Quantitative EPR: Some of the most difficult problems

Abstract

Abstract. Many of the errors in EPR concentration measurements result from the inability of an analyst to use the reference sample that is identical to the sample to be analyzed in a number of substantial respects. Generally, it is not correct to compare directly the intensities of the signals from two samples that contaln different paramagnetic centers, orare different in matrix material, dimensions, or position in the cavity. It is necessary to evaluate the resulting systematic errors and correct them. The paper is devoted to the analysis of the foUowing sources of errors which frequently occur in such situations: nonuniformity of the cavity fields; different extent of microwave field perturbation by samples with different dielectric constants; inequality of the transition probabilities corresponding to the used spectral lines of the samples under comparison. Theoretical and semiempirical calculations enable one to determine the magnitudes of the errors from these sources and to obtain the appropriate eorrection factors in many specific cases.

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

“…The modern systems of data acquisition and processing (double integration and baseline correction) easily and accurately yield the integrated area of the spectrum. Several books and reviews confirm that the most practical system for determining the spin concentration of a sample is to compare the integrated area of its spectrum with that of an appropriate standard [6][7][8][9][10][11][12][13]. Numerous paramagnetic materials are used as primary, secondary and reference standards [13].…”

Analysis of the Radial and Longitudinal Effect in a Double TE104 and a Single TE102 Rectangular Cavity

“…Figure 4 shows, in a polar coordinate system, how the normalized experimentally observed peak-to-peak height of the first-derivative EPR signal, I pp , varies with anticlockwise rotation ( ʦ ͗0, 2͘) of a pointlike sample in the central ( y-z) plane of the cavity (with x ϭ 0 mm) on the orbits with radii, , equal to 0, the sample position in the cavity center, 1, 2, 3, 4, and 5 mm around the x axis of the first cavity of the double TE 104 rectangular cavity. The agreement between theory and experiment is very good in all cases and any minor deviations can be attributed to the presence of the sample access holes (7,30), to the possible imperfection in the resonator shape (6,15), and to other possible nonuniformities in both the microwave and modulation fields (1,6,7), which were not included in the original empirical model. 4b corresponds to equivalent measurements when the cavity contains a variabletemperature, double-wall quartz Dewar (in this case, the maximum radius is restricted to 3 mm).…”

“…The radial effect has been precisely analyzed by Casteleijn et al (2) using a small DPPH sample about 1 mm in length and by Nagy and Plaček (6) using a pointlike sample, a small crystal of the stable nitroxide radical TEMPOL less than 0.1 mm in size, which was rotated in the horizontal planes around the vertical axis of the cavity in concentric circles with various radii. Unfortunately, the theoretical calculations using a strict mathematical description based on the fundamental Maxwells' equations and the Biot-Savarts' law give poor agreement between the theoretical values and experimental data (15). This phenomenon has been previously neglected in the literature.…”

“…The I pp dependence on rotation angle and for the different radii was found to be a complicated function with a significant increase of the signal intensity when the sample approached the right-left side walls of the cavity and a decrease when the sample approached the front-back walls of the cavity. Together with known polynomial coefficients, the average value of the signal intensity of the sample can be calculated by integrating these splines over the sample shape and its position in the cavity (6,15). Nagy and Plaček (6) suggested that this unusual I pp dependence was due to the high nonuniformity of the modulation field produced by the pair of Helmholz coils, which are mounted in the left and right side walls of the microwave cavity.…”

Electron paramagnetic resonance and mass spectrometry: Useful tools to detect ultraviolet light induced skin lesions on a molecular basis – A short review

“…However, a quantification is possible and Yordanov et al did pioneering work in this field [42][43][44][45]. In addition to an appropriate internal standard like Mn 2+ or DPPH a proper sample positioning within the resonator is one of the most important requirements for quantification.…”