Electron paramagnetic resonance (EPR) is the method of choice to investigate and quantify paramagnetic species in many applications in materials science, biology, and chemistry. In these fields, typical sample states include thin films and solutions. Of particular interest are dynamic processes in solution. Their investigation, however, is limited by the form factor of the utilized microwave (MW) resonators as the entire process needs to be confined to the resonator. The EPR-on-a-Chip (EPRoC) dipstick device circumvents these limitations by integrating the entire EPR spectrometer into a single microchip, covered with a protective coating that enables the operation of the EPRoC submerged directly in the sample solution, thereby expanding the accessible sample environments for EPR measurements. In this approach, instead of a MW resonator, the coil of a voltage-controlled oscillator (VCO) with a size of a few hundred micrometers is simultaneously used as MW source and detector. As a test for the EPRoC dipstick and its protective coating, differently charged electrolyte solutions of a Vanadium redox flow battery with pH < 1 were investigated with the EPRoC and compared to conventional EPR results. The same linear relationship of EPRoC and EPR signal intensities with respect to the state of charge (SOC) was found, so that these experiments serve as proof-of-principle for a quantitative EPRoC dipstick device operating in a harsh sample environment. Dipstick EPRoC with its inherent fast rapid scan (RS) capability using frequency sweeps allows the investigation of processes, in which time resolution is important. It has been shown recently that the SNR per allocated acquisition time can significantly be enhanced, if measurement time for obtaining a full spectrum is the limiting factor, such as in oximetry. In combination with a permanent magnet, of which a first prototype will be presented, the EPRoC dipstick may find its way beyond the laboratory as a quantification tool for paramagnetic species in solution.

In this tutorial I will use some of our recent applications to discuss how we test structural hypotheses by site directed spin-labelling and pulse dipolar EPR spectroscopy (PDS). The tutorial is aimed at broader audience of EPR spectroscopists who are interested in spin-labelling and PDS as a methodology to test structural models, but do not (yet) have extensive experience with this. I will cover a general strategy including advantages and disadvantages of different spin labels for PDS and experimental aspects, both in the wet lab and at the spectrometer. Finally, I will share our take on data analysis and its robustness.

High field pulsed EPR should offer significantly better absolute and concentration sensitivity compared to lower frequencies, operate over larger percentage bandwidths, whilst maintaining all the standard advantages associated with high frequency resolution and working at higher energy scales. The fact that this level of performance is not always fully seen in practice is often associated with instrumental challenges at mm-wave and sub-mm-wave frequencies. However, this also opens up new opportunities as technology evolves. In this tutorial I will try to give an overview of some important ideas in high field EPR, and mm-wave instrumentation, concentrating on design concepts and applications, which are not always emphasized in the literature.