Atmospheric Pressure Photoionization Mass Spectrometry. Ionization Mechanism and the Effect of Solvent on the Ionization of Naphthalenes

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116

The ionization mechanism of negative ion-direct analysis in real time (NI-DART) has been investigated using over 42 compounds, including fullerenes, perfluorocarbons (PFC), organic explosives, phenols, pentafluorobenzyl (PFB) derivatized phenols, anilines, and carboxylic acids, which were previously studied by negative ion-atmospheric pressure photoionization (NI-APPI). NI-DART generated ionization products similar to NI-APPI, which led to four ionization mechanisms, including electron capture (EC), dissociative EC, proton transfer, and anion attachment. These four ionization mechanisms make both NI-DART and NI-APPI capable of ionizing a wider range of compounds than negative ion-atmospheric pressure chemical ionization (APCI) or negative ion-electrospray ionization (ESI). As the operation of NI-DART is much easier than that of NI-APPI and the gas-phase ion chemistry of NI-DART is more easily manipulated than that of NI-APPI, NI-DART can be therefore used to study in detail the ionization mechanism of LC/NI-APPI-MS, which would be a powerful methodology for the quantification of low-polarity compounds. Herein, one such application has been further demonstrated in the detection and identification of background ions from LC solvents and APPI dopants, including water, acetonitrile, chloroform, methylene chloride, methanol, 2-propanol, hexanes, heptane, cyclohexane, acetone, tetrahydrofuran (THF), 1,4-dioxane, toluene, and anisole. Possible reaction pathways leading to the formation of these background ions were further inferred. One of the conclusions from these experiments is that THF and 1,4-dioxane are inappropriate to be used as solvents and/or dopants for LC/NI-APPI-MS due to their high reactivity with source basic ions, leading to many reactant ions in the background. [7] and atmospheric pressure photoionization (APPI) [8,9], these new ionization methods are especially successful in the analysis of compounds on a variety of surfaces, including concrete, human skin, currency, airline boarding passes, fruits, vegetables, cloth, drug tablets, and biological tissues without sample preparation. Of these new ionization methods, DART is particularly interesting due to its distinct ionization mechanism, while the others have close relationships with their respective traditional API methods.DART ionization begins with a stream of gas, usually helium, which is electrically discharged to produce ions, electrons, and metastable species. After heating and removal of charged particles, this stream of gas exits the DART source into the open air, and is able to ionize chemicals by instant contact, therefore permitting the analysis of gases, liquids, and solids. Although the DART ionization mechanisms are not yet fully understood, it has been proposed [1] that in the positive ion (PI) mode, metastable helium atoms induce Penning ionization of ambient water in the open air, generating protonated water clusters, mostly H 5 O 2 ϩ , which further ionize analytes through chemical reactions. In the negative ion (NI) mode; however,...

Section: Analysis Of Lc Solvents and Appi Dopantsmentioning

confidence: 99%

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confidence: 99%

Ionization mechanism of negative ion-direct analysis in real time: A comparative study with negative ion-atmospheric pressure photoionization

Song

Dykstra

Yao

et al. 2009

129

116

“…The aim of this work is to study, in detail, the relevant reactions so that LC/EC-APPI-MS development can be better guided toward analysis of low polarity compounds in the future. The basic principle of APPI is the formation of a molecular radical cation caused by the absorption of a photon by a molecule, and is accompanied by the formation of a thermal electron [9]. In dopant assisted negative ion-APPI, the ionization process is initiated by thermal electrons.…”

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confidence: 99%

“…The basic principle of APPI is the formation of a molecular radical cation caused by the absorption of a photon by a molecule, and is accompanied by the formation of a thermal electron [9]. In dopant assisted negative ion-APPI, the ionization process is initiated by thermal electrons.…”

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confidence: 99%

Negative ion-atmospheric pressure photoionization: Electron capture, dissociative electron capture, proton transfer, and anion attachment

Song

Wellman

Yao

et al. 2007

To better guide the development of liquid chromatography/electron capture-atmospheric pressure photoionization-mass spectrometry (LC/EC-APPI-MS) in analysis of low polarity compounds, the ionization mechanism of 19 compounds was studied using dopant assisted negative ion-APPI. Four ionization mechanisms, i.e., EC, dissociative EC, proton transfer, and anion attachment, were identified as being responsible for the ionization of the studied compounds. The mechanisms were found to sometimes compete with each other, resulting in multiple ionization products from the same molecule. However, dissociative EC and proton transfer could also combine to generate the same [M Ϫ H] Ϫ ions. Experimental evidence suggests that O 2 Ϫ· , which was directly observed in the APPI source, plays a key role in the formation of [M Ϫ H] Ϫ ions by way of proton transfer. Introduction of anions more basic than O 2 Ϫ· , i.e., C 6 H 5 CH 2 Ϫ , into the APPI source, via addition of di-tert-butyl peroxide in the solvent and/or dopant, i.e., toluene, enhanced the deprotonation ability of negative ion-APPI. Although the use of halogenated solvents could hinder efficient EC, dissociative EC, and proton transfer of negative ion-APPI due to their EC ability, the subsequently generated halide anions promoted halide attachment to compounds that otherwise could not be efficiently ionized. With the four available ionization mechanisms, it becomes obvious that negative ion-APPI is capable of ionizing a wider range of compounds than negative ion chemical ionization (NICI), negative ion-atmospheric pressure chemical ionization (negative ion-APCI) or negative ion-electrospray ionization (negative ion-ESI). (J Am Soc Mass Spectrom 2007, 18, 1789 -1798) © 2007 American Society for Mass Spectrometry I t is well known that liquid chromatography mass spectrometry (LC/MS) favors the analysis of polar compounds, as acid-base solution reactions are the most commonly observed ionization mechanism when using electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Traditionally, gas chromatography mass spectrometry (GC/MS) using both electron ionization (EI) and chemical ionization (CI) is employed for the analysis of volatile and thermostable nonpolar compounds. However, it can become difficult to analyze nonvolatile and/or thermo-labile nonpolar compounds via hyphenated techniques of chromatography and MS.In the year 2000, atmospheric pressure photoionization (APPI) [1] was developed as a complement to LC/ESI-MS and LC/APCI-MS. Presently, two fundamentally different APPI sources are commercially available [1,2]. Syagen Technology (Tustin, CA) produces PhotoMate, and Applied Biosystems/MDS SCIEX (Concord, Ontario, Canada) markets PhotoSpray. In both designs, photons emitted by a krypton discharge lamp are used to initiate a series of gas-phase reactions, which finally lead to analyte ionization. However, they differ in that PhotoMate utilizes a design to enhance direct APPI, while PhotoSpray implements a design using a dopant, usually tolu...

“…APPI is a valuable complement to atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) and is helping to extend the range of molecules that can be efficiently ionized. However, it is fair to say that the growth in usage of APPI and development of new applications is outpacing work to understand the fundamental mechanism of PI at elevated pressures [10,11,12].…”

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confidence: 99%

Mechanism of [M + H]⁺ formation in photoionization mass spectrometry

Syage¹

2004

150

157

In this paper we examine the mechanism of [M ϩ H] ϩ (henceforth MH ϩ ) formation by direct photoionization. Based on comparisons of the relative abundance of M ϩ and MH ϩ ions for photoionization of a variety of compounds M as vapor in air versus in different solvents, we conclude that the mechanism is M ϩ h 3 M ϩ ϩ e Ϫ followed by the reaction M ϩ ϩ S 3 MH ϩ ϩ S(ϪH). The principal evidence for molecular radical ion formation M ϩ followed by hydrogen atom abstraction from protic solvent S are: (1) Nearly exclusive formation of M ϩ for headspace ionization of M in air, (2) significant relative abundance of MH ϩ in the presence of protic solvents (e.g., CH 3 OH, H 2 O, c-hexane), but not in aprotic solvents (e.g., CCl 4-), (3) observation of induced equilibrium oscillations in the abundance of MH ϩ and M ϩ , and (4) correlation of the ratio of MH ϩ /M ϩ to reaction length in the photoionization source. Thermodynamic models are advanced that explain the qualitative dependence of the MH ϩ /M ϩ equilibrium ratio on the properties of solvent S and analyte M. Though the hydrogen abstraction reaction is endothermic in most cases, it is shown that the equilibrium constant is still expected to be much greater than unity in most of the cases studied due to the very slow reverse reaction involving the very low abundant MH ϩ and S(ϪH) species. (J Am Soc Mass Spectrom 2004, 15, 1521-1533