Ex vivo analysis of irradiated fingernails: chemical yields and properties of radiation-induced and mechanically-induced radicals.

A qualitative and quantitative analysis of the radicals underlying the radiation-induced signal (RIS) in fingernails was conducted in an attempt to identify properties of these radicals that could be used for biodosimetry purposes. A qualitative analysis of RIS showed the presence of at least three components, two of which were observed at low doses (<50 Gy) and the third required higher doses (>500 Gy). The low dose signal, obtained by reconstruction, consists of a 10 gauss singlet at g = 2.0053 and an 18 gauss doublet centered at g = 2.0044. Based on the initial slope of the dose-response curve, the chemical (radical) yields of the radicals giving rise to the singlet and doublet were 327 (+/-113) and 122 (+/-9) nmol J-1 (standard error, SE), respectively. At doses below 50 Gy, the singlet signal is the dominant component. Above this dose range, the signal intensity of the singlet rapidly dose-saturates. At doses <50 Gy, there is a small contribution of the doublet signal that increases in its proportion of the RIS as dose increases. A third component was revealed at high dose with a spectral extent of approximately 100 gauss and displayed peaks due to g anisotropy at g = 2.056, 2.026, and 1.996. The total radical yield calculated from the initial slope of the dose-response curve averaged 458 +/- (116) nmol J-1 (SE) in irradiated nail clippings obtained from six volunteers. Such high yields indicate that nails are a strong candidate for biodosimetry at low doses. In a comparison of relative stabilities of the radicals underlying the singlet and doublet signals, the stability of the doublet signal is more sensitive to the moisture content of the nail than the singlet. This differential in radical stabilities could provide a method for removing the doublet signal under controlled exposures to high humidities (>70% relative humidity). The decay of the singlet signal in RIS varies with exposure of a nail clipping to differing ambient humidities. However, long exposures (>6 h) to relative humidities of 72-94% results in singlet intensities that approach 7.0 +/- (3.2)% (standard deviation) of the original intensities in an irradiated nail. This result suggests the existence of a subpopulation of radicals underlying the singlet signal that is relatively insensitive to decay under exposure of nails even to high humidities. Therefore, exposures of an irradiated nail clipping under controlled humidities may provide a method for estimating the exposure dose of the nail that is based on the intensity of the signal of the humidity insensitive radical population underlying the singlet signal.