Fig. 2.
18O heavy isotope incorporation. 18O-Labeled VLVLDTDYKK2+ from β-lactoglobulin detected by LTQ-Orbitrap and triple quadrupole instrument. A, LTQ-Orbitrap MS spectrum. B, extracted ion chromatogram of fully labeled (18O2), singly labeled (18O1), and unlabeled (16O2) peak from LTQ-Orbitrap. C, SRM extracted ion chromatogram of fully 18O2-labeled transitions (solid lines) and light unlabeled transitions (dashed lines in zoom-in view) from LC-TSQ. Green, y6; blue, y7; red, y8 ion, respectively. The intensities of the light transitions are near the noise level.