Atomic emission spectroscopy (AES), more commonly referred to as emission spectroscopy, is a spectroscopic technique which examines the wavelengths of photons emitted by atoms or molecules during their transition from an excited state to a lower energy state. Each element emits a characteristic set of discrete wavelengths according to its electronic structure; by observing these wavelengths the elemental composition of the sample can be determined. AES detectors operate by passing a continuous stream of target vapor to a burner where it is combusted by a hydrogen flame, resulting in excited state ions. The photoemission from the flame is passed to a diffraction grating and the resulting emission spectrum is then detected by a photo detector array. In practice, AES detectors are used to detect the presence of specific target elements and are not suitable for screening unknowns. Chemicals of interest must contain elements that are observable by AES in order to be detected.