Challenges of Continuous Wave EPR of Broad Signals—The Ferritin Case

Abstract

The study of continuous wave (cw) electron paramagnetic resonance (EPR) spectra still poses a challenge for very broad signals, especially when the spectrum extends over a large part of the accessible field range. The difficulties derive from instrumental challenges, because of insufficient modulation depth and the need to apply measurement conditions that enhance cavity background. The biggest problem, however, is how to define a baseline such that spectral distortions are minimized. Conventional methods rely on a suitable choice of points outside the range of the signal of interest to perform a polynomial interpolation. These methods are effective in most cases where the signal of interest comprises only a narrow range of magnetic field (narrow features). In this study, a novel method of baseline correction for broad signals is proposed and compared to conventional methods. It takes into account that there are only few anchor points for the baseline. The method is applied to the signal of the iron-storage protein ferritin. The ferritin signal is a broad band that extends from zero to 0.8 T. An approach is developed by which this broad signal is analyzed reliably. The method is also extended to the case where the broad signal is superimposed on narrow signals and enables to extract the parameters of both types of signals in a fitting pipeline.