Ion-Molecule Reactions by Photoionization Techniques

Chupka, W. A.

doi:10.1007/978-1-4757-0088-6_3

Cited by 18 publications

(7 citation statements)

References 57 publications

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“…To convert eq 4 into the rate of dopant ion production, the fate of the excited-state intermediates, D*, must first be considered: predissociation, fluorescence, collisional quenching,°and°autoionization°may°all°occur° [25][26][27].…”

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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

J. Am. Soc. Mass Spectrom.

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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...

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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

J. Am. Soc. Mass Spectrom.

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“…7 show that fragmentation at threshold occurs from parent ions initially formed in the 2E 1U excited state and at somewhat higher energies in the 2 7 show a large kinetic shift of about 1.9 eV which implies, in the framework of RRKM theory, a very high density of states for the active molecular ion which could only be attained in one or more of the lowest electronic states. The shift of metastable and fragment curves upward in energy by about 0.55 £*" for Cgtyj is approximately as expected from the increased density of states of the perdeuterated ion while the positions of the 2 E, and ^Bŝ tates in the photoelectron spectrum remain very nearly unchanged.…”

Section: Recent Tests Of Qetmentioning

confidence: 95%

Photoionization and Fragmentation of Polyatomic Molecules

Chupka

1974

Chemical Spectroscopy and Photochemistry in the Vacuum-Ultraviolet

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“…The interactions W~m are the same as in (7) since we adopt the Condon approximation and furthermore assume that the ionization cross section does not depend on the energy E' [36]. We have discretized the continuum, i.e.…”

Section: ~ Dt" U+(t-t")w+~tt" Co -E') 0 T" ×~ Dt" U(t"--t')w~a(tmentioning

confidence: 99%

Femtosecond time-resolved molecular multiphoton ionization and fragmentation of Na2: experiment and quantum mechanical calculations

Engel

Baumert

Meier

et al. 1993

Z Phys D - Atoms, Molecules and Clusters

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Abstract. We present a comparison between experimental and theoretical results for pump/probe multiphoton ionizing transitions of the sodium dimer, initiated by femtosecond laser pulses. It is shown that the motion of vibrational wave packets in two electronic states is probed simultaneously and their dynamics is reflected in the total Na~ ion signal which is recorded as a function of the time delay between pump and probe pulse. The time dependent quantum calculations demonstrate that two ionization pathways leading to the same final states of the molecular ion exist: one gives an oscillating contribution to the ion signal, the other yields a constant background. From additional measurements of the Na+-transient photofragmentation spectrum it is deduced that another ionization process leading to different final ionic states exists. The process includes the excitation of a doubly excited bound Rydberg state. This conclusion is supported by the theoretical simulation.

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Ion-Molecule Reactions by Photoionization Techniques

Cited by 18 publications

References 57 publications

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

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

Photoionization and Fragmentation of Polyatomic Molecules

Femtosecond time-resolved molecular multiphoton ionization and fragmentation of Na2: experiment and quantum mechanical calculations

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