Fig. 2.

Label-free LC/MS data consist of individual MS spectra accumulated over (retention) time. Stacked side by side, these spectra form two-dimensional maps. In these maps, individual peptides being eluted from the column give rise to sets of peaks across multiple spectra. Feature-finding algorithms can identify features, which can be defined as all mass-spectrometric signals (peaks) caused by the same peptide. Elution profiles have ideally a Gaussian shape, but they can be significantly distorted. The projection of a feature along the m/z axis accordingly corresponds to the isotope profile of the peptide.