No routines for application¶
Why does EPR spectroscopy play such a minor role in investigating urgent questions of high interest? Climate change and developing new energy materials both for conversion and storage is only one of the aspects where EPR spectroscopy would be a perfect fit, due to charges being ubiquitous in these materials.
One answer is clearly physics: For NMR, you only need a proton (in the simplest case), and most samples will contain some NMR-active nuclei. Hence NMR spectroscopy is routinely applied as major analysis tool in chemistry and chemical and pharmaceutical industry. Similarly, combining FTIR and Raman spectroscopies, there is no molecule left that you can not investigate with either method. EPR spectroscopy, in contrast, requires an unpaired electron spin, i.e. either a radical or some high-spin state such as triplets or metal clusters.
However, from own experience, the real problem is not physics, but a fundamental lack of routine tools and processes as well as a common understanding in the community how to apply EPR spectroscopy in a rather routine fashion to a large series of different questions. Nearly every lab has its own protocols, programs, and niches - and the result is all-too-often a lack of proper analysis of the data. Colleagues from other departments and collaboration partners are often astonished that there is no state-of-the-art protocol and analysis routine for basic characterisation of a new sample. If EPR spectroscopy is to play a role in answering the big questions of today – and there is ample opportunity for it – this needs to change.
Four problems of state-of-the-art EPR spectroscopy can be identified that prevent its more widespread and successful application to many fields. These problems will be detailed each in its own section, and it is the declared goal of this project to provide solutions for each of the problems described.
Routine analysis of a sample using NMR or mass spectrometry doesn’t take more than a day at most. EPR spectroscopy rather counts weeks, and the problem is not so much the measurement time but rather the effort needed to analyse and interpret the results that cannot be left to the inexperienced user or collaboration partner.
EPR spectroscopy is intrinsically complicated. This is not only due to the underlying physics, but due to mostly overlapping broad lines. Individual contributions can usually not simply be disentangled, and often, the only chance to extract parameters from spectra is by fitting spectral simulations. Compare this with the relative ease of interpreting NMR or optical spectra. Additionally, setting up experiments and choosing the appropriate parameters is usually left to the operator and requires experience and thorough understanding of both, hardware and underlying physics.
A state-of-the-art research-grade pulsed EPR spectrometer costs about a million EUR/USD/GBP. This is far more than many other methods (compare, e.g., optical or FTIR devices). Sure enough, NMR spectrometers are pretty expensive as well, but far more useful for analysis and applicable to a vast area of research.
A few tools have become de-facto standard in EPR spectroscopy, such as the EasySpin toolbox for spectral simulations. However, routine protocols for reliable data acquisition, processing, and analysis are mostly missing. The quality of too many published papers speaks of its own. Having virtually every group develop their own very basic tools (mostly small scripts) does not necessarily improve the situation. Additionally, all too often EPR investigations will not yield results due to lack of thorough analysis and the ability to extract the necessary parameters from the data.