QUANTITATIVE EPR - SOME OF THE MOST DIFFICULT PROBLEMS

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 contain different paramagnetic centers, or are 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 following 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 correction factors in many specific cases.