A New Detection System of Saturation Transfer Electron Paramagnetic Resonance Spectroscopy by a Fourier Transformation Technique

A new detection system for measurement of accurate saturation transfer electron paramagnetic resonance (ST-EPR) spectra has been developed. The principle of this system is that the EPR absorbance is recorded digitally over a wide frequency range without filtering, then processed numerically by a Fourier transformation technique without any phase sensitive detector which is used in conventional ST-EPR spectroscopy. It is possible to acquire the first and second harmonic signals at in-phase and 90° out-of-phase simultaneously. To obtain very high accuracy of phase determination, the modulation signal is also recorded and processed digitally simultaneously. To improve the signal/noise ratio, an accumulation technique is adopted. Consequently, in-phase and 90° out-of-phase for 100 kHz modulation can be easily corrected within the ±0.5° accuracy, and the phase can be changed quantitatively. Experimentally it is shown that 90° out-of-phase has to be set within 90 ± 0.5° to suppress the out-of-phase signal under 1% of in-phase signal in the absence of saturation. The errors in the ST-EPR parameters and τ-values are also estimated in the case of improper phase angles.