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Analytica Chimica Acta,
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Description: The response of the cavity to the movement of a concentrically positioned ''over full-length cavity'' planar sample of variable length, L=30, 40, and 50mm, and width, w=4, 6, and 8mm, along the vertical, common sample-cavity x-axis in a Bruker single TE"1"0"2 rectangular cavity (longitudinal effect) has been analysed. The observed dependence of the EPR signal intensity, Ipp, on the sample position, x, showed the following: (i) A plateau, over which the Ipp value remained constant within experimental error of 0.62-2.48%, for sample length of 30 and 40mm. (ii) An ''sloping plateau'' region, which could be approximated by a linear function (correlation, R=0.83-0.93), for sample length of 50mm. (iii) An additional oscillating component superimposed on the ''sloping plateau'' region. The trends of these dependencies with the plateau and ''sloping plateau'' region were independent of the planar sample width, w, and its angular position, @f, in the rectangular cavity. But, the EPR signal amplitude on the plateau and ''sloping plateau'' region obtained for the sample positioned at angles, @f=0^o and @f=180^o (which corresponded to the experimental configuration, in which the planar sample area is parallel to the waveguide in the microwave cavity) was ca 1.8 times higher compared to those obtained for the sample positioned at angles @f=90^o and @f=270^o (which corresponded to the experimental configuration, in which the planar sample area is perpendicular to the waveguide). Theoretical predictions of the experimental dependencies of the EPR signal intensity versus sample position along the common sample-cavity x-axis can be calculated using the ''sine-squared'' function (given by longitudinal integral) modified for an ''over full-length cavity'' planar sample, and the correlation between observed and theoretically computed dependencies is very good. Each of the above-mentioned phenomenon could be a serious source of significant errors in quantitative EPR spectroscopy. For maximum accuracy in quantitative EPR an ''over full-length cavity'' planar sample should be compared to a reference sample of identical length which must be identically positioned in the microwave cavity. .
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