Identification of citrullinated peptides. MS1 (left) and MS/MS (right) spectra of two different fibrinogen-derived peptides (DSGEGDFLAEGGGVR in panel A and NRGDSTFESKSY in panel B) from synovial fluid samples. In the MS1 spectra, the peak(s) corresponding to the respective peptide are marked with their mass-to-charge ratio (m/z) and charge state, the peak of which the MS/MS spectrum is shown and is marked with the red m/z value. In the corresponding MS/MS spectra, y-ions are marked in blue, b-ions are marked in red and all other annotated ions are marked in green. Please note that parent ions are not present in the MS/MS spectra generated with the instrumentation and settings used. MS, mass spectrometry; MS/MS, tandem mass spectrometry.