Electron bombardment flow

Effusive beam technique, 157-158 Electron bombardment flow radiolysis, 238 Electrospray ionization and ionic clusters, 168 Enantiomers, separation techniques, 154-155 Enantioselectivity of enzymes, 148 Enthalpy-entropy compensation plots, 261 Enthalpy of activation, and quantum tunneling, 67, 70-71... 

The above experiments are generally difficult to perform and the interpretation of the results may not necessarily be straightforward. The low abundance of the neutral products collected and the likelihood of mass spectral interference between reagents and products make these techniques applicable only to special cases. An independent approach to this problem has been proposed by Marinelli and Morton (1978) who have used an electron-bombardment flow reactor allowing in principle for larger collection of neutral products followed by glc and mass spectral analysis. 

Further support for the idea that cationic nucleophilic displacement occurs with inversion of configuration has been advanced by Hall et al. (1981). The study of reaction (55) in an electron-bombardment flow reactor at reagent pressures below 10 3torr, followed by neutral product analysis (Marinelli and Morton, 1978), reveals that these reactions also occur via backside attack. This is in disagreement with the original suggestion of Beauchamp et al. (1974) who proposed a frontside displacement in the case of t-butyl alcohol. 

Second, for the elucidation of ion structures by a procedure which involves trapping of the carbocation of interest, R+, via halide abstraction from a suitable donor, R X, to form a neutral, RX, which is subsequently isolated and characterized. From the structure of the neutral product RX, that of the precursor ion, R+, can be confidently inferred. This procedure is usually carried out by means of either the electron bombardment flow (EBFlow)2 or the radiolytic techniques3. 

Ring closure of nascent CH2=CHCH2CH2CHf within ion-neutral complexes has been studied using a specially designed Electron Bombardment Flow (EBFlow) reactor, schematically drawn in Figure 3. This apparatus has the advantage that the conditions under which ions are formed and react (70 eV electron impact pressure <0.001 mbar) closely parallel those in mass spectrometer sources. The neutral product yields are routinely interpreted with reference to the ionic products observed by the mass spectrometry. Hypotheses based on EBFlow results for ion neutral complexes are further tested by comparison with mass spectrometry. 

Figure 3, Schematic of an Electron Bombardment Flow (EBFiow) reactor. The magnetic field maintains the electron beam along the axis of the solenoid. The beam s negative charge prevents cations from migrating to the walls, and all charged species are dumped into the differentially pumped region downstream of the clown cap, where they are pumped away and do not contaminate the neutrals collected in the cold trap, which come from the EBFiow reaction vessel.

Fig. 13.5. Tip annealing methods, (a) Electron bombardment. A filament, biased negatively with regard to the tip, emits electrons to heat up the tip. (b) Resistive heating by a W filament. The tip is spotwelded to the filament. After heating, the tip is removed from the chamber and detached from the filament, and put into the vacuum chamber quickly, (c) Using the tip shank as the heating element. The tip is made in touch with a thicker W wire, which is connected to a power supply. Current flows through the tip shank to the ground.

An alternative method of creating atoms and radicals in excited electronic states is the use of discharge methods in fast flow systems. This topic has recently been reviewed by Kolts and Setser [70]. Metal atoms have been produced in excited states by simple d.c. discharges [71] as well as by optical pumping [72]. Electronically excited inert gas and metal atoms may be produced by electron bombardment [73, 74]. 

The techniques of Flowing Afterglow, where the flow of gases is submitted to ionization by electron bombardment " ... 

Sharp and Paterson [41] used a Perkin Elmer filament pyrolysis unit fitted in a Perkin Elmer Ell gas chromatograph (pyrolysis temperature control 250-550 °C). The gas chromatographic column used is a 1.7 m x 3 mm od stainless steel column packed with 30% m/m silicone oil (Embaphase) on acid washed Celite, operated at 80 "C with a helium flow rate of 30 ml/min. The column effluent is split in the ratio 2 1 between a flame ionisation detector and an API MS12 mass spectrometer equipped with a glass fit type of molecular separator at 150 C. Mass spectra are scanned from m/e 200 to 20 at 8 seconds per decade under standard electron bombardment conditions, electron energy 70 eV, emission current 500 pA, accelerating voltage 8 kV and source temperature 200 °C. 

Two of the emission sources for d - c IT excitation, electric discharges in flowing He-N2"H2 (D2) mixtures and electron bombardment of NH3, also excite the d b system, the vibrational bands of which show single P and R branches characteristic of a transition. The spectra of NH and ND generated by electric discharges were... 

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