Figure 2.

We suggest to perform this scheme of Label-free quantification useful for metabolic research. (A) Peptide signals are detected at the MS1 level and are distinguished from chemical noise/background by their characteristic isotopic pattern. (B) These patterns are then followed via the retention time dimension and are used to rebuild a chromatographic elution-profile of the mono-isotopic peptide mass. (C–E) The total ion current of the peptide signal is then integrated and used as a quantitative measurement of the original peptide concentration. For each detected peptide, all isotopic peaks are first found and the charge state is then assigned. Label-free can be carried out via Fourier Transform Ion Cyclotron Resonance (FTICR) or Orbitrap.