When calcified tissues like bone or tooth, enamel, are exposed to ionizing radiation, the EPR spectrum is dominated by the so-called "asymmetric EPR signal near g = 2". This signal is essentially stable for thousands of years and has applications in EPR dosimetry, EPR dating and detection of irradiated food.
Some ten to twenty years ago this signal was assigned to a single radical, either to CO₃^3- or CO₂^-. An extensive comparative EPR study performed in our laboratory on both biological and synthetic apatites, and on carbonate containing single crystals, convincingly proved that the signal is composite and mainly due to two or CO₂^- radicals. Moreover, other carbonate derived radicals like CO₃^3-, CO₃^-, CO^-, O^- and O₃^- may also contribute to the EPR spectrum. For the spectrum decomposition the applicability of the statistical MLCFA (Maximum Likelihood Common Factor Analysis) techniques has been demonstrated.
For some ions it was possible to identify their exact location by means of powder ENDOR studies using the "orientation selection principle" (O^-, CO₃^3-, O₃^-). The principal g-values of most radicals discussed in this work depend only weakly on the host lattice. The EPR work on CO₂^-, CO₃^- and O₃^- in alkali halide single crystals is very illustrative in this respect.
The possible consequences of the composite character of the asymmetric EPR signal near g = 2 for EPR applications in dosimetry, dating and food irradiation, are discussed.