Intensive fragmentations into b/y- and c-sequence ions have been reported for Substance P ionized under dopant-assisted atmospheric pressure photoionization (APPI) conditions. It was postulated that the fragmentation comes from the reaction of the doubly charged peptide with photoelectrons, released upon photoionization of the dopant, through an electron capture or transfer dissociation-type mechanism. In order to investigate further the origin of each fragment ion type, a tunable VUV synchrotron radiation source coupled to a modified APPI source has been used. Using this set-up, the photon energy can be scanned continuously below and above the photoionization threshold of the dopant, i.e. toluene, probing the ionization/fragmentation processes occurring on Substance P peptide at the atmospheric pressure interface of the mass spectrometer.
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