Figure 1 - Extracting quantitative data from mass spectra.

Mass spectra are acquired, resulting in isotope clusters for each peptide (doubly charged peptide, black in top graph). As the peptide elutes from the column, the signal is sampled several times, forming the black curve in the right panel. The area under the black curve is the XIC, a measure that is proportional to the peptide's abundance. The red isotope cluster is a heavy isotope–labeled analog of the black peptide, 4 Da higher in mass and present at 85% of the unlabeled peptide. The 85% ratio can also be determined by comparing the areas under the red and black curves. Several thousand such peptide XICs can be extracted from a data file obtained in typical complex mixture analysis.