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Short answer: EPR (electron paramagnetic resonance) spectroscopy, also called ESR (electron spin resonance), detects species that contain unpaired electrons, such as organic radicals and many transition-metal centres. A sample sits in a magnetic field, absorbs microwave radiation at resonance, and the position, splitting and shape of the signal report on the electron’s environment.
EPR, ESR and electron spin resonance: the same technique
EPR spectroscopy, ESR spectroscopy, electron spin resonance spectroscopy and electron paramagnetic resonance spectroscopy are different names for one method. LibreTexts describes EPR (also known as ESR) as the study of species with unpaired electrons. It is the electron-spin counterpart of nuclear magnetic resonance: where NMR excites nuclear spins, EPR excites electron spins (see the NMR spectrometer guide for the nuclear side). Zavoisky first observed it in 1944 in Kazan, and Bleaney at Oxford observed it independently (Wikipedia, secondary source).
How an EPR spectrum is read
The resonance condition and the g-factor
Resonance occurs when the microwave energy hν equals gβB, where β is the Bohr magneton and B the magnetic field. LibreTexts gives the practical form g = 71.4484 × ν(GHz) / B(mT). A free electron has g = 2.00232. Organic radicals sit close to that value (about 1.99 to 2.01; the methyl radical is 2.0026), while transition-metal complexes span roughly 1.4 to 3.0 (copper(II) acetylacetonate is 2.13). Because g depends on the electron’s environment, it works as a fingerprint.
Worked check (arithmetic from the formula above): at 9.5 GHz, a species with g = 2.0023 resonates at B = 71.4484 × 9.5 / 2.0023 ≈ 339 mT, which is about 3,390 G (1 T = 10,000 G). That agrees with the roughly 0.33 T field quoted for X band.
The derivative display
Phase-sensitive detection means spectra are normally shown as the first derivative of absorption. The zero crossing marks the signal centre, which is where you read g. Treating the peaks of a derivative trace as absorption maxima is a common reading error.
Hyperfine splitting
Coupling to nuclei splits a signal. For N equivalent nuclei of spin I, LibreTexts gives 2NI + 1 lines; for I = 1/2 nuclei the intensities follow Pascal’s triangle. Its examples: the methyl radical (three equivalent protons) shows 4 lines in a 1:3:3:1 ratio, and sets of different nuclei multiply, so the H2C(OCH3) radical gives 12 lines.
Continuous-wave vs pulsed EPR, and microwave bands
Continuous-wave (CW) EPR uses field modulation and yields first-derivative spectra. Pulsed EPR applies pulses of roughly 10 to 100 ns and measures relaxation (for example inversion recovery) and dephasing (Hahn echo). Most measurements are made near 9 to 10 GHz; per Wikipedia the common bands are X (about 10 GHz, 0.33 T), Q (about 35 GHz, 1.25 T) and W (about 95 GHz, 3.5 T), with X and Q most common and higher bands improving resolution.
Pulsed methods are not always the better choice. Mitchell et al. (Biophysical Journal, 2013) note that pulsed Fourier-transform EPR is impractical for spin adducts because their T2* is below 100 ns, shorter than most pulsed-EPR dead times. In the same study, rapid-scan EPR gave a signal-to-noise ratio more than 40 times that of CW for the same acquisition time under their conditions, in buffer and bacterial suspensions; that is a result for specific conditions, not a general rule.
Direct detection, spin labels and spin traps: which approach fits what you want to measure
| What you want to study | Approach | Source |
|---|---|---|
| A stable radical or metal centre already in the sample (for proteins: tyrosyl, tryptophanyl, glycyl radicals, semiquinones, flavins; Cu, Ni, Co, Mn, Mo, Fe, Cr, heme, Fe-S clusters) | Direct detection of the native paramagnetic centre | University of Colorado Boulder biochemical instrument core |
| A molecule with no paramagnetic centre of its own, such as nucleic acids or lipids | Site-directed spin labelling with a nitroxide label | University of Colorado Boulder biochemical instrument core |
| A short-lived radical such as hydroxyl, superoxide or a carbon-centred radical | Spin trapping, which converts it into a longer-lived adduct | University of Colorado Boulder core; Mitchell et al. 2013 |
For superoxide, Mitchell et al. report that it is too short-lived for direct EPR. The spin trap BMPO forms a BMPO-OOH adduct with a reported half-life of about 23 minutes, and DMPO has long been the standard trap. What you observe is the adduct, not the original radical, so interpretation has to account for that.
Where EPR matters in labs and facilities
Applications listed by Wikipedia include radicals and paramagnetic complexes, catalysis intermediates, radicals in batteries and electrochemistry, spin labels in membranes, dating of geological material and teeth, alanine dosimetry (about 1 Gy to 100 kGy) and vanadium in crude oil. The Boulder core states that samples can be measured under near-physiological conditions, with as little as 4 µL of 25 to 100 µM sample, and that users supply their own EPR tubes or capillaries. Instrument access, tube choice and sample preparation are decisions for your own facility manager and the instrument’s documentation.
Common misunderstandings
- EPR is not a general-purpose spectroscopy. It sees only species with unpaired electrons, and the radical must exceed the detection limit (Wikipedia).
- Water can be a problem. Water absorbs microwaves, which constrains aqueous sample handling (Wikipedia).
- The trace is a derivative. Read g at the zero crossing, not at a peak (LibreTexts).
- A trapped adduct is not the original radical. Spin-trap spectra report on the adduct (Mitchell et al. 2013).
- Complex spectra are hard to interpret. Overlapping hyperfine patterns multiply line counts, as the 12-line example shows (LibreTexts, Wikipedia).
Frequently asked questions
Is EPR the same as ESR?
Yes. EPR (electron paramagnetic resonance) and ESR (electron spin resonance) name the same technique, which studies species with unpaired electrons.
How does EPR differ from NMR?
Both are magnetic resonance methods, but EPR excites electron spins with microwaves at about 0.33 T (X band), whereas NMR excites nuclear spins. EPR therefore needs unpaired electrons in the sample.
What does the g-value tell you?
It identifies the electronic environment: about 2.00 for most organic radicals and roughly 1.4 to 3.0 for transition-metal complexes (LibreTexts).
How many lines will a hyperfine pattern show?
For N equivalent nuclei of spin I, 2NI + 1 lines. Three equivalent protons give 4 lines at 1:3:3:1.
Related guides
For neighbouring techniques see fluorescence spectroscopy, Raman spectrometers, mass spectrometers and the organic spectroscopy database; background is in physical chemistry.
This page is general technical information for researchers and is not instrument-operation or safety guidance; follow your facility’s procedures.
Sources
- LibreTexts Chemistry, EPR – Interpretation (read 2026-11-08).
- Wikipedia, Electron paramagnetic resonance (secondary source, read 2026-11-08).
- University of Colorado Boulder, EPR spectroscopy, biochemical instrument core (read 2026-11-08).
- Mitchell et al., Biophysical Journal 2013;105(2):338-342, PMC3714875 (read 2026-11-08).
