Flowing afterglow mass spectrometry

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). 

Freitas and O Hair133 studied the gas-phase reactivity of aniline and other nucleophiles toward the methoxymethyl cation, CH3 — 0+=CH2, an ambident electrophile, using flowing afterglow mass spectrometry. For aniline, two reaction channels were observed addition followed by elimination of methanol with concomitant [M + CH]+ ion formation and adduct formation, viz. [M + CH3OCH2]+. 

Authors note The period of the late 1960s to 1980 saw an explosion in ion-molecule chemistry too large to document here and included high-pressure mass spectrometry, flowing afterglow mass spectrometry, and more. 

Fig. 4.7 Schematic diagram of a flowing afterglow mass spectrometry setup. The system is setup to perform reactions with H3O+. Other primary ions can be generated by other types of ion sources. (Adapted from ref [121] and Smith, D. panel, R Selected ion flow tube mass spectrometry (SlFT-MS) for online trace gas analysis. Mass Spectrom Rev. 24, 2005, 661-700] with permission of Wiley. 2001 and 2005, Wiley, Ltd.)...

Smith D, Spanel P. On-line determination of the deuterium abundance in breath water vapour by flowing afterglow mass spectrometry with applications to measurements of total body water. Rapid Commun Mass Spectrom. 2001 15 25-32. 

Tan BK, Smith D, Spanel P, Davies S J. Dispersal kinetics of deuterated water in the lungs and airways following mouth inhalation real-time breath analysis by flowing afterglow mass spectrometry (EA-MS). J Breath Res. 2010 4 017109. 

Tandem mass spectrometry H6,i46) both stationary 116> and flowing afterglow-methods 118,147) and drift tube techniques U6> have also been applied to some of the clustering reactions. Results for the gas-phase solvation of H+ by H2O and NH3 generally agree well with the values obtained by high pressure mass spectro-metric observations 148). 

In the present review, a new variation on an existing experimental method will be used to show how accurate unimolecular dissociation rate constants can be derived for thermal systems. For example, thermal bimolecular reactions are amenable to study by use of several, now well-known, techniques such as (Fourier transform) ion cyclotron resonance spectrometry (FTICR), flowing afterglow (FA), and high-pressure mass spectrometry (HPMS). In systems where a bimolecular reaction leads to products other than a simple association adduct, the bimolecular reaction can always be thought of as containing a unimolecular... 

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... 

Introduction 198 Experimental techniques 200 Ion cyclotron resonance spectrometry 201 Flowing afterglow 203 High pressure mass spectrometry 204 General features of gas-phase ion-molecule reactions 204 Gas-phase SN2 reactions involving negative ions 206 Thermochemical considerations 206 General aspects of gas-phase SN2 reactions 207 Stereochemistry 209... 

Three basic techniques, and variations thereof, have been used in recent years to study aspects of gas-phase ion-molecule reactions pertinent to organic systems they are ion cyclotron resonance spectrometry, flowing afterglow, and high pressure mass spectrometry. The essential feature of these techniques is that ions produced under vacuum are allowed to undergo from few to many collisions with neutrals before they are neutralized at the walls of the instrument. 

Gas-phase acid-base studies are usually performed by using one of the following techniques high-pressure mass spectrometry (HPMS), chemical ionization mass spectroscopy (CIMS) with mass-analysed ion kinetic energy spectroscopy/collision induced dissociation (MIKES/CID), flowing afterglow (FA) or ion cyclotron resonance (ICR) spectrometry. For a brief description of all methods, Reference 8 should be consulted. 

The gas phase acid/base properties of molecules have been subject to equilibrium or bracketing measurements employing mass spectrometric techniques like ion cyclotron resonance (ICR) [4], Fourier transform ion cyclotron resonance (FT-ICR) [5,6], Flowing afterglow (FA) and Selected ion flow tube (SIFT) [7], and high pressure mass spectrometry (HPMS) [8]. Proton transfer between neutral molecules are then investigated by measurements of reactions... 

Gas-phase acidities and basicities for many organic compounds are now available, primarily due to the development within the past decades of three new experimental techniques pulsed high-pressure i.e. 0.1... 1300 Pa) mass spectrometry (HPMS) [22, 23, 118], the flowing afterglow (FA) technique with a fast-flowing gas like helium in the pressure range of ca. 10 . .. 10 Pa [119], and pulsed electron beam, trapped ion cell, ion cyclotron resonance (ICR) spectrometry, carried out at ca. 10 ... 10 Pa [24-26, 115]. 

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. 

A few examples are given in the table below of reactions of PH3 with other atoms or radicals and of ion-molecule reactions, where PHg was not detected but was assumed to have been formed (abbreviations k = rate constant, f = vibrational excitation, MW = microwave, UHF = ul-trahlgh frequency, FP=flash photolysis, LIF=laser-induced fluorescence, MS=mass spectrometry, ICR = ion cyclotron resonance, FA = flowing afterglow). 

Relative proton affinities are determined by measuring the enthalpy of proton transfer from one base B to a second base B (equation 6), using high-pressure mass spectrometry, the SIFT/flow afterglow or ion cyclotron resonance techniques. This overlap method achieves an ordering of proton affinities, but to obtain absolute values these must be anchored on some independent measurement. 

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