Free radicals were originally detected using simple analytical techniques that were based on the experiments by Paneth. Modern detection methods include a variety of spectral methods. For example, absorption spectroscopy is relatively simple way to detect radicals in the gas phase. A another method is based on spectrometry. This technique works by measuring ionization energy of the free radicals and can be used to quantitatively measure radical concentration. The best technique is considered to be electron paramagnetic resonance spectroscopy which can characterize radicals in liquids, solids, or gases. Furthermore, this method is very sensitive and can be used to gain information on the structure of the detected radicals.