Signal Area Measurements in EPR

Although the sensitivity and dynamic range of EPR are high, the precision of concentration measurements has often been low. Consequently, EPR spectroscopy has a reputation as a rather nonquantitative method. Interlaboratory comparisons of signal area measurements many years ago yielded very discouraging results (1). Since then there have been important improvements in instrumentation and useful quantitative studies of the spectrometer-sample interaction. It is now realistic to aspire to much greater accuracy in EPR signal area measurements. In many research problems EPR is the only technique that can provide desired information. Of the five fundamental types of information that can be obtained by EPR (0-value, electron-electron and electron-nuclear spin-spin coupling, line shape, intensity, and relaxation times), this review is concerned only with intensity. Often it is necessary to answer the question: "How many spins are present?" In metalloenzyme studies, for example, it could be important to know whether all of the copper and iron atoms in a particular system yield EPR signals. The relative concentrations of metal species and organic free-radical species in a system could also be crucial to interpretation of a mechanism. Although it seems almost too trivial to mention, it is important to know whether the EPR signal being displayed represents all of the species in the sample or only an impurity or decomposition product. Lack of attention to order-of-magnitude quantitation can lead to publication of impurity or decomposition spectra even in cases in which the bulk of the sample has an EPR spectrum but with greater line width, or to publication of background spectra as spectra of the sample. These egregious errors can be avoided, and