Atomic Fluorescence(AF) is a technique used for elemental analysis. Energy absorbed by an atom will cause an electron to move from the preferred level (ground state) to a higher energy level (excited state), and as the electron returns to the ground state energy is released as fluorescence. The technique is most effective for elements that readily form hydrides (e.g. arsenic and selenium), or are volatile at or near ambient temperature (e.g. mercury) as these elements can be easily stripped from solution and carried to the detector in gaseous phase free from molecular interferences. The technique is used almost exclusively for ultra-trace detection of mercury in environmental samples, with detection limits in the sub-part per trillion range. AF instruments have limited availability. Typically atomic emission and atomic absorption instruments are used to acquire elemental data with comparable sensitivity.