For mercury released directly as atomic vapour, different direct transfer systems are used. In an open dynamic system the liberated analyte is transported by a carrier gas through the sampling or excitation source and swept away. In an open static system the equilibrated gaseous phase is forced into the absorption cell by displacement, e.g. by water. During measurement the gaseous phase is thus static. In a closed system the analyte and carrier gas are circulated through the absorption cell and the generator vessel until equilibrium between the liquid and gaseous phases is established.