Influence of the movement of cylindrical samples with variable internal diameter and variable length along the x-axis of a double TE104 and a single TE102 rectangular cavity on the EPR signal intensity: A sample shape study

The response of the cavity to the movement of cylindrical samples with inte rnal diameters from 0.7 to 4 mm and lengths from 5 to 50 mm along the x-axi s of the Bruker double TE104 and single TE102 rectangular cavity has been a nalyzed. Independently of sample internal diameter, the experimentally obse rved dependences of the electron paramagnetic resonance (EPR) signal intens ity versus sample position in the cavity showed the following: (i) a sharp maximum for sample lengths from 5 to 20 mm; (ii) a "plateau", over which th e signal intensity remained constant within experimental errors of 0.47-1.1 6%, for lengths from 30 to 40 mm; and (iii) a "sloping plateau" region, whi ch could be approximated by the linear function (correlation r = 0.96-0.98) for the 50 mm sample. Theoretical predictions of the experimental dependen ces of the signal intensity versus sample position in the cavity were calcu lated with the "modified" and "'revised" sine-squared function, and the cor relation between observed and theoretically computed dependences is very go od. Additionally, the experimental dependence of the signal intensity versu s the sample internal diameter and length for cylindrical samples situated at the position in the cavity at which the signal intensity was a maximum w as likewise numerically approximated by the surface fitting with the Lorent zian cumulative additive function (correlation r = 0.999). The experimental dependence of the signal intensity versus the sample internal diameter for the given sample length is nonlinear. The samples with internal diameters of 0.7 and 1 mm gave the total maximum of signal intensity for the 40 mm sa mple, however, the samples with internal diameters of 2, 3 and 4 mm gave th e total maximal value of signal intensity, which was identical for both the 30 and 40 mm samples. The experimental dependence of the EPR signal intens ity versus the sample volume clearly showed that the samples with identical volumes, however, with different shapes, can give significantly different signal intensities (with differences ca. 200-400%). Then, the comparison of cylindrical samples with identical volumes but different shapes may be a s erious source of significant errors in quantitative EPR spectroscopy Cylind rical samples to be compared should he of identical shape. Accurate and pre cise positioning of each sample in the microwave cavity is essential.