Utilizing the inherent electrolysis in a chip-based nanoelectrospray emitter system to facilitate selective ionization and mass spectrometric analysis of metallo alkylporphyrins |
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Authors: | Email author" target="_blank">Gary?J?Van?BerkelEmail author Vilmos?Kertesz |
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Institution: | (1) Organic and Biological Mass Spectrometry Group, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6131, USA;(2) Present address: Physics and Chemistry Department, Szechenyi Istvan University, Egyetem Square 1, Gyor, 9026, Hungary |
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Abstract: | A commercially available chip-based infusion nanoelectrospray ionization system was used to ionize metallo alkylporphyrins
for mass spectrometric detection and structure elucidation by mass spectrometry. Different ionic forms of model compounds
(nickel (II), vanadyl (II), copper (II), and cobalt (II) octaethylporphyrin) were created by using two different types of
conductive pipette tips supplied with the device. These pipette tips provide the conductive contact to solution at which the
electrolysis process inherent to electrospray takes places in the device. The original unmodified, bare carbon-impregnated
plastic pipette tips were exploited to intentionally electrochemically oxidize (ionize) the porphyrins to form molecular radical
cations for detection. Use of modified pipette tips, with a surface coating devised to inhibit analyte mass transport to the
surface or slow the kinetics of the analyte electrochemical reactions, was shown to limit the ionic species observed in the
mass spectra of these porphyrins largely, but not exclusively, to the protonated molecule. Under the conditions of these experiments,
the effective upper potential limit for oxidation with the uncoated pipette tip was 1.1 V or less, and the coated pipette
tips effectively prevented the oxidation of analytes with redox potentials greater than about 0.25 V. Product ion spectra
of either molecular ionic species could be used to determine the alkyl chain length on the porphyrin macrocycle. The utility
of this electrochemical ionization approach for the analysis of naturally occurring samples was demonstrated using nickel
geoporphyrin fractions isolated from Gilsonite bitumen. Acquiring neutral loss spectra as a means to improve the specificity
of detection in these complex natural samples was also illustrated. |
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