The concept of magnetic resonance

Spectroscopy requires a source of radiation, a sample, and a detector magnetic spectroscopy additionally requires an external magnetic field. The term spectroscopy implies that at least one of these four elements is variable, or tunable, in some way or other, and that one measures the amount of radiation absorbed by the sample as a function of this variable. For example, the source generates radiation with energy 

EPR spectrometers use radiation in the giga-hertz range (GHz is 109 Hz), and the most common type of spectrometer operates with radiation in the X-band of micro-waves (i.e., a frequency of circa 9-10 GHz). For a resonance frequency of 9.500 GHz (9500 MHz), and a g-value of 2.00232, the resonance field is 0.338987 tesla. The value ge = 2.00232 is a theoretical one calculated for a free unpaired electron in vacuo. Although this esoteric entity may perhaps not strike us as being of high (bio) chemical relevance, it is in fact the reference system of EPR spectroscopy, and thus of comparable importance as the chemical-shift position of the II line of tetra-methylsilane in NMR spectroscopy, or the reduction potential of the normal hydrogen electrode in electrochemistry. 

A derived Si-unit for magnetic held is the gauss, which is defined in tesla units as 

The human mind appears to have a preference for employing units that force the value of things into small numbers (i.e., of the order of the number of fingers on two human hands). Therefore, EPR spectroscopists prefer the gauss over the tesla an EPR linewidth of, say, 8 gauss somehow sounds easier to deal with than a linewidth of 0.0008 tesla. Alternatively, some prefer a linewidth of 0.8 mT (milli-tesla). In this vein, Equation 2.4 is frequently written in the practical form 

As an added advantage of using this unit one immediately sees what the wavelength of the employed radiation is 

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Figure 2.1 illustrates the concept of magnetic resonance in EPR spectroscopy. The sample is a system that can exist in two different states with energies that are degenerate (i.e., identical) in the absence of a magnetic field but that are different in the presence of a field—for example, a molecule with a single unpaired electron. 

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