Targeted proteomics to quantify SEPT11 enrichment in urea fractions. (a) MS/MS scan of the precursor ion (m/z 979.0) corresponding to a unique SEPT11 peptide (amino acids 400-418). (b) MS/MS product ions selected for quantitation in light (top) and heavy (bottom) peptide forms. (c) Extracted ion chromatogram of targeted spectra for each of the product ions in (b). The spiked heavy- (red) and endogenous light (black) peptides have identical retention times and are overlaid. For each product ion, enrichment is calculated as a ratio of light to heavy signal of the area under each curve.