Tandem Mass Spectrometry Using the Atmospheric Pressure Electron Capture Dissociation Ion Source

Atmospheric pressure electron capture dissociation (AP-ECD) is an emerging technique capable of being adopted to virtually any electrospray mass spectrometer, without modification of the main instrument. To date, however, because the electron capture reactions occur in the ion source, AP-ECD has been limited by its apparent inability to select precursors prior to fragmentation, i.e., to perform tandem mass spectrometry (MS/MS) experiments. In this paper we demonstrate a novel AP-ECD-MS/MS method using an AP-ECD source on a Xevo G2-S quadrupole time-of-flight (Q-TOF) mass spectrometer from Waters Micromass. The method takes advantage of the tendency for electron capture reactions to generate charge-reduced "ECnoD" products, species that have captured an electron and have had a covalent bond cleaved yet do not immediately dissociate into separate products and so retain the mass of the precursor ion. In the method, ECnoD products from the AP-ECD source are isolated in the quadrupole mass filter and induced to dissociate through supplemental activation in the collision cell, and then the liberated ECD fragment ions are mass analyzed using the high-resolution TOF. In this manner, true MS/MS spectra may be obtained with AP-ECD even though all of the precursors in the source are subjected to electron capture reactions in parallel. Here, using a late-model Q-TOF instrument otherwise incapable of performing electron-based fragmentation, we present AP-ECD-MS/MS results for a group of model peptides and show that informative, high-sequence-coverage spectra are readily attainable with the method.