ABSTRACT
The EPR spectrometric equivalent of the 280-nm measurement would be to “blindly” monitor the amplitude of any feature in an EPR spectrum as the flag of some state of matter, using a fixed set of spectrometer settings. This black box type of relative concentration monitoring with EPR, in which we do not care about detailed interpretation of spectra as long as they can be assigned to a (bio)chemical species, allows in principle for considerable sloppiness on the part of the spectrometer operator: a signal can be overmodulated, partially saturated, or deformed by filtering as long as the spectrometer settings are kept constant such that the suboptimal measuring conditions are reproducible. This implies that we can go for maximal signal-to-noise (of a deformed signal), that is, for an improved detection limit in EPR spectrometry compared to optimized conditions for EPR spectroscopy. Thus, monitoring an EPR amplitude from a dilute sample as a function of some external thermodynamic or kinetic parameter can provide quite valuable biochemical information even when the spectra are not particularly “publication quality.” As always, however, there is a catch: in multicenter molecules subject to changing external conditions, the intrinsic properties (e.g., the spin-lattice relaxation rate) of the paramagnet under monitoring may be influenced by another paramagnet nearby, and so the apparent amplitude of a partially saturated signal may change even if the spin concentration does not. For these situations craftsmanship of a well-informed operator is obviously at least as important as in regular detection of high-quality spectra.
