Terminal ions in weak atmospheric pressure plasmas. Applications of atmospheric pressure ionization to trace impurity analysis in gases

The positive APCI-mass spectra in air of linear (n-pentane, n-hexane, n-heptane, n-octane), branched [2,4-dimethylpentane, 2,2-dimethylpentane and 2,2,4-trimethylpentane (i-octane)], and cyclic (cyclohexane) alkanes were analyzed at different mixing ratios and temperatures. The effect of air humidity was also investigated. Complex ion chemistry is observed as a result of the interplay of several different reagent ions, including atmospheric ions O 2 ϩ• , NO ϩ , H 3 O ϩ , and their hydrates, but also alkyl fragment ions derived from the alkanes. Some of these reactions are known from previous selected ion/molecule reaction studies; others are so far unreported. The major ion formed from most alkanes (M) is the species [M Ϫ H] ϩ , which is accompanied by M ϩ• only in the case of n-octane. Ionic fragments of C n H 2nϩ1 ϩ composition are also observed, particularly with branched alkanes: the relative abundance of such fragments with respect to that of [M Ϫ H] ϩ decreases with increasing concentration of M, thus suggesting that they react with M via hydride abstraction. The branched C 7 and C 8 alkanes react with NO ϩ to form a C 4 H 10 NO ϩ ion product, which upon collisional activation dissociates via HNO elimination. The structure of t-Bu ϩ (HNO) is proposed for such species, which is reasonably formed from the original NO ϩ (M) ion/molecule complex via hydride transfer and olefin elimination. . For air pollution control, different types of reactors and power supplies are being developed and tested for production of nonthermal plasmas by means of electrical discharges [3]. Following an initial phase devoted to prove the feasibility and competitiveness of the process in terms of energy and cost efficiency, research is presently focusing on the characterization of the chemistry-i.e., the products being released and the underlying chemical mechanisms-to develop a more efficient and cleaner process [5].In nonthermal plasmas, which are conveniently produced by electric corona discharges in air at atmospheric pressure, high-energy electrons induce ionization, excitation, and dissociation of the bulk gas molecules, N 2 and O 2 . The resulting reactive species, ionic and neutral as well as thermalized electrons, can attack molecules of organic pollutants present in the air, such as hydrocarbons and other volatile organic compounds (VOCs), and initiate a chain of reactions leading eventually to their oxidation. It is generally accepted that nonthermal plasma-induced VOC oxidation proceeds via VOC-derived organic radicals, R • , which are trapped by molecular oxygen and undergo further reactions as described for their tropospheric oxidation [6], leading eventually to CO 2 . As for the origin of such VOC-derived radicals, i.e., the nature of the initiation steps of nonthermal plasma-induced VOC decay, reactions of VOC molecules with neutrals and radicals, notably O( 3 P) and • OH, are usually envisioned. However, increasing numbers of literature reports suggest that in some cases ionic initiation steps might prevail [7][8][9...

Section: Cyclohexanementioning

confidence: 99%

A mass spectrometry study of alkanes in air plasma at atmospheric pressure

Marotta

Paradisi

2008

“…The primary ions are assumed to be distributed uniformly throughout the cylindrical volume defined by the cross-sectional area of the vapor conduit (d ϭ 0.7 cm), and the width of the intersecting photon beam (w Ϸ 0.4 cm); thus, V Ϸ 0.15 cm 3 . For the sake of simplicity, following°the°example°of°Siegel°and°Fite° [35],°we assume a single universal recombination rate constant of about 10 Ϫ6 cm 3 s Ϫ1 . A typical volumetric flow rate is ϳ120 cm 3 s Ϫ1 (7.0 l min Ϫ1 ).…”

Section: Ion Lossmentioning

confidence: 99%

Effects of solvent flow, dopant flow, and lamp current on dopant-assisted atmospheric pressure photoionization (DA-APPI) for LC-MS. Ionizationvia proton transfer

Robb

Blades

2005

In this paper, the effects of solvent flow, dopant flow, and lamp power on proton transfer ionization in dopant-assisted (DA) atmospheric pressure photoionization (APPI) are investigated. A broad theoretical framework is presented, describing the primary photoionization process, the formation of protonated-solvent cluster ions, and the balance between analyte ion creation via proton transfer and loss via recombination. The principal experimental test system utilized methanol as the solvent, toluene as the dopant, and acridine as the analyte. Comparisons are made between acridine and a less basic compound, 9-methylanthracene (9-MA). Experimental determinations of the trends in the analyte MH ϩ signal and the total ion current (TIC) with variations in the subject parameters are provided. Experimental results and theory demonstrate that both the analyte signal and the TIC approach asymptotic limits with increases in dopant flow and/or lamp current (two factors which dictate the rate of photoion generation). The data show that these limits are lowered at higher solvent flow rates. These results are attributed to the recombination loss process, the rate of which increases with the second power of ion concentration. We deduce that the recombination rate constant increases with solvent flow rate, a consequence of the growth of ion-solvent clusters. Cluster growth is also believed to be a factor in the dramatic loss of sensitivity for 9-MA that occurs as the solvent flow is raised, because larger protonated-solvent cluster ions have greater solvation energies and may be unreactive with compounds having low gas-phase basicity and/or low solvation energy. (J Am Soc Mass Spectrom 2005, 16, 1275-1290) © 2005 American Society for Mass Spectrometry P hotoionization (PI) is the latest means of ionization to be incorporated into atmospheric pressure ionization (API) sources for liquid chromatography-mass spectrometry (LC-MS). The original motivation for the development of atmospheric pressure photoionization (APPI) sources was the demand for a method or device capable of expanding the range of compounds amenable to LC-MS to include nonpolar compounds not readily ionized by either electrospray [1,2] or atmospheric pressure chemical ionization (APCI) [3,4]. In recent years, two approaches towards utilizing PI at atmospheric pressure have emerged: dopant-assisted (DA) APPI [5] and direct APPI [6]. Recent review papers provide details of the two APPI-MS methods [7,8]. This paper is concerned mainly with DA-APPI.As is often the case for new technologies, the practical application of DA-APPI has outpaced the development of detailed knowledge regarding the mechanisms responsible for its performance. DA-APPI relies upon gas-phase ion-molecule reactions to place a charge on neutral analytes, so it is especially important that these reactions be well understood. The groups of Kostiainen and Bruins have completed several studies of the reaction chemistry of DA-APPI and the effects of solvent and dopant composition on the ionization effici...

“…In such cases, a single ion-electron and ion-ion recombination coefficient approximately equal to 1 ϫ 10 Ϫ6 cm 3 -s Ϫ1 is assumed [14].…”

Section: Functional Dynamicsmentioning

confidence: 99%

“…APCI is effected by a sequence of electron-and ion-molecule reactions initiated by electrons that are generally produced via either a corona discharge [12] or a ␤-emitter such as 63 Ni [1,13]. Because the equilibrium distribution in such reactions under normal APCI operating conditions often favors terminal ions that are characteristic of trace impurities rather than major components of the support gas [14], the ionization method has proven to be highly sensitive in analytical applications. For the same reason, however, APCI can be subject to matrix effects from compounds that compete for charge.…”

mentioning

confidence: 99%

Factors influencing the analytical performance of an atmospheric sampling glow discharge ionization source as revealed via ionization dynamics modeling

Goeringer

2003

A kinetic model is developed for the dynamic events occurring within an atmospheric sampling glow discharge that affect its performance as an ion source for analytical mass spectrometry. The differential equations incorporate secondary electron generation and thermalization, reagent and analyte ion formation via electron capture and ion-molecule reactions, ion loss via recombination processes, diffusion, and ion-molecule reactions with matrix components, and the sampling and pumping parameters of the source. Because the ion source has a flow-through configuration, the number densities of selected species can be estimated by applying the steady-state assumption. However, understanding of its operation is aided by knowledge of the dynamic behavior, so numerical methods are applied to examine the time dependence of those species as well. As in other plasma ionization sources, the ionization efficiency is essentially determined by the ratio of the relevant ion formation and recombination rates. Although thermal electron and positive reagent ion number densities are comparable, the electron capture/ion-molecule reaction rate coefficient ratio is normally quite large and the ion-electron recombination rate coefficient is about an order of magnitude greater than that for ion-ion recombination. Consequently, the efficiency for negative analyte ion formation via electron capture is generally superior to that for positive analyte ion generation via ion-molecule reaction. However, the efficiency for positive analyte ion formation should be equal to or better than that for negative analyte ions when both ionization processes occur via ion-molecule reaction processes (with comparable rate coefficients), since the negative reagent ion density is considerably less than that for positive reagent ions. Furthermore, the particularly high number densities of thermal electrons and reagent ions leads to a large dynamic range of linear response for the source. Simulation results also suggest that analyte ion number densities might be enhanced by modification of the standard physical and operating parameters of the source. . In these and other applications, a number of methods have been utilized for generation of ions from compounds of interest. Electron ionization (EI) has been employed for formation of positive ions in situations for which air is leaked directly into the mass spectrometer [5,6]. To enhance ionization selectivity over that exhibited by EI, single-and multi-photon ionization techniques have been used for determination of targeted aromatic compounds in automobile exhaust [7][8][9][10]. More widely used approaches for direct sampling are based on atmospheric pressure chemical ionization (APCI) [11] which is capable of making positive or negative ions. APCI is effected by a sequence of electron-and ion-molecule reactions initiated by electrons that are generally produced via either a corona discharge [12] or a ␤-emitter such as 63 Ni [1,13]. Because the equilibrium distribution in such reactions under normal APCI operati...