Pumps deliver the sample and solvent through the columns and the system. Detectors record the exit of fractions of the sample and count the number of molecules of a certain MW. The computer controls the test automatically, records the results, and calculates the different MW averages. The GPC system contains a number of different instruments that work together to provide the optimum system performance. Figure 17.3 shows a schematic of a gel permeation chromatograph with the basic components.