Effects of solvent flow,dopant flow,and lamp current on dopant-assisted atmospheric pressure photoionization (DA-APPI) for LC-MS. Ionizationvia proton transfer |
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Authors: | Robb Damon B Blades Michael W |
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Institution: | (1) Chemical Analysis Group and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands; |
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Abstract: | 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+ 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. |
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