Pulsed Electron-Electron Double Resonance (PELDOR) and Electron Spin Echo Envelope Modulation (ESEEM) spectroscopy in bioanalysis

Electron Paramagnetic Resonance (EPR) has a long history with the first spectrum being reported in 1945 (Zavoisky 1945) and is adopted widely, in its original continuous wave (CW) form, as a tool for the analysis of both biological systems and materials bearing unpaired electrons. The related technique of “Nuclear Magnetic Resonance” (NMR) developed rapidly thanks to the technological advances that enabled the use of pulsed radiofrequency and Fourier Transformation (FT). In EPR, the challenges posed by needing to generate short and very powerful microwave pulses and the fast relaxation times, of the electron spin, led to a much slower adaptation of pulsed excitation schemes. Nevertheless, early experiments using an NMR spectrometer with a greatly reduced magnetic field had shown the feasibility of pulse EPR (Blume 1958). It was not until the availability of commercial systems in the 1980s that this technique began to be applied more widely outside laboratories focused on the development of instrumentation. Over the last decades the application of pulse EPR has seen a very wide variety of systems studied from biological systems to those of materials science. This development was paralleled by a constant innovation in EPR instrumentation and methodology. Recent success in using arbitrary waveform generators (AWGs) and the commercial implementation and user uptake indicates that EPR is just entering a new era with tremendous opportunities offered by bespoke excitation schemes.