High pressure mass spectrometer

The thenuodynamic quantities are derived from equilibrium measurements as a fiinction of temperature. The measurements are frequently made in a high-pressure mass spectrometer [107]. The pertinent equation is In... 

Kebarle P 1988 Pulsed electron high pressure mass spectrometer Techniques for the Study of Ion-Molecule Reactions ed J M Farrar and W FI Saunders (New York Wiley-Interscience)... 

Method (b) corresponds to the usual method of investigating ion-molecule reactions in a high pressure mass spectrometer although charge exchange with slow ions is used instead of electron impact. After preliminary work (9, 23), the method was fully developed by Szabo 20, 21, 22). 

The important and stimulating contributions of Kebarle and co-workers 119 14 > provide most of the data on gas-phase solvation. Several kinds of high pressure mass spectrometers have been constructed, using a-particles 121>, proton- 123>, and electron beams 144> or thermionic sources 128> as primary high-pressure ion sources. Once the solute A has been produced in the reaction chamber in the presence of solvent vapor (in the torr region), it starts to react with the solvent molecules to yield clusters of different sizes. The equilibrium concentrations of the clusters are reached within a short time, depending on the kinetic data for the... 

The instruments used for the experimental work detailed in this review are several high-pressure mass spectrometers (HPMS) and a Fourier transform ion cyclotron resonance spectrometer (FTICR). Each of the instruments was constructed, to a considerable degree, in-house at the University of Waterloo, and each contains features unique to its type of apparatus. The instruments in general and the unique features of the Waterloo apparatus in particular are described below. 

Figure 1. Schematic of the reverse geometry double-focusing high-pressure mass spectrometer.

B. The Pulsed High-Pressure Mass Spectrometer (PHPMS).231... 

Stone and coworkers determined the /3-silicon effect in w-alkyl- and aryl-substituted carbenium ions20 and vinyl cations21 by measuring in a high-pressure mass spectrometer the thermodynamic data for the association of various alkenes and alkynes with trimethylsilylium ion. From their measured thermodynamic data they calculated, by using equations 13 and 14, the /S-silyl stabilization energies listed in Table 1. 

Concerning the first field of application, the kinetics and equilibrium constants for several halide transfer reactions (equation 1) were measured in a pulsed electron high pressure mass spectrometer (HPMS)4 or in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR)5. From measurements of equilibrium constants performed at different temperatures, experimental values were obtained for the thermochemical quantities AG°, AH° and AS° for the reaction of equation 1. The heat of formation (AH°) of any carbocation of interest, R+, was then calculated from the AH0 of reaction and the AH° values of the other species (RC1, R Cl and R +) involved. 

The free energy for electron attachememt (AGa°) was determined for perfluorobenzenes C6F5X (X = F, Cl, Br, CF3, COMe, CHO, CN, N02, C6F5, COC6F5) by measuring the electron transfer equilibria with some reference anions A- (A was primarily S02) in a pulsed high pressure mass spectrometer (equation 29)273. 

Additional evidence for the occurrence of gas-phase intermolecular and intramolecular nucleophilic substitution was obtained in an investigation on the reactivity of mono- and dichlorophenols within the high-pressure mass spectrometer source under conditions of argon-enhanced negative ion mass spectrometry282. It was shown that the reactant anions involved in these processes are derived exclusively from the chlorophenols and not from possible impurities such as residual oxygen, water, etc. Thus, for example, the formation of an abundant [2M - H - Cl]" adduct was attributed to an intermolecular nucleophilic Cl displacement by an [M - H]" chlorophenoxide ion282. 

The equilibrium constant for the case X = Cl and i = 1 agrees well with that obtained in a pulsed electron beam high pressure mass spectrometer, the other data have not been previously reported. 

The data thus obtained have been supplemented by relative heats of formation obtained by the study of proton, hydride, and halide transfer equilibria [Eqs. (3, 4)] in a high-pressure mass spectrometer, flow tube, or ion cyclotron resonance spectrometer [20]. 

The first ion-molecule reactions studied in the gas phase under quasiequilibrium conditions in a high-pressure mass spectrometer were... 

The rate constant at 86 K for the formation of Hs (or D5) has been measured using a high-pressure mass spectrometer.219 Comparison of the result with that at 300 K indicates that the reaction ... 

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. 

A striking feature of the ionic reaction scheme for ethyl chloride which is evident from examining the reactions above is the fact that virtually all of these processes lead either to the protonated molecule ion or to other precursors of this product. In addition, the protonated species is efficiently converted to the C4Hi0C1+ ion hence, one expects that at higher pressures there will be essentially only this one ionic species present in any appreciable quantity. This conclusion is also supported by the high pressure mass spectrometer data discussed below. 

The assignment of the well depth for H20 + H2O was speculative and based on the failure to observe H20 H2O in chemical ionization experiments. This value may well be incorrect since the ion mje = 36 (assigned the structure H30 OH) has been observed both in a flowing afterglow and in an ultra-high-pressure mass spectrometer at 300°K (see Chapter 8, Section 2.4). 

Several high-pressure mass spectrometers were used at Alberta a high-pressure alpha-particle instrument capable of operating at pressures up to 200-300 Torr, " a 100-keV proton-beam mass spectrometer, and a 4-keV pulsed electron-beam instrument. 

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