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Origin of an EPR signal

Because of electron-nuclear mass differences, the magnetic moment of an electron is substantially larger than the corresponding quantity for any nucleus, so that a much higher electromagnetic frequency is needed to bring about a spin resonance with an electron than with a nucleus, at identical magnetic field strengths. For example, for the field of 3350 G shown above, spin resonance occurs near 9388.2 MHz for an electron compared to only about 14.3 MHz for H nuclei. (For NMR spectroscopy, the corresponding resonance equation is h ν = g N μ N B 0 {\displaystyle h\nu =g_{\mathrm {N} }\mu _{\mathrm {N} }B_{0}} where g N {\displaystyle g_{\mathrm {N} }} and μ N {\displaystyle \mu _{\mathrm {N} }} depend on the nucleus under study.)