Peter Moens

AN EPR STUDY OF INTACT AND POWDERED HUMAN TOOTH ENAMEL DRIED AT 400C

An EPR study of human tooth enamel dried at 400°C is presented. Enamel blocks as well as powdered samples were investigated. The discussion deals mainly with three different spectral components, i.e., a CO33-and two different CO2-signals. Using the anisotropic enamel block spectra, a convincing differentiation between the latter two radicals was possible. The first CO2-signal shows no dependence on the orientation of the enamel blocks, disappears from the spectrum upon heating, and was assigned to a surface radical. The second CO2-component is mainly responsible for the angular variation of the enamel blocks and is assigned to a bulk position. For the CO33-ion, the (pseudo) angular variation of its isolated spectrum is presented and discussed. By means of the results presented in this study, earlier interpretation problems are considerably reduced.

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.

An EPR spectrum decomposition study of precipitated carbonated apatites (NCAP) dried at 25°C: Adsorption of molecules from the atmosphere on the apatite powders

Abstract The effect of storage under ambient conditions on the Electron Paramagnetic Resonance (EPR) spectrum of X-irradiated sodium and carbonate containing synthetic apatites has been studied. A first series of samples was X-irradiated shortly after preparation and drying at 25°C and investigated by means of EPR. The observed spectra were decomposed in terms of five theoretical curves representing an O− radical, two CO3− radicals (surface and bulk) and two CO2− radicals (surface and bulk). Afterwards, a second series of the same samples which was stored under ambient conditions for a long period, was also X-irradiated and examined with EPR. The same five radicals were found, but in different relative amounts. It appeared that the relative contributions of the two carbon containing surface radicals increased in comparison with the corresponding bulk radicals. This is explained by an adsorption of molecules from the atmosphere on the surface of the apatite powder.

Location and Motion of Isotropic Paramagnetic Centres in Synthetic Monohydrocalcite

The isotropic CO-2 and CO-3 centres in CaCO3H2O were examined with electron spin resonance (ESR), electron-nuclear double resonance (ENDOR) and thermogravimetric analysis (TGA). From the absence of anisotropy and the small value of the 1H ENDOR splitting ( ~0.3 MHz), the effect of crushing and/or thermal annealing on the ESR and TGA spectra, it is concluded that both isotropic centres are located in the occluded water, surrounding the constituent crystallites of the spherulites. ESR line width and 13C hyperfine coupling parameter measurements yield approximate values for the rotation activation energy (ΔE( CO3-<ΔE( CO2-) with ΔE~ 0.2 eV) and for the vibrational frequency ( ~2.1012 Hz for CO2-).

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.

New method for threshold voltage extraction of high-voltage MOSFETs based on gate-to-drain capacitance measurement

This letter reports on the extraction of the threshold voltage of laterally diffused MOS transistors. A clear analysis of the device physics is performed, highlighting the correlation between the change of the electron charge distribution along the channel and the device capacitance variations when the gate voltage is swept. Using numerical simulations, it is shown that the peak of the gate-to-drain capacitance is related to the transition of the surface from weak to moderate inversion in the intrinsic MOS transistor at the location of the maximum doping concentration, which corresponds to the threshold voltage of the device according to the MOS theory. Comparison between conventional I/sub D///spl radic/g/sub m/ extraction and the new proposed capacitance peak method is performed on both technology computer-aided design simulations and measurements in order to confirm the new experimental technique and related theory.

An electron nuclear double resonance and electron spin resonance study of isotropic CO2- and SO2- radicals in natural carbonates

Natural samples often exhibit isotropic electron spin resonance (ESR) signals at g=2.0006, (CO2-) and g=2.0057 (SO2-). Electron nuclear double resonance (ENDOR) data were collected for various calcitic or aragonitic samples of marine origin. The small and isotropic H-1 hyperfine ENDOR splitting (250-350 kHz) is explained by a locally disordered water environment for these species. As for the isotropic CO2- and CO3- radicals in synthetic monohydrocalcite and apatite, the radicals are most probably located in the occluded water embedded in these samples, and not in the crystal matrix.

31P and1H powder ENDOR study of ozonide radicals in carbonated apatites, synthesized from aqueous solutions

An O3− defect in Na+ CO32− containing apatite powder has been investigated with ENDOR after X-irradiation. The powder, synthesized by a hydrolysis of octo-calciumphosphate (OCP) and Na2CO3 was dried at 25°C until constant weight was reached. At low temperatures, both31P and1H ENDOR spectra were recorded for different settings of the magnetic field (i.e., when the magnetic field is swept through the EPR O3− spectrum). The ENDOR powder spectra were thoroughly analyzed using computer simulations based on the “orientation selection principle”. Interactions with two types of protons and two types of31P nuclei could be resolved. In this way, a detailed model could be established for the O3− ion in the hydroxyapatite lattice. The defect is located between two successive vacant hydroxyl sites. The axis connecting the two outer oxygen atoms (gy-axis) of the O3− ion is found to be along the hexagonalc-axis of the lattice. The twofold axis of the defect ion (gz-axis) is parallel to theb-axis of the lattice.

1H and31P ENDOR of the isotropic CO2 − signal atg=2.0007 in the EPR spectra of precipitated carbonated apatites

The isotropic EPR signal atg=2.0007 in X-irradiated carbonated apatites, precipitated from aquaeous solutions and dried at 25°C, is investigated with Electron Nuclear Double Resonance (ENDOR). The23Na,31P and1H ENDOR results indicate that the precursor of this radical is most probably located in an aqueous phase entrapped between the crystallites (the so-called occluded water). Other experimental features, such as the correlation between the appearance of the signal in the EPR spectrum and the presence of some residual (adsorbed and/or occluded) water in the sample seem to strengthen our hypothesis.