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Generation of ion-bound solvent clusters as reactant ions in dopant-assisted APPI and APLI

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Abstract

We provide experimental and theoretical evidence that the primary ionization process in the dopant-assisted varieties of the atmospheric pressure ionization methods atmospheric pressure photoionization and atmospheric pressure laser ionization in typical liquid chromatography–mass spectrometry settings is—as suggested in the literature—dopant radical cation formation. However, instead of direct dopant radical cation–analyte interaction—the broadly accepted subsequent step in the reaction cascade leading to protonated analyte molecules—rapid thermal equilibration with ion source background water or liquid chromatography solvents through dopant ion–molecule cluster formation occurs. Fast intracluster chemistry then leads to almost instantaneous proton-bound water/solvent cluster generation. These clusters interact either directly with analytes by ligand switching or association reactions, respectively, or further downstream in the intermediate-pressure regions in the ion transfer stages of the mass spectrometer via electrical-field-driven collisional decomposition reactions finally leading to the predominantly observed bare protonated analyte molecules [M + H]+.

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Notes

  1. Purcell et al. [1] dissolved bitumen samples in perdeuterated toluene and subjected the mixture directly to high-resolution APPI Fourier transform ion cyclotron resonance analysis. Deuterated toluene donated a deuteron to only 10–15 % of the even-electron ions formed from pyridinic nitrogen model compounds.

  2. E r = E/N, where E is the electrical field strength (V/cm) and N is molecular number density (molecule per cubic centimeter), resulting in the unit volt square centimeter per molecule. More conveniently, 1 Td (townsend) = 10-17 V cm2/molecule.

  3. There is also a very good and comprehensive tutorial on buffer gas cooling of ion beams by Moore [20]. There one actually learns why ground loops in combination with high-voltage surges may result in a good number of bottles of beer being wasted.

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Acknowledgments

The instrument briefly described in this work was partly developed at the University of Wuppertal, Germany, to be finally installed at IEK-7 of Research Center Jülich, Germany. S.K., V.D., H.K., and T.B. are indebted to the director of the IEK-7, Martin Riese, and to Fred Stroh of IEK-7 for strongly supporting this work by a 1-year loan of the instrument to the University of Wuppertal. S.K. acknowledges a graduate student research fellowship from Bruker Daltonics, Bremen, Germany. S.A. acknowledges a graduate student research fellowship from Research Center Jülich. Part of this work was funded through grant Be 2124/6-1 of the German Research Foundation.

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Authors and Affiliations

  1. Physical and Theoretical Chemistry, Institute for Pure and Applied Mass Spectrometry, University of Wuppertal, Gaussstrasse 20, 42097, Wuppertal, Germany

    Sonja Klee, Valerie Derpmann, Hendrik Kersten & Thorsten Benter

  2. Institute of Energy and Climate Research: Stratosphere (IEK-7), Forschungszentrum Juelich GmbH, Wilhelm-Johnen-Straße, 52428, Jülich, Germany

    Sascha Albrecht

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  1. Sonja Klee
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  2. Sascha Albrecht
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Correspondence to Thorsten Benter.

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Published in the topical collection Photo Ionisation in Mass Spectrometry with guest editor Ralf Zimmermann.

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Klee, S., Albrecht, S., Derpmann, V. et al. Generation of ion-bound solvent clusters as reactant ions in dopant-assisted APPI and APLI. Anal Bioanal Chem 405, 6933–6951 (2013). https://doi.org/10.1007/s00216-013-7114-8

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