About ESR

ESR stands for Electron Spin Resonance.

It is a phenomenon and a corresponding spectroscopic method that enables the study of various attributes of paramagnetic substances (and even more, see below). It is also often called Electron Paramagnetic Resonance (EPR). The word 'Spin' in ESR emphasizes that the paramagnetic moments detected by the method are associated mainly by uncompensated (i.e. unpaired) electron spins. ESR spectroscopy is able to detect paramagnetic spins with an extreme high sensitivity: at room temperature the signal of a sample with roughly 1011 paramagnetic spins is readily measurable. The molar concentration of such a number of paramagnetic molecules in a solution with a volume of 0.1 cm3 is in the order of only 1 nM (10-9 mol/L)! And even higher sensitivities can be attained by performing the measurement at lower temperatures.

ESR spectrometers,

even though based on the same principle of operation (to be outlined below), come in various shapes, sizes and prices. The most capable and versatile ones (which we prefer to call research grade ones here) are extremely expensive devices (with a unit price easily reaching the order of 105 – 106 euros), and can therefore be afforded mainly by large research institutions only. Not surprisingly then, usually only a couple of research grade spectrometers are in operation on the national level, as is the case for example in Hungary. More affordable and compact spectrometers are offered to serve mainly particular purposes, such as higher education or the study of free radicals for example.

Research grade ESR spectrometers,

can also be advantageously used to obtain information on relevant properties (e.g., magnetic anisotropy, Curie temperature) of materials exhibiting spontaneous magnetization such as ferri- and ferromagnetic materials. The resonance phenomenon detected in this case is called Ferromagnetic Resonance (FMR). In contrast with ESR measurements whose interpretation in general requires the consideration of quantum physics, the theory of FMR can essentially be treated in the frame of classical physics. It may sound then somewhat paradoxical that in practice it is ESR that is often capable to paint a clear and readily comprehensible picture of the studied material, whereas the information as provided by FMR is usually rather complex. At the same time, ESR spectrometers can show an amusingly high sensitivity when used for the detection of ferromagnetic resonance. Minute amounts (even below 0.001 g) of ferromagnetic materials can be readily investigated in practice, in a routine manner.

When the size of the ferromagnetic particles decreases below the order of a few tens of nanometers, a new phenomenon called superparamagnetism sets in, which involves the fast relaxation of the particles' magnetic moment. When FMR of such superparamagnetic nanoparticles are recorded, it is sometimes referred to as Superparamagnetic Resonance (SPR) in order to emphasize the presence of effects (e.g. narrowing of the resonance spectrum) associated with superparamagnetism. Nevertheless, be it SPR, FMR or EPR/ESR, it is measured with the same equipment (i.e. the ESR spectrometer) and essentially in the same way.

Diverse fields

of the physical, chemical, biological and medical sciences can benefit from the information accessible by the means of an ESR spectrometer: materials science, geology, archaeology, complex chemistry, radical reaction chemistry, chemical reaction kinetics, food chemistry, cell membrane biology, biochemistry, pharmacology, medical viral infection diagnosis, gene therapy and cancer therapy, and even plant taxonomy, just to name a few of the numerous examples. Obviously, ESR is a spectroscopic method with a uniquely wide range of applicability. The efficient utilization of ESR in these fields, however, requires expertise simultaneously concerning the spectrometric method and the application field in question. ESR spectroscopists therefore come from various fields, and they can have for example biology, chemistry or physics as their original background. Quite often, researchers coming from different fields work together in ESR laboratories such as to complete each other's knowledge in order to enable a more comprehensive use of their ESR spectrometer. Beside knowledge, the application of ESR in all these diverse fields can certainly also do with a great deal of creativity.

The method of ESR spectroscopy

relies on the monitoring of the microwave EM (electromagnetic) radiation absorption properties of the studied material. However, as microwave EM radiation sources do not offer wide-enough frequency variability, during the ESR measurement a fixed microwave frequency is applied, and it is the microwave absorption spectrum of the material that is tuned continuously over a wide-enough range by the application of an external magnetic field. The absorption of the microwave is then monitored while sweeping the applied field over a given range, thereby observing the spectrum as a function of the external magnetic field rather than as a function of the microwave frequency. ESR spectrometers are usually operated in conjunction with a given microwave frequency range, but there are several different microwave frequency ranges for which ESR spectrometers are available. The microwave X-band is the one that is most often used, usually in between 9 and 10 GHz. At these frequencies it is usually enough to apply less than 1 T external magnetic field to make the microwave frequency coincide with a peak in the EM absorption spectrum of the studied material. ESR with microwave bands centered at higher frequencies (e.g. Q-band at around 34 GHz) requires the application of larger magnetic fields, but it offers - among others - the advantage of higher resolution in the recorded spectra. ESR used in conjunction with lower microwave frequencies (notably L-band around 1 GHz) can be advantageous as well: due to the lower loss and higher penetration depth of longer-wavelength EM radiation, the absorption characteristics of larger-volume samples can also be investigated. At L-band even a living mouse can serve as the subject of an ESR study.

For more detailed information

about the method of ESR spectroscopy we refer to the various links available at ESR.HU. The Wikipedia and University pages linked under the menu item ESR Links above are a good starting point. Useful information and further links may also be found at the websites of ESR equipment suppliers and the ESR societies. For even more in-depth information we refer to the publications whose list is accessible via the Libraries menu item. For information on up-to-date scientific research visit the websites of the various laboratories featured at ESR.HU, and check out their material accessible via the Posters menu item.

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