of gas composition during technological processes, monitoring of indoor air, and investigation of VOCs emitted by living organisms. 

Selected ion flow tube mass spectrometry (SIFT-MS) is an analytical technique used for direct and quantitative determination of VOCs in mixtures of gases. SIFT-MS was introduced in 1976 by N. G. Adams and D. Smith. The technique can be applied for parallel real-time monitoring of a few substances [126]. A scheme of the SIFT-MS system is presented in Fig. 14.9 [127]. The principle of gas mixture analysis is based on the reaction of reagent ions with molecules of analyte within a specific time (a few milliseconds). In this method, chemical ionization is applied reagent ions are generated in the ion source by a suitable ionization gas (nitrogen, oxygen, or water vapor). Of aU the obtained ions, only cations of the desired m/z... 

A more sophisticated, though less common version of PTR-MS is selected ion flow tube mass spectrometry (SIFT-MS). In this technique, a mixture of reagent ions is generated in a gas discharge ion source and then a Q mass filter is used to select one reagent ion, which is then injected into an inert carrier gas (usually He), for... 

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. 

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

---