Spin trapping is a technique employed in the study of free radicals that are either unstable or terminate rapidly at ambient temperatures.
Spin trapping involves the addition of a diamagnetic radical scavenger (the spin trap) to a reaction mixture containing a radical or radicals of interest. A reactive free radical adds to the scavenger, forming a long-lived paramagnetic adduct. This product can then be studied by EPR spectroscopy and the identity of the species adducted can be inferred from EPR spectral characteristics such as an observed g value or a hyperfine-coupling pattern.
One of the most commonly used spin traps is alpha-phenyl N-tertiary-butyl nitrone (PBN).
The use of radical-addition reactions to detect short-lived radicals was first proposed by E. G. Janzen in 1965.