Studies on the uncertainties in quantitative EPR estimations due to the construction of the cavities used Original Research Article

The magnitude of the continuous wave electron paramagnetic resonance (CW-EPR) response of a given sample is governed by several cavity parameters. Two of them are the distribution of the modulation magnetic flux density and the microwave magnetic component over the sample length. Their mutual configuration could have a significant influence on quantitative EPR estimations. In the present paper, we report the results obtained with 14 different cavities of TE102, TE104, TM110 and TM011 modes commercially available and used in many EPR laboratories with respect to the magnitude of their EPR response as a function of the sample length. The results show that due to their technical construction the widely used cavities in CW-EPR spectroscopy could be classified into three types in which: (i) the distribution of the microwave magnetic component in the cavity coincides with the modulation magnetic flux; (ii) the modulation magnetic flux is wider than the microwave magnetic component in the cavity. In this type, the magnitude of the EPR response along the sample length is governed only by the distribution of the microwave magnetic component in the cavity, (iii) the modulation magnetic flux is narrower than to the microwave magnetic component in the cavity. In the cavities of this type, the magnitude of the EPR response is governed by the distribution of the modulation magnetic flux. Whereas, in both former types, the EPR response corresponds to the full length of the sample along the z-axis of the cavity, an EPR silent zone appears in the latter type. In some of them this zone takes ca. 40% of their full length. Therefore, the assumption that the EPR active zone corresponds to the full cavity length (along the sample axis) is not valid for the latter type cavities. Their use will give 2–3% error within a series of samples, but the results will be burdened by a systematic error and could not be compared with the data obtained with another cavity even on the same spectrometer. In view of this, we should be familiar with the features of every cavity before using it for quantitative EPR estimations. Two ways are recommended to overcome the effect of EPR silent zones of an unknown cavity in order to get comparable results in quantitative EPR estimations: (i) to use samples of less than 5–6 mm length, symmetrically situated with respect to the cavity center in consecutive procedures of estimations independently of the cavity type, (ii) in the cases of simultaneous recording of unknown sample and a standard they both should be homogeneously mixed or coaxially situated along their full length.