Electron spin resonance (ESR)

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Electron spin resonance (ESR) can be used to detect and characterize radicals. The method is also known as EPR (for Electron Paramagnetic Resonance). Due to its high sensitivity, it is also possible to detect decay curves of the radical density on material surfaces after plasma treatment. Additionally, this method can be used for larger surface areas in imaging mode.


Zeeman splitting: Splitting of the energy levels of the electron in the magnetic field.
Zeeman splitting: Splitting of the energy levels of the electron in the magnetic field.
Electron-Spin-Resonance signal.
Electron-Spin-Resonance signal.

Radicals are characterized by unpaired electrons and thus by a spin (±1/2), which in turn is associated with a magnetic moment. In a magnetic field, the energy of the electron depends on whether it is aligned parallel or antiparallel to the magnetic field (so-called Zeeman splitting, see diagram). If the sample is exposed to microwave radiation whose quantum energy corresponds to the Zeeman splitting, resonant absorption occurs.

The local magnetic field in which a radical is located results from an external field applied to the sample, and from the magnetic field of the nuclei of the neighboring atoms whose spin is not equal to 0, such as N14. Therefore, the chemical environment of the ESR-active species (i.e., the radicals) can be inferred from the ESR spectra: similar to NMR.
Practically, the sample is placed in a microwave resonator that is in a uniform magnetic field. The MW frequency is kept constant, and the magnetic field is scanned over a certain range. During this process, the MW absorption is measured.

Particular advantages of this method include its high sensitivity and the ease of measuring the absolute number of spins in the sample.

Requirements for test material

  • Liquids, e.g.:
    solutions, suspensions, blood, ...
  • Solids, e.g.:
    cells, tissues, pastes, powders, textiles, (packaging) films, membranes, semiconductor wafers, ...

Measuring equipment

© Fraunhofer IGB
Magnettech Miniscope MS 200.
Magnettech Miniscope MS 200

MS-5000 (formerly Magnettech, now Bruker)

Magnettech Miniscope MS 200


Technical data/measurement capabilities

  • Working frequency: X-band (approx. 9.5 GHz)
  • Sensitivity: 5x1010 spins/mT (5x109 spins/G)
  • Microwave power: 1 μW - 100 mW
  • Temperature range: - 170 °C to + 200 °C



  • Investigation of radicals in polymers, for example on plasma-treated surfaces
  • Detection of chemical functions on surfaces using spin labeling
  • Investigations of biological samples
  • Crystal defects (point defects)
  • Paramagnetic transition metals


Life Sciences
Reactive oxygen and nitrogen species
(RONS), oxidative stress, photodynamic

Environmental Toxicology
Generation of radicals by pollutants

Food Chemistry and Pharmacy

Antioxidant properties of foods, radiation-induced radicals, alanine dosimetry

Radical polymerization, UV stability, temperature stability

Radical protection efficacy, e.g. UV absorbers in sunscreen, etc.