Quantitative Electron Paramagnetic Resonance Measurements Research Articles

Identification of intra‐grain and grain boundary defects in polycrystalline Si thin films by electron paramagnetic resonance

AbstractWe investigate the characteristics of intra‐grain and grain boundary defects in polycrystalline Si films, by employing quantitative electron paramagnetic resonance measurements on liquid phase crystallized layers with an average grain size of 200 µm and tailored solid phase crystallized Si layers with similar intra‐grain morphology but systematically varied grain sizes between 0.25 µm and 1 µm. The defect characteristics are found to be composed of two distinctive g ‐values of g = 2.0055 and 2.0032, which are attributed to grain boundary defects and intra‐grain defects, respectively. Additional hydrogenation leads to a reduction of the overall defect concentration, while a rapid thermal annealing process primarily heals intra‐grain defects.magnified image(© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evaluation of the binding of oxovanadium(iv) to human serum albumin

The understanding of the biotransformations of insulin mimetic vanadium complexes in human blood and its transport to target cells is an essential issue in the development of more effective drugs. We present the study of the interaction of oxovanadium(iv) with human serum albumin (HSA) by electron paramagnetic resonance (EPR), circular dichroism (CD) and visible absorption spectroscopy. Metal competition studies were done using Cu(II) and Zn(II) as metal probes. The results show that V(IV)O occupies two types of binding sites in albumin, which compete not only with each other, but also with hydrolysis of the metal ion. In one of the sites the resulting V(IV)O-HSA complex has a weak visible CD signal and its X-band EPR spectrum may be easily measured. This was assigned to amino acid side chains of the ATCUN site. The other binding site shows stronger signals in the CD in the visible range, but has a hardly measurable EPR signal; it is assigned to the multi metal binding site (MBS) of HSA. Studies with fatted and defatted albumin show the complexity of the system since conformational changes, induced by the binding of fatty acids, decrease the ability of V(IV)O to bind albumin. The possibility and importance of ternary complex formation between V(IV)O, HSA and several drug candidates - maltol (mal), picolinic acid (pic), 2-hydroxypyridine-N-oxide (hpno) and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone (dhp) was also evaluated. In the presence of maltol the CD and EPR spectra significantly change, indicating the formation of ternary VO-HSA-maltol complexes. Modeling studies with amino acids and peptides were used to propose binding modes. Based on quantitative RT EPR measurements and CD data, it was concluded that in the systems with mal, pic, hpno, and dhp (V(IV)OL(2))(n)(HSA) species form, where the maximum value for n is at least 6 (mal, pic). The degree of formation of the ternary species, corresponding to the reaction V(IV)OL(2) + HSA -->/<-- V(IV)OL(2)(HSA) is hpno > pic ≥ mal > dhp. (V(IV)OL)(n)(HSA) type complexes are detected exclusively with pic. Based on the spectroscopic studies we propose that in the (V(IV)OL(2))(n)(HSA) species the protein bounds to vanadium through the histidine side chains.

Effect of phosphorylation and pretreatment with high hydrostatic pressure on radical processes in maize starches with different amylose contents

Waxy maize (amylopectin with trace amounts of amylose) and Hylon VII starch containing mostly amylose were phosphorylated in acidic or alkaline medium to produce mono- or distarch phosphates, respectively. The changes in the starch structure and properties occurring upon phosphorylation and high pressure treatment (650 MPa/9 min) were analyzed using X-ray diffraction (XRD), differential scanning calorimetry (DSC), polarized light microscopy (LM) and electron paramagnetic resonance (EPR) spectroscopy. Quantitative EPR measurements revealed that incorporation of phosphorus into the starch structure increased tendency of the starch to be thermally degraded with formation of radicals, the effect being more pronounced in the case of monostarch phosphates than in distarch phosphates. Pretreatment of the starch with high hydrostatic pressure resulted in diminishing of the number of radicals generated thermally in monostarch phosphates.

Genesis of V4+ in heteropoly compounds CsxH4-xPVMo11O40 during thermal treatment, rehydration and oxidation of methanol studied by EPR spectroscopy

The development and structural changes of V4+ and Mo5+ centers in caesium salts and the acid form of molybdovanadophosphorus heteropoly compounds CsxH4−xPVMo11O40 ·nH2O (x=0, 3, 4) of Keggin structure were investigated by electron paramagnetic resonance (EPR) after preparation, thermal treatment in oxygen, reduction by methanol and catalytic oxidation of methanol. The various types of paramagnetic centers are found to characterize the different states and structural transitions of the heteropoly anion. In addition, the EPR spectra reflect well the different reducibility, thermal stability and mobility of vanadium and Keggin (“ lattice”) oxygen of the Cs4, the Cs3H and the H4 compounds. The V4+ centers in the Cs4 salt are different in structure, lower in concentration and thermally more stable than those in the acid form. The reduction of vanadium during thermal treatment due to the loss and oxidation of coordinated (“ lattice”) oxygen to O2 is observed in the acid and the Cs3H salt only, but not in the Cs4 compound. As deduced from quantitative EPR measurements and manganometric redox titrations, between 0.5 and 8% of the vanadium is present as V4+ after preparation and thermal treatment in oxygen, whereas the majority of vanadium is reduced by reaction with methanol. V4+ substitutes Mo atoms in the Keggin unit in the fully exchanged Cs salt (Cs4). The presence of the bulky caesium counter-cations prevents the removal of vanadium from the Keggin unit to the secondary structure and thus the formation of vanadyl bridged polymers.

Electron paramagnetic resonance signal intensity of a line-like sample with a variable length situated at an arbitrary position along the common sample-cavity axis. Theoretical prediction versus experimental measurement

The integral equations used for the theoretical prediction of the relative EPR-signal intensity of variable length line-like samples situated at the central position of a microwave cavity have been computed numerically for the cases where: (i) the microwave field is non-uniform but the modulation field is uniform, and (ii) the microwave and modulation fields are non-uniform. In both cases, stress is placed on the variable length of the sample and the length of the active part of the microwave cavity. The theoretically predicted and experimentally observed electron paramagnetic resonance (EPR) signal intensity are in a very good agreement for sample lengths from 1.3 to 50 mm. However, for the cases where the sample length is greater than that of the cavity, the theoretical equations and experimental results can only be reconciled if the cavity is assumed to have an active length, * a=40 mm, considerably greater than the actual length of the cavity, a=23.5 mm. Comparison of the theoretically predicted and experimentally obtained EPR-signal intensities indicate that those parts of the sample situated in the upper and lower sample access holes of the rectangular cavity can contribute significantly to the measured signal intensity. The possible influence of this phenomenon as a significant source of error in quantitative EPR measurements is discussed. The integral equations have been generalised to describe the signal intensity for line-like sample positioned at any point along the common sample-cavity axis.

A simple alignment procedure for the precision positioning of the sample at arbitrary points of the intracavity space of a TE104 double rectangular cavity

A significant source of errors in quantitative electron paramagnetic resonance measurements is incorrect sample positioning in the microwave cavity. In this article a modification to our previously described sample alignment tool which allowed the precise and reproducible positioning of cylindrical samples along the vertical central axis of the rectangular cavity is described. These revisions allow cylindrical sample movement in the horizontal (y,z) plane which is perpendicular to the cavity x axis. Using this upgrade, the new sample setup procedure is now able to position samples precisely and reproducibly at arbitrary points in the intracavity space.

Quantitative electron paramagnetic resonance (EPR) spectrometry with a TE 104 double rectangular cavity Part 2. Analysis of sample and TE 104 cavity error sources associated with the movement of line-like samples into the TE 104 cavity

In this study the sample- and double TE 104 cavity-associated error sources have been analysed. It was found that in quantitative EPR experiments, the EPR signal intensity of identical full-length cavity line-like samples, which were packed by the same procedure with the same powdered material into identical sample tubes, then precision inserted into the same positions within the double TE 104 cavity using a special sample insertion procedure, and measured using the same instrumental parameters, could be obtained with experimental errors of about 3–5%. However, if the procedures for packing and inserting the powdered samples into the cavity were not used, the associated experimental errors could be over 20%. The errors associated with the movement of a full-length cavity line-like sample (30 mm length) into the double TE 104 cavity have been analysed. This analysis has shown that the experimental dependence of the EPR signal intensity upon the movement of the sample along the x-axis of the double TE 104 cavity exhibited a 12.5 mm wide plateau over which the EPR signal intensity remained constant within an experimental error 0.5–0.8%. Because the centre of the plateau coincided with the situation in which centre of the sample was positioned in the cavity centre, this position is recommended for quantitative EPR measurements. The existence of the plateau in the case of full-length cavity line-like samples was found to be the principal difference when compared with the corresponding dependence for point-like samples.

Simple setup for quantitative electron paramagnetic resonance measurements

Simple setup for quantitative electron paramagnetic resonance measurements