In certain reactions in solution a @T06451@ @R05066@ will interact with a @D01668@ @R05190@ to form a more @P04516@ radical. The product radical accumulates to a concentration where detection and, frequently, identification are possible by EPR/ESR @S05848@. The key reaction is usually one of @A00512@; the @D01668@ @R05190@ is said to be a 'spin trap' and the @P04516@ product radical is then the 'spin @A00138@'. The procedure is referred to as spin @T06492@, and is used for @MT06831@ reactions involving the intermediacy of @R05180@ radicals at concentrations too low for direct observation. Typical spin traps are C[email protected]@ and @N04164@, to which reactive radicals will rapidly add to form nitryl radicals. A quantitative development, in which essentially all reactive radicals generated in a particular system are intercepted, has been referred to as 'spin counting'. Spin @T06492@ has also been adapted to the interception of radicals generated in both gaseous and solid phases. In these cases the spin adduct is in practice transferred to a liquid solution for observation in order to facilitate interpretation of the EPR/ESR spectra of the radicals obtained.