Asphaltenes in Crude Oil

Electron Paramagnetic Resonance (EPR) measures the concentration and composition of free radicals in a sample. Samples can be either liquid, solid or gas. Free radicals are atomic or molecular species with unpaired electrons that can be highly reactive. There are also many stable free radicals, such as melanin in hair and ultramarine blue pigment. Many transition and rare earth metals have unpaired electrons, and are EPR active. Some minerals (eg amethyst, smoky quartz and fluorite) receive their color from unpaired electrons, and are also EPR active.