Pulsed high-pressure mass

Williamson, D.H. Knighton, W.B. Grimsrud, E.P. Effect of Buffer Gas Alterations on the Thermal Electron Attachment and Detachment Reactions of Azu-lene by Pulsed High Pressure Mass Spectrometry. Int. J. Mass Spectrom. 2000, 795/796,481-489. 

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. 

At the higher pressures of other ion-molecule techniques, such as flowing afterglow or pulsed high-pressure mass spectrometry," both of which operate with a bath gas pressure of about 1 torr, collisions of such an excited intermediate with the bath gas occur on a nanosecond to microsecond time-scale, in competition with the unimolecular dissociation rate. For these techniques, ions that are the... 

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

Understanding the behavior of organic bases in solution requires some knowledge of their gas phase (intrinsic) basicities (proton affinities (PA)). These can be determined by ICR methods or by variable-temperature pulsed high-pressure mass spectrometry. Both methods afford basicities (termed thermodynamic vs. kinetic basicity), which have been compared in (91JOC179). 

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. 

Three new experimental techniques, developed within the past decades, now make it possible to study ionic reactions in the gas phase as well. These are pulsed ion-cyclotron-resonance (ICR) mass spectrometry, pulsed high-pressure mass spectrometry (HPMS), and the flowing afterglow (FA) technique [469-478 see also the references given in Section 4.2.2]. Although their approaches are quite independent, the results obtained for acid/base and other ionic reactions agree within an experimental error of 0.4... 1.3 kJ/mol (0.1... 0.3 kcal/mol) and are considered as reliable as those obtained in solution. 

PHPMS pulsed high pressure mass spectrometry... 

Electron affinities of molecules are of interest not only in gas-phase reactions, e.g. in negative chemical ionization mass spectrometry, but also in the field of condensed-phase chemistry. It is characteristic that negative ions are by far not studied to the same level of detail as the corresponding positive ions. However, during the last decade a large number of EA determinations based on measurements of electron transfer equilibria utilizing pulsed high-pressure mass spectrometry have been reported . ... 

High-pressure mass spectrometry and pulsed high-pressure mass spectrometry have also been a source of thermochemical data for organometaiiic compounds (e.g., proton affinities, electron affinities, and bond dissociation enthalpies). The essential difference between both techniques is the fact that in the former the reactants are produced outside the collision cell. 

In spite of the advantages cited above, ion mobility spectrometers operating at atmospheric pressure have been used infrequently to obtain physical chemical data, kinetic and thermodynamic, in the study of ion/molecule chemistry. In this chapter, an overview is given on the type of information obtainable from ion mobility studies at atmospheric pressure and the variety of experimental methods employed in such studies. The data obtained under weU-defined conditions agree favorably with those from other more frequently used methods, for example (i) pulsed high pressure mass spectrometry (PHPMS), which is operated at well-defined temperatures but at pressures ca 200 times lower than IMS and (ii) FT-ICR and ion trap mass spectrometers, which are operated under vacuum. 

Thermochemical data (AH and AG in kJ/mol, AS in J mor K" ) were derived from pulsed, high-pressure mass spectrometric studies of the association reactions N2H +X2 where X = H2 or N2 ... 

---