Flowing afterglow mass spectrometer

The sequential removal of H and H+ from isobutene-type structural units (so-called H2+ abstraction ) was also used to generate the radical anion of non-Kekule benzene , i.e. l,3-dimethylenecyclobutane-l,3-diyl (39) (Scheme 11). As shown by Hill and Squires161, this highly unusual, distonic C(,II(, isomer can be produced in pure form by reaction of O with 1,3-dimethylenecyclobutane (38). Working in a flowing afterglow mass spectrometer, subsequent reactions were again used to characterize this radical anion and differentiate it from other ( VdL, isomers. 

Figure 7. Schematic diagram of a flowing-afterglow electron-ion experiment. The diameter of flow tubes is typically 5 to 10 cm and the length is 1 to 2 meters. The carrier gas (helium) enters through the discharge and flows with a velocity of 50 to 100 m/s towards the downstream end of the tube where it exits into a fast pump. Recombination occurs mainly in the region 10 to 20 cm downstream from the movable reagent inlet, at which the ions under study are produced by ion-molecule reactions. The Langmuir probe measures the variation of the electron density in that region. A differentially pumped mass spectrometer is used to determine which ion species are present in the plasma.

The complementary techniques for determining rate constants for thermal electron attachment, detachment, and dissociation are the flowing afterglow, the microwave technique, the ion cyclotron resonance procedures, the swarm and beam procedures, the shock tube techniques, the detailed balancing procedures, the measurement of ion formation and decay, and the high-pressure mass spectrometer procedures. In all cases the measurement of an ion or electron concentration is made as a function of time so that kinetic information is obtained. In the determination of lifetimes for ions, a limiting value of the ion decay rate or k is obtained. 

Figure 6, The flowing afterglow-tandem mass spectrometer. Hypervalent anions are formed and cooled in the flow tube, and collision-induced dissociation occurs in the quadrupole-octopole-quadrupole tandem mass spectrometer.

Most of the studies in this decade were carried out with conventional single source mass spectrometers, which limited the kind and accuracy of the information. During the next decade, however, various sophisticated techniques for the study of ion—molecule reactions, such as tandem mass spectrometers, photoionization sources, pulsed sources, flowing afterglow and drift tube methods, crossed and merging beams and ion cyclotron resonance, have been developed. Much detailed information on various aspects of ion—molecule reactions has accumulated, and this has consequently stimulated the theoretical studies as well. This decade was, so to speak, the second epoch in the history of ion—molecule studies. 

Reaction (102) has also been searched for in a tandem mass spectrometer [95] and a flowing afterglow experiment [164]. In both studies it was not observed, in agreement with the result of single source mass spectrometer experiments. These studies set the upper limits of the cross-section for reaction (102) at 6 x 10 cm (for He energies from 1 to 10 eV) and at 10" cm (for thermal energy reactions), respectively. 

Apart from the proton transfer reactions discussed in Section II, phosphorus species undergo a range of other ion-molecule reactions in the gas phase. The types of instruments which have been used to study ion-molecule reactions of phosphorus species include ion cyclotron resonance (ICR) mass spectrometers and the related FT-ICR instruments, flowing afterglow (FA) instruments and their related selected-ion flow tubes (SIFT) and also more conventional instruments This section is divided into four topics (A) positive ion-molecule reactions (B) negative ion-molecule reactions (C) neutralization-reionization reactions and (D) phosphorus-carbon bond formation reactions. 

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