The most widely employed systems today are the mechanical valves and the DEANS valveless pressure switch (named after its inventor, D.R. Deans, around 1981). Both switching systems are commercially available. Valves could present problems such as dead volumes, sample adsorption and bad resistance to high temperatures as well to the heating–cooling process if installed inside the GC oven (leaks). If possible, the valve should be placed in a vacant detector oven. Valves are easy to use and relatively cheap. The valveless switch eliminates these problems but it is more time consuming to fit. The flow direction during the heart-cut in the DEANS three ‘live T-pieces’ is caused by pressure changes on the different ‘Ts’, and for this reason, many pressure adjustments are necessary before being operational.