PHPMS

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. 

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

With respect to the practical considerations of gas flow and vacuum requirements, the PHPMS experiment might, upon cursory consideration, appear to be easily extended into the VHP region. That is, several MS-based analysis techniques routinely use ion source pressures of 1 atm. However, when an attempt to increase the pressure within a PHPMS ion source is made, the factors that do become problematic are those related to the subtle principles on which the method is based. Most importantly, the PHPMS method requires that the fundamental mode of diffusion be quickly established within the ion source after each e-beam pulse, so that all ions are transported to the walls in accordance with a simple first-order rate law while the IM reactions of interest are occurring. This ensures that a constant relationship exists between the ion density in the cell and the detected ion signal. The rates of the IM reactions can then be quantitatively determined from the observed time dependencies of the reactant ion signal because the contribution of diffusion to the time dependencies are well known and easily accounted for. 

Figure 3. Chloride reactant ion decay curves obtained by the PHPMS ion source shown in Figure 2 for the n2 nucleophilic displacement reaction, CP + CHjBr CH3CI + Br", in methane buffer gas at a pressure of 62 torr and a temperature of 125 °C. Cl" is made by electron attachment to CCI4 (partial pressure = 0.06 mtorr). For curve A, no CH3Br was present so that the loss of Cl" was determined only by the physical processes, diffusion and/or ion-ion recombination. For curves B and C, the concentration of CH3Br was 1.1 x 10 and 2.1 x 10 moiecules/cm, respectively.

One additional point of concern in the extension of the PHPMS technique to elevated pressures is that the criteria normally considered to be necessary for accurate aperture sampling of a high-pressure ion source will probably be violated. A condition of molecular flow through the aperture is generally required for accurate sampling. Molecular flow occurs when the critical dimension (such as the diameter of an orifice or the width of an exit slit) is equal to or smaller than the... 

The TRAPI was developed by Matsuoka and co-workers " and has been used to determine the rate constants of about a dozen IM reactions at atmospheric pressure. As a first approximation, the TRAPI experiment might be described as an atmospheric pressure version of the PHPMS with initial ionization caused by a pulsed X-ray source. The X-rays cause relatively even ionization throughout the 6.4-cm ion source volume by penetrating through thin sections of the ion source walls formed by 25- im thick molybdenum foil. A 16-pm ion-sampling aperture is located at the center of one of these thin walls. The ions that pass through this aperture are measured by an associated mass spectrometer as a function of time after the X-ray pulse. 

Even though the TRAPI and the PHPMS both involve a pulsed high-pressure ion source, they are fundamentally different methods in that they are based on different underlying principles. In the TRAPI method, ions are transported through the sampling aperture by convective flow with the buffer gas, rather than by diffusion through the buffer gas. Also, second-order ion-ion or ion-electron recombination... 

Note The acronyms used here are OSPED (optical spectroscopy in a pulsed electrical discharge), FAMS (flowing afterglow mass spectrometry), SIFT (selected ion flow tube), TRAPI (time-resolved atmospheric pressure ionization mass spectrometry), PHPMS (pulsed high-pressure ionization mass spectrometry), ICRMS (ion cyclotron resonance mass spectrometry), and ADO (averaged dipole orientation collision rate theory). 

PHPMS pulsed high pressure mass spectrometry... 

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. 

PHPMS data from reference Hiraoka, K. Yamabe, S. J. Chem. Phys. 1989, 90, 3268-3273. 

Note Values in brackets are PHPMS data from reference NIST. NIST Chemistry Webbook. 1998. 

The graph of k" vs [RBr] is a straight line of slope kj and the value for 24 was dnplicated in a different experiment involving mass spectrometric detection with the drift tnbe acting as a reactor with the shatter open continuously. The intensities of Cr and Br" were measnred as a fnnction of RBr concentration with reaction time the same as for the pulsed mode a plot of ln(/g //(, + /g ) vs [RBr] yielded a straight line of slope -k24t. The resnlts obtained for the four alkyl bromides by the area method and the mass spectrometric method are compared in Table 13.3 with those obtained from a PHPMS stndy completed at ca 4 nun Hg [39],... 

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