  Diamond has been extensively investigated recently due to a wide range of
potential applications of nitrogen-vacancy (NV) defect centers existing in a
diamond lattice. The applications include magnetometry and quantum information
technologies, and long decoherence time ($T_2$) of NV centers is critical for
those applications. Although it has been known that $T_2$ highly depends on the
concentration of paramagnetic impurities in diamond, precise measurement of the
impurity concentration remains challenging. In the preset work, we show a
method to determine a wide range of the nitrogen concentration ($n$) in diamond
using a wide-band high-frequency electron spin resonance and double
electron-electron resonance spectrometer. Moreover, we investigate $T_2$ of the
nitrogen impurities and show the relationship between $T_2$ and $n$. The method
developed here is applicable for various spin systems in solid and
implementable in nanoscale magnetic resonance spectroscopy with NV centers to
characterize the concentration of the paramagnetic spins within a microscopic
volume.
