A @[email protected] @[email protected] @[email protected] (typically a nitryl radical) that is attached to a part of a @[email protected] whose microscopic environment is of interest and may be revealed by the electron spin @[email protected] (ESR) spectrum of the spin @[email protected] When a simple @[email protected] molecular entity is used in this way without covalent @[email protected] to the molecular entity of interest it is frequently referred to as a 'spin probe'.