Sub Parts-Per-Million Mass Measurement Accuracy of Intact Proteins and Product Ions Achieved Using a Dual Electrospray Ionization Quadrupole Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

High mass measurement accuracy (MMA) is demonstrated for intact proteins and subsequent collision-induced dissociation product ions using internal calibration. Internal calibration was accomplished using a dual electrospray ionization source coupled with a hybrid quadrupole Fourier transform ion cyclotron resonance (Q-FT-ICR) mass spectrometer. Initially, analyte ions generated via the first electrospray (ESI) emitter are isolated and dissociated in the external quadrupole. This event is followed by a simultaneous switch to the calibrant ion ESI emitter and a disablement of the isolation and activation of the external quadrupole such that a broad m/z range of calibrant ions are accumulated before injecting the analyte/calibrant ion mixture into the ICR cell. Two different internal calibrant solutions were utilized in these studies to evaluate this approach for the top-down characterization of melittin and ubiquitin. While external calibration of protein fragments resulted in absolute MMA greater than 16 ppm, internal standardization significantly improved upon the MMA of both the intact proteins and their products ions which ranged from −2.0 ppm to 1.1 ppm, with an average of −0.9 ppm. This method requires limited modification to ESI-FT-ICR mass spectrometers and is applicable for both positive and negative ionization modes.