Understanding the dosimetric powder EPR spectrum of sucrose by identification of the stable radiation-induced radicals.

Sucrose, the main component of table sugar, present in nearly every household and quite radiation sensitive, is considered as an interesting emergency dosemeter. Another application of radiation-induced radicals in sugars is the detection of irradiation in sugar-containing foodstuffs. The complexity of electron paramagnetic resonance (EPR) spectra of radicals in these materials, as a result of many hyperfine interactions and the multi-compositeness of the spectra of individual sugars, complicate dose assessment and the improvement of protocols for control and identification of irradiated sugar-containing foodstuffs using EPR. A thorough understanding of the EPR spectrum of individual irradiated sugars is desirable when one wants to reliably use them in a wide variety of dosimetric applications. Recently, the dominant room temperature stable radicals in irradiated sucrose have been thoroughly characterised using EPR, electron nuclear double resonance (ENDOR) and ENDOR-induced EPR. These radicals were structurally identified by comparing their proton hyperfine and g-tensors with the results of Density Functional Theory calculations for test radical structures. In this paper, the authors use the spin Hamiltonian parameters determined in these studies to simulate powder EPR spectra at the standard X-band (9.5 GHz), commonly used in applications, and at higher frequencies, up to J-band (285 GHz), rendering spectra with higher resolution. A few pitfalls in the simulation process are highlighted. The results indicate that the major part of the dosimetric spectrum can be understood in terms of three dominant radicals, but as-yet unidentified radicals also contribute in a non-negligible way.

Multi-frequency electron paramagnetic resonance study of irradiated human finger phalanxes.

Electron paramagnetic resonance (EPR) is often used in dosimetry using biological samples such as teeth and bones. It is generally assumed that the radicals, formed after irradiation, are similar in both tissues as the mineral part of bone and tooth is carbonated hydroxyapatite. However, there is a lack of experimental evidence to support this assumption. The aim of the present study was to contribute to that field by studying powder and block samples of human finger phalanxes that were irradiated and analyzed by multi-frequency EPR. The results obtained from bones are different from the ones obtained in enamel by several respects: the ordering of the apatite crystallites is much smaller in bone, complicating the assignment of the observed CO2- radicals to a specific location, and one type of CO3(3-) radical was only found in enamel. Moreover, a major difference was found in the non-CO2- and non-CO3(3-) signals. The elucidation of the nature of these native signals (in bone and tooth enamel) still represents a big challenge.

Effect of different sterilisation methods on the properties of bioadhesive powders and ocular minitablets, and clinical evaluation.

The purpose of this study was to evaluate the influence of gamma-irradiation and dry heat sterilisation on the properties of a bioadhesive powder mixture containing ciprofloxacin and its corresponding ocular minitablets. The molecular weight characteristics of drum dried waxy maize starch (DDWM), employed as major component of the bioadhesive formulation, the decay kinetics of radicals, the rheological properties of the bioadhesive polymers and the microbial activity of ciprofloxacin were studied. The influence of the different sterilisation methods on the characteristics of the ocular minitablets was investigated by measuring the crushing strength, the friability, and the in vitro release of ciprofloxacin from the minitablets. Finally, the clinical value of the selected sterilised minitablets was evaluated in seven healthy volunteers. Both sterilisation methods similarly affected the properties of the bioadhesive formulation by inducing stable radicals and decreasing the molecular weight of DDWM, although no changes in the microbiological activity of ciprofloxacin were measured. An obvious influence of both sterilisation methods was observed in the in vitro release study. The crushing strength and friability of the minitablets were not significantly influenced by gamma-irradiation. Based on these data, gamma-irradiation was more adequate as sterilisation method for the bioadhesive ocular minitablets than dry heat sterilisation, because it affected the least the physical properties of the minitablets. Therefore, the gamma-sterilised minitablets were selected for an in vivo evaluation in seven volunteers. The concentration of ciprofloxacin in the tear film remained above its MIC value for the most common ocular pathogens for at least 8 h. Consequently, the gamma-irradiated minitablets containing ciprofloxacin can be considered as a promising formulation to treat bacterial keratitis and conjunctivitis.

ENDOR-assisted study of the stable EPR spectrum of X-irradiated alpha-L-sorbose single crystals: MLCFA and simulation decomposition analyses.

After X irradiation of single crystals of alpha-L-sorbose at 295 K, previous electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EI-EPR) results have indicated the formation of at least 10 different free radicals, and also that conceivably each carbon in the pyranose ring is a possible radical center. The radicals appear to be formed mostly by net H-abstraction reactions followed by standard elimination (e.g. beta-OH elimination) reactions or proton shifts, in turn leading to ring opening and fragmentation. In the present work, EPR spectra were recorded at room temperature with the external magnetic field along each of the three crystallographic axes subsequent to careful annealing at different temperatures using a high-temperature cavity. Each of the three sets of spectra was subjected to a maximum likelihood common factor analysis (MLCFA) that contributed to a better understanding of the spectral decays. Furthermore, the most stable spectra were simulated by optimization of previous ENDOR and EI-EPR results. The optimized EPR parameters resulted in excellent simulations of the experimental stable sorbose spectra and hence provided an improved insight of their spectral compositions.

Some recent multi-frequency electron paramagnetic resonance results on systems relevant for dosimetry and dating.

Electron Paramagnetic Resonance (EPR) applications like e.g. EPR dosimetry and dating, are usually performed at X-band frequencies because of practical reasons (cost, sample size, etc.). However, it is increasingly recognized that the radiation-induced EPR signals are strongly composite, what might affect dose/age estimates. A few recent examples from both the dosimetry and dating field, illustrating the problems, will be presented. The involved spectra are mainly due to carbonate-derived radicals (CO2-, CO3(3-), etc.). Measurements at higher microwave frequencies are often recommended to improve the insight into the spectra and/or the practical signal quantification. Recent results at Q- and W-band frequencies will show that a multi-frequency approach indeed opens many interesting perspectives in this field but also that each frequency may have specific (dis)advantages depending on the EPR probe and application involved. The discussion will concern carbonate-containing apatite single crystals, shells, modern and fossil tooth enamel.

Multifrequency EPR study of carbonate- and sulfate-derived radicals produced by radiation in shells and corallite.

Shells of two sea mollusks (Venus sp.), pearl oyster (Meleagrina vulgaris) and corallite (white coral) were exposed to ionizing radiation (gamma and X rays) and then examined by EPR spectroscopy in X, Q and W band. The resulting spectra were analyzed and the g values of the EPR lines in the multicomponent spectra were determined. The increased resolution in Q- and W-band spectra allowed us to assign the observed lines to CO(2)(-) ion radicals (isotropic and orthorhombic), SO(2)(-) isotropic, SO(3)(-) (isotropic and axial), and Mn(2+) species. The assignments were confirmed by simulations of the spectra. Practical implications for the use of Q and/or W band in low-dose quantitative EPR measurements for dating and for accidental dose estimation are discussed.

X- and Q-band electron paramagnetic resonance of CO2- in hydroxyapatite single crystals.

Both X- and Q-band electron paramagnetic resonance (EPR) research has been conducted using slightly carbonated hydroxyapatite (HAp) single crystals after exposure to ionizing radiation. Below a temperature of 90 K, O(-) and CO(2-) radicals were detected, whereas at room temperature only CO(2-) spectra could be observed. The O(-) ion has previously been investigated in high-purity HAp single crystals, whereas EPR spectra of CO(2-) in HAp single crystals have not been reported. Both paramagnetic defects exhibit EPR angular variations in planes containing the c axis of the crystal from which spin Hamiltonian parameters were derived. Arguments are given for the presence of two CO(2-) defects in the irradiated HAp single crystals.

A decomposition study of the EPR spectrum of irradiated sucrose

In general, the EPR spectra of irradiated sugars are very complex because of their multicomponent character. In this study we applied a multivariate statistical method called MLCFA, maximum likelihood common factor analysis, and it predicted at least six components contributing to the total EPR spectrum of irradiated sucrose. Three dominant components have already been isolated in an irradiated sucrose single crystal using electron nuclear double resonance (ENDOR) and ENDOR induced EPR (EI-EPR). Results of EPR simulations based on the ENDOR data are in a reliable agreement with the experimental EPR spectra of irradiated sucrose single crystals.

EPR spectrum deconvolution and dose assessment of fossil tooth enamel using maximum likelihood common factor analysis.

In order to determine the components which give rise to the EPR spectrum around g = 2 we have applied Maximum Likelihood Common Factor Analysis (MLCFA) on the EPR spectra of enamel sample 1126 which has previously been analysed by continuous wave and pulsed EPR as well as EPR microscopy. MLCFA yielded agreeing results on three sets of X-band spectra and the following components were identified: an orthorhombic component attributed to CO2-, an axial component (CO3(3-)), as well as four isotropic components, three of which could be attributed to SO2-, a tumbling CO2- and a central line of a dimethyl radical. The X-band results were confirmed by analysis of Q-band spectra where three additional isotropic lines were found, however, these three components could not be attributed to known radicals. The orthorhombic component was used to establish dose response curves for the assessment of the past radiation dose, D(E). The results appear to be more reliable than those based on conventional peak-to-peak EPR intensity measurements or simple Gaussian deconvolution methods.

Decomposition study of the electron paramagnetic resonance spectrum of irradiated alanine.

Recent Electron Paramagnetic Resonance (EPR) studies on alanine powders as a function of irradiation dose and temperature on the one hand and single crystal Electron Nuclear DOuble Resonance (ENDOR) studies on the other hand, showed the presence of at least three radicals contributing to the total alanine EPR spectrum. The latter spectrum obtained after irradiation at room temperature (RT), is dominated by the well-known stable-alanine-radical (SAR) CH3C*HCOO-, also denoted R1. Appropriate heating of irradiated alanine causes the relative contribution of R1 to decrease, resulting in a spectrum mainly caused by the H-abstraction radical CH3C*(NH3)COO-, denoted R2. Although the EPR spectrum of these two radicals could be satisfactorily simulated, their influence on dose reconstruction has not been reported yet. Therefore, a detailed Maximum Likelihood Common Factor Analysis (MLCFA) study has been performed on EPR spectra from polycrystalline alanine samples, after irradiation and heat treatments. Conclusions concerning the number of contributing radicals and their influence on the RT irradiated alanine EPR spectrum will be made.

Effect of temperature on the electron paramagnetic resonance spectrum of irradiated alanine.

Polycrystalline samples of the amino acid L-alpha-alanine have been irradiated with X rays at both room temperature and higher temperatures. The electron paramagnetic resonance (EPR) spectra of alanine powder irradiated at room temperature are dominated by the well-known room-temperature-stable alanine radical CH3C*HCOOH. Upon heating of room-temperature-irradiated alanine powder, a strong decay of the signal was observed, and the features of the spectrum recently ascribed to a second stable radical in alanine irradiated at room temperature become more pronounced, providing an experimental isolation of this second alanine radical. In combination with the high-temperature experiments, a multivariate statistical decomposition method, maximum likelihood common factor analysis, was used to determine the number of components in irradiated alanine powder which behave differently as a function of temperature. The EPR components found in the present study are compared with simulations using earlier EPR and ENDOR single-crystal data.