Paul Matthys

The contribution of CO3(3-) and CO2- to the ESR spectrum near g = 2 of powdered human tooth enamel.

SummaryThe ESR spectrum near g=2 of powdered human tooth enamel from upper central incisors and lower canines was studied as a function of microwave power, irradiation, and storage time. The results clearly demonstrate that the ESR spectrum is composite with at least five paramagnetic species contributing to the signal. The main stable component is assigned to CO2−. Two other components arise from CO33− radicals, one of which is demonstrated to be the same center as is present on a phosphate site in sodium- and carbonate-containing calciumapatite.

EPR of carbonate derived radicals: Applications in dosimetry, dating and detection of irradiated food

After exposure of biological (tooth enamel, bone, …) and synthetic apatites to ionizing radiation, the so-called “asymmetric EPR signal nearg = 2” is formed. Although this signal is being used in EPR dosimetry, dating and detection of irradiated food for many years already, its composite character and the precise nature of the radicals contributing to the spectrum are still insufficiently known and/or recognized. For some fifteen years already, the EPR group in Ghent is gaining extensive experience on the radicals present in calcified tissues and model systems like synthetic apatites, calcites and single crystals doped with carbonate. It will be shown that the majority of radicals in calcified tissues are carbonate derived, e.g., CO2−, CO3−, CO33− while also phosphate derived radicals like PO42− and oxygen species (O−, O3−) have been identified with EPR and/or ENDOR. For the EPR applications mentioned above, the most important type of radicals is CO2t- (g values ranging from 2.0035 to 1.9970). A second type of radicals which is very intriguing but still badly known, exhibits a spectrum atg values around 2.0045. It is very apparent in tooth enamel below doses of 1 Gy, it has been observed in certain fossil teeth in a very prominent way and also in irradiated food containing bone (e.g., frog legs). It will be shown that the organic origin of this signal can be questioned. The importance of other radicals like CO33t- and CO3t- for EPR applications will also be discussed.

Effect of carbonate content on the ESR spectrum near g=2 of carbonated calciumapatites synthesized from aqueous media

SummaryThe ESR spectrum of X-irradiated carbonated apatites synthesized at low temperature was studied as a function of their carbonate content. Using13C-enriched samples, four different carbonate-derived radicals and a surface O− ion could be identified. Isotropic CO3− and CO2− ions are present at a B site in the apatite lattice, and anisotropic CO3− and CO2− radicals are located at the surface of the crystallites. Only the isotropic ESR signals increase with increasing carbonate content. The anisotropic signal ascribed to a surface CO2− radical is mainly responsible for the so-called asymmetric ESR signal near g=2. It is argued that this surface signal may still be composite and caused by several very similar CO2− ions. The consequences for phenomenological ESR studies of calcified tissues are discussed.

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.

Adsorption of carbonate-derived molecules on the surface of carbonate-containing apatites

Carbonated apatites synthesized at low temperatures were dried until constant weight at 25 °C. Two 99%13C-enriched samples were examined with EPR after X-irradiation. The observed spectra are interpreted in terms of five paramagnetic radicals, i.e. on O–, two CO–3 and two CO–2 radicals. As a result of the 13C hyperfine coupling, practically no signals were found in the region around g= 2, where EPR signals originating from 12C-containing radicals (commonly called 12C signals) are to be expected. Afterwards, the apatite powders were spread out in order to ensure a good contact of the apatite surface with the surrounding atmosphere. The samples were again X-irradiated and re-examined with EPR. Strong 12C signals were detected and attributed to surface radicals. Two different paramagnetic species were tentatively identified as CO–2 and CO– radicals adsorbed on the apatite surface. The g values for these radicals are g1= 2.0030, g2= 2.0015, g3= 1.9973 for the CO–2 radical and g1= 2.0058, g2= 2.0041, g3= 2.0023 for the assumed CO– radical.

31 P and 1H powder ENDOR and molecular orbital study of a CO33– ion in X-irradiated carbonate containing hydroxyapatites

An X-irradiated synthetic carbonate-containing apatite powder is examined with EPR and ENDOR. At low microwave powers, the room-temperature EPR spectrum contains a major contribution of a signal with g values: gx= 2.0045, gy= 2.0034 and gz= 2.0014. In a related 13C-enriched sample, the radical was shown to exhibit a hyperfine interaction with one carbon nucleus. The 13C hyperfine tensor values are: Ax= 263 MHz, Ay= 263 MHz and Az= 423MHz. The radical is assigned to a CO33– molecular ion. It is demonstrated by means of CNDO/II and INDO calculations that by lowering the symmetry of the CO33– ion from C3v to Cs, an orthorhombic g tensor can be obtained. However, the deviation from axial symmetry for the 13C hyperfine tensor is so small that it is not measurable on a powder specimen. The thus-calculated spin-Hamiltonian parameters are in very good qualitative and quantitative agreement with the experimental ones, adding strong evidence for the assignment of the observed signal to a CO33– radical.At low temperatures, both 31P and 1H ENDOR spectra are recorded for different settings of the magnetic field (i.e. when the magnetic field is swept through the EPR CO33– spectrum). By a careful analysis of the ENDOR powder spectra using computer simulations based on the ‘orientation-selection’ principle, a detailed model for the CO33– ion could be proposed. In this way, it is established unambiguously that the CO33– ion substitutes for a phosphate group in the hydroxyapatite lattice, with a vacancy on the nearest hydroxy-group site. In addition, some deductions can be made about the substitution mechanism according to which the precursor of the CO33– radical (i.e. a carbonate ion) is incorporated into the apatitic lattice.

The effect of carbonate content and drying temperature on the ESR-spectrum near g = 2 of carbonated calciumapatites synthesized from aqueous media

SummaryThe ESR spectrum of X-irradiated carbonated apatites precipitated from aqueous solutions was studied as a function of their carbonate content and drying temperature. When the latter increases from 25 to 400°C, the ESR spectrum is gradually modified and becomes similar to the spectrum of carbonated apatites, synthesized at high temperatures by solid state reactions. The latter ESR spectrum is dominated by CO33−-contributions whereas the spectrum of precipitated samples dried at 25°C can mainly be interpreted in terms of CO2−, CO3−, and O− ions. The behavior of these earlierreported CO2−, CO3−, and O− centers is now studied as a function of drying temperature. In addition, the Spin Hamiltonian parameters of the CO33− centers are determined and some other new paramagnetic radicals are discussed. It is shown that a CO32− ion at a phosphate lattice site (B-type substitution) may give rise to either a CO2−, CO3−, or CO33− radical on X-irradiation, depending on the sample preparation conditions. A surface CO32− ion may cause a surface CO2−, CO3−, or O− radical. From the reported results it is not unambiguously clear whether the CO33− ion detected in the samples with the relatively lowest carbonate content should be located on the surface or on a hydroxyl lattice site (A-type substitution). An important result is that the absolute concentration of the B-type CO33− ion increases with increasing carbonate content as was also the case for the earlier reported B-type radicals (isotropic CO2− and CO3−). On the other hand, the absolute concentration of the surface radicals decreases with increasing carbonate content. The reported results show that similar deconvolution techniques can be applied in the future for the study of ESR spectra of calcified tissues. This will allow a more efficient phenomenological investigation of the latter.

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.

Comparative X- and Q-Band EPR Study of Radiation-Induced Radicals in Tooth Enamel

Abstract Vanhaelewyn, G. C. A. M., Sadlo, J., Matthys, P. F. A. E. and Callens, F. J. Comparative X- and Q-Band EPR Study of Radiation-Induced Radicals in Tooth Enamel. Radiat. Res. 158, 615–625 (2002). Human tooth enamel blocks and powders that were either unheated or heated prior to X irradiation at room temperature were investigated by means of Q-band electron paramagnetic resonance (EPR). It was found that the EPR spectra of unheated human tooth enamel consist mainly of two different anisotropic CO2− signals, as was suggested previously from an X-band study of analogous samples. In the present study, the two CO2− radical contributions could be differentiated convincingly by comparing the anisotropic Q-band spectra of heated and unheated enamel blocks. One type of CO2− is probably located in the bulk of the apatitic microcrystallites that constitute the enamel, and it appears in both heated and unheated samples. The other type is presumably located in an intercrystallite position and appears mainly in the unheated samples. Clear differences between g values in the Q-band spectra of heated and unheated enamel suggest that the CO2− radicals in the bulk exhibit larger g anisotropy than those in intercrystallite positions. Isotropic CO2− signals and contributions that may be from CO33− and CO− radicals have also been detected. However, the present work focuses mainly on the CO2− signals and discusses potential and/or real difficulties that may be encountered in applications of EPR dosimetry using calcified tissues.

A study of the composite character of the ESR spectrum of alanine

Abstract Both L and DL-alanine ESR powder spectra have been studied in the dose range of 1–10 6 Gy. By using Maximum Likelihood Common Factor Analysis (MLCFA), it has been demonstrated that the ESR spectrum of L-alanine is at least 3-fold composite over the whole dose range considered. For DL-alanine, in certain dose ranges only two ESR components could be detected. Possible consequences for the application and optimization of alanine dosimetry are discussed.