Figure 1.

Quantitative MS-based proteomics. (a) Analysis of complex peptide mixtures by LC-MS². Peptide mixtures are resolved by liquid chromatography, ionized through electrospray and resolved by MS¹. Selected peptides are fragmented by collision with an inert gas and the resulting MS² spectra are recorded. (b) Quantitative proteomics strategies. In the SILAC technique, isotope-labeled peptide intensities (I) are compared in the MS¹ spectra. For 'label-free' quantitation, intensities of peptides are compared between different runs. Alternatively, standard peptides are spiked into the mixture to yield calibration for absolute peptide abundances. R refers to the ratio between either heavy and light peptides (SILAC panel) or ion intensities between different runs (label-free quantitation).