Etienne Boesman

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.

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.

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.

EPR study of carbonate derived and ozonide radicals in carbonated apatites synthesized from aqueous-solutions.

In this study, a series of Na+ and12CO32− containing apatites synthesized by a hydrolysis of octo-calciumphosphate (OCP) and dried at 25°C until constant weight, were examined with EPR after X-irradiation. A variety of different paramagnetic radicals was formed, giving rise to composite and hence complex EPR powder spectra. The spectra were successfully decomposed into their basic components using a multivariate statistical technique. The different components could be identified unambiguously. In this way, it was found that an O−, an O3−, a CO3−, two types of CO2− and two types of CO33− ions were formed upon X-irradiation. Also resonances from atomic hydrogen were observed. The most striking feature is the presence of the ozonide ion, which is only rarely observed in irradiated hydroxyapatites. A comparison is made between the EPR spectra of apatite samples prepared by hydrolysis of OCP on the one hand, and those of samples prepared by hydrolyzing monetite, and the tooth enamel spectrum on the other hand.

Electron-Nuclear Double Resonance of Human Tooth Enamel Heated at 400°C

Abstract. Powdered tooth enamel heated at 400°C has been examined with electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIEPR) methods and compared with hydroxyapatite synthesized at high temperature. The EPR spectrum of the tooth enamel sample consists mainly of signals assigned to O− and CO2− radicals, in contradistinction to the synthetic apatite sample, where signals of O− and CO33− were detected. The ENDOR results of heated enamel show that the O− radical is located at an A site and interacts with three unequivalent sets of 31P nuclei and two unequivalent protons. This supports the model proposed for synthetic apatite. The EIEPR results show that the 31P and 1H ENDOR resonances originate from both O− and CO2− radicals and that the 23Na and 19F resonances originate only from a CO2− radical located probably at the surface.

Electron nuclear double resonance of stable Rh centers in solution-grown NaCl single crystals

Unlike the photographically important silver halides, large NaCl single crystals can be grown from solution. In such NaCl crystals, with doping comparable to practical AgCl and AgBr microcrystals, three stable Rh centers were detected and studied by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). The primary center was identified as a nonlocally compensated [RhCl6]4− complex, at variance with earlier reported EPR work. By lack of sufficient EPR and/or ENDOR evidence, the identity of the two other Rh related centers with orthorhombic and isotropic symmetry respectively, remains rather speculative.

Electron paramagnetic resonance and electron nuclear double resonance of NaCl:Rh2+: A comparison between similarly doped Bridgman- and solution-grown single crystals

In a recent electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) study several new rhodium related centres in a Bridgman-grown crystal were reported. For two centres a detailed microscopic model was established. The primary Rh2+ centre was identified as a [RhCl6]4− complex with a single charge compensating Na+ vacancy in a next nearest neighbour position. A second centre was identified as a rhodium dimer. Two other Rh2+ centres, with apparent axial symmetry, remained unidentified. In the present study a detailed comparison between Rh2+ defects in Bridgman- and solution-grown NaCl crystals is presented. One of the previously unidentified Rh2+ centres is shown not to be axial but to have orthorhombic-I symmetry and is identified as a cis-[RhCl6]4− complex with two next nearest neighbour Na+ vacancies. For the primary centre additional ENDOR interactions confirm the model mentioned above. Finally a comparison is made between the temperature stability of the primary and orthorhombic-I centre in Bridgman-grown and similarly doped solution-grown NaCl crystals.

ELECTRON PARAMAGNETIC RESONANCE OF SELENIUM DEFECTS IN NACL

Abstract An EPR study of two new selenium centres in NaCl is presented. The paramagnetic centres are formed after appropriate doping and exposure to X-rays at room temperature. By enriching the selenium dopant with77Se it is shown that these defects involve one and two selenium nuclei, respectively. For the first defect a Se− ion is a plausible model, the other one is attributed to a Se2− ion in a divacancy site. Both centres are compared with the analogous centres in sulphur-doped NaCl and KCl. For the first time the selenium atomic parameters γeγn( 8π 3 )|ψ(0)|2, γeγn〈r−3〉s and γeγn〈r−3〉l are determined from experimental 77Se hyperfine data.