Mosaic artifacts

Finally, the minimally required number of molecular orientations (steps in cos0 and in < >) is determined experimentally by inspection of trial simulations as illustrated in Figure 6.5 on the now familiar high-spin heme spectrum too few orientations cause so-called mosaic artifacts, which must be eliminated by increasing the step numbers. In this particular example of the high-spin heme from Figure 6.2, the gx and -values are relatively close, and 25 steps in (p suffice, but gz is well separated and the number of steps in cos0 must be increased beyond 1000 to fully eliminate mosaic artifacts. 

Note that mosaic artifacts can also occur physically in real spectra when a real powder sample of a model compound exhibits microcrystallinity and thus contains too few different molecular orientations. This phenomenon is rare in X-band EPR and is usually easily solved by grinding the sample in a mortar it is, however, not at all uncommon even for extensively ground samples in high-frequency EPR with single-mode resonators where the sample size is orders of magnitude less than that of an X-band sample. 

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