EXPERIMENTAL FIGURE 3-47 LC-MS/MS is used to identify the proteins in a complex biological sample. A complex mixture of proteins in a biological sample (e.g., an isolated preparation of Golgi organelles) is digested with a protease. The mixture of resulting peptides is fractionated by liquid chromatography (LC) into multiple, less complex, fractions, which are slowly but continuously injected by electrospray ionization into a tandem mass spectrometer. The fractions are then sequentially subjected to multiple cycles of MS/MS until the masses and sequences of many of the peptides have been determined and used to identify the proteins in the original biological sample through comparison with protein databases.