As far as I understand, the integral intensities of the following EPR spectra:
1. The EPR spectrum of a TEMPO radical solid grain with one exchange narrowed line.
2. EPR spectrum with 3 lines of the same grain dissolved in some solvent.
should be exactly the same, provided that the resonator Q-factor, other parameters of the experiment remains unchanged and the whole samples are located in the central region of the resonator.
Are there any effects that can lead to a difference in the integral spectra intensity in the 1st and 2nd cases?
Dear Sergei
Yes, there are plenty of reasons for the difference. First of all, spin concentrated systems are characterized by a broad distribution of relaxation times including very short ones. Those spins with short relaxation times appear thus in CW-EPR experiment as a very broad signal which can be missed during integration. Consequently, the observable amount of spins will be less.
Another reason would be that close pairs of spins with noticeable exchange integral may form EPR-silent singlet (S=0) pairs. They will thus also disappear in the experiment.
For spin counting, a better choice is a spin-diluted system with a narrow line, i.e. case number 2.
Sergei V. Kuzin
Sergei, thanks for the clear answer!
It's very interesting how significant these effects can be. Unfortunately, I have not yet been able to find any experimental or theoretical articles devoted to these issues.
Yes, of course, it is desirable to carry out quantitative measurements in conditions when the spins are diluted. However, in our experiments with real systems, a situation often arises when both isolated and exchange-coupled spins are simultaneously present in the samples. I would like to understand how much we can trust standard quantitative measurements in the case of exchange narrowed spectra. It seems that we ourselves will have to find out this experimentally using model systems.
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