Quantitative EPR spectroscopy: Comparison between primary standards and application to MgO-MnO and α-Al2O3-Cr2O3 solid solutions

Abstract

To study their reliability as primary standards in the quantitative EPR spectroscopy, a large series of pure paramagnetic compounds with known spin concentrations, whose spectra vary considerably in intensity, shape, structure and overall width are compared. The paramagnetic species examined as pure solid compounds and solutions, were free radicals (DPPH and TEMPO), vanadyl and Cul l ions (S = 1/2), Cr" (S = 3/2) and Mu ll (S = 5/2) ions. The quantitative EPR findings suggest that all the S = 1/2 paramagnetic compounds investigated and MnSO4 • H2O (S = 5/2) are reliable primary standards. By contrast, none of the pure Cr" compounds proved useful as primary standards because of their large fine-structure terms or high Néel temperature that invalidated the simple Curie law. Application of quantitative EPR in the study of dilute MgO-MnO and a-Al203 Cr203 solid solutions, focussing on the circumstances making paramagnetic species undetectable, is reported. In MgOMnO solid solutions of high surface area, detection problems arising from the variation of local site symmetry can be circumvented and almost all Mn 2 are detected only by reducing the surface area. In concentrated a-Al2O3-Cr203 solid solutions, magnetic interactions lead to paramagnetic species being undetectable.

Reactivity of some vanadium oxides: An EPR and XRD study

“…The absolute concentration of the paramagnetic species was determined from the integrated area of the spectra recorded at RT, using as standard the Varian Strong Pitch, SP (5 Â 10 15 spin cm À1 ). This secondary standard was accurately calibrated with a series of primary standards, including solid VOSO 4 Á 5H 2 O and VO(acac) 2 [25]. The number of spin cm À1 , N, of sample 'a' and standard 'b', obeying the Curie law, are related by the equation:…”

Boosting Supercapacitor Performance of TiO2 Nanobelts by Efficient Nitrogen Doping

“…Electron paramagnetic resonance (EPR) spectroscopy was performed to confirm the presence of unpaired electrons in oxygen vacancies. For precise determination of the G‐values, in this study we have employed MgO powders containing Mn 2+ ions (0.1 % w/w, g=2.0064) as reliable standard samples for g‐value calibration, which have been extensively used for quantitative EPR spectrometry . d‐TiO 2 NBs present a strong EPR signal (Figure b), which are assigned to the O 2− adsorption owing to the Ti 3+ sites on the surface of d‐TiO 2 NBs.…”

“…For precise determination of the G-values, in this study we have employed MgO powders containing Mn 2 + ions (0.1 % w/ w, g = 2.0064) as reliable standard samples for g-value calibration, which have been extensively used for quantitative EPR spectrometry. [31,32] d-TiO 2 NBs present a strong EPR signal (Figure 2b), which are assigned to the O 2À adsorption owing to the Ti 3 + sites on the surface of d-TiO 2 NBs. No signal was seen for the TiO 2 NBs sample.…”

Effects of Natural Antioxidant Agents on the Bitumen Aging Process: An EPR and Rheological Investigation

“…The spin concentration could be estimated by taking the ratio of area of the sample to that of a standard containing a known number of radicals. Therefore, the concentration of paramagnetic centers in both the asphaltene and maltene fractions of bituminous specimens were obtained by double integration of the experimental first-derivative spectrum and compared with EPR spectrum of a known amount of a MgO-MnO solid solution used as standard [36,37] (spin density = 6.83 × 10 15 spin/g). The g-value (Landé factor) was determined from the condition hν = gβB r = g s βB s , from which g = g s B s /B r where ν is the frequency of the used microwave radiation, β the Bohr magneton for electron, g s = 1.9810 is the g-value for the standard [38,39], B s and B r are the magnetic field values of standard and sample, respectively.…”