Fluorimetry and phosphorimetry are both luminescence-based spectroscopic methods used for the determination of substances that absorb electromagnetic radiation and subsequently emit light. Although both methods depend on the emission of radiation from an excited species, they differ mainly in two important aspects: the nature of the electronic transition involved and the time and conditions of emission measurement. The first major difference is related to the electronic transition responsible for emission. In fluorimetry, fluorescence occurs when a molecule absorbs radiation and an electron is promoted from the ground-state singlet level to a higher singlet excited state. The excited electron then returns rapidly from the excited singlet state to the ground-state singlet level, generally by an allowed transition. Because this transition is highly probable, fluorescence occurs very rapidly, usually within about (10^{-9}) to (10^{-7}) seconds after excitation. The emitted radiation is therefore produced almost immediately after the exciting radiation is absorbed. Fluorescence generally involves transitions between electronic states of the same spin multiplicity, such as (S_1 \rightarrow S_0). In phosphorimetry, on the other hand, the molecule may undergo intersystem crossing from an excited singlet state to a triplet state. The electron changes its spin during this process. Emission then occurs when the electron returns from the excited triplet state to the ground-state singlet state, represented as (T_1 \rightarrow S_0). This transition is spin-forbidden and is therefore much less probable than the transition responsible for fluorescence. As a result, phosphorescence is considerably slower than fluorescence. Thus, the first aspect in which the two techniques differ is the type of electronic transition: fluorescence normally involves an allowed