Choosing reference samples for EPR concentration measurements. Part 1. General introduction and systems of S = 12

Paramagnetic centers in the reference samples used in electron paramagnetic resonance (EPR) spectrometric concentration measurements often differ in nature from the centers to be determined in unknowns. Quantitations based on such an approach can be correct only if the differences in specific intensities of the two signals, that is, the inequality of the signal areas per paramagnetic particle, is taken into account. This series of papers provides tabulated values that characterize the specific intensities of X-band EPR signals for paramagnetic species of various types in solid samples. The data were obtained by quantum-mechanical calculations comprising exact diagonalization of the spin Hamiltonian matrices. One can generally use the tabulated values to calculate the correction factor when a dissimilar reference sample has to be employed. Examination of this data can also help in selecting a reference sample whose paramagnetic centers provide an EPR signal very close in specific intensity to the signal of the analyte, no matter how much these two paramagnetic centers differ in nature. With such a reference sample, no correction is necessary at all. This first part of the series provides a general introduction and the data for the simplest class of paramagnetic species with the effective spin of 12.