The production of a prep-scale gas chromatographic unit can be increased by several orders of magnitude if the following rules are followed. a. Use helium, or better hydrogen, at a flow-rate two to three times larger than the one corresponding to the maximum efficiency at very low sample size. b. Select a temperature and column outlet pressure so that the Valentin equation is fulfilled. (Temperature equals boiling point of main compound at average column pressure.) c. Inject important sample size so that the maximum partial pressure of the sample in the carrier gas at column inlet is smaller than 30% of the total pressure, while the width of the injection band is large enough to allow the overlapping of the elution bands of the compounds to prepare. An intermediate cut will be recycled, but the production of pure compounds is larger than if the bands do not overlap. A time-based programmer to control the trap valves is necessary. d. Eliminate compounds or impurities with long retention time either by distillation or by a first GC step using short columns and large samples. e. Superpose injections so that the first impurities of an injection are eluted in the same time as the last ones of the previous injection. Of special Importance is the fact that it is possible to prepare large amounts of pure compounds from bands which show very little or even no resolution between the compounds of the mixture. The productivity of a 5-inch diameter column may reach in optimum conditions 10 to 100 pounds per day depending on the difficulty of the problem.