EPR spectroscopy

What is EPR spectroscopy used for?

EPR is commonly used to monitor chemical reactions. As it can detect compounds with unpaired electrons, it is vital for detecting free radicals that often only appear as transient intermediates in a reaction. This can be used to understand reaction dynamics, which is essential when researching areas such as chemical kinetics and catalysis.

EPR also has a wide range of uses in the biological sciences, where cells, enzymes, and other macromolecules are often tagged with “markers” that will allow EPR to probe specific areas of their structure. This also allows the method to be used in pharmaceutical studies to monitor the actions of drugs in vitro.

In archaeology, electron paramagnetic resonance can help determine the age and origin of samples based on specific chemical indicators that are EPR-sensitive. EPR can also be used to examine irradiated samples to determine the degree to which they have been affected by radiation exposure.

How does EPR spectroscopy work?

Certain atomic particles have a fundamental natural property referred to as “spin”. One such particle with discreet spin properties is the electron. When electrons exist in pairs, their spins naturally counter each other. However, if a chemical species contains unpaired electrons, it is possible to analyze their spins through EPR.

To achieve this, the sample is exposed to a strong magnetic field. This causes unpaired electrons to act like tiny magnets, and align their spins with or against the magnetic field. These alignments can be seen as discreet energy levels that the spins exist in.

The sample is then exposed to microwave radiation. The electrons absorb the energy, which causes their spins to switch energy levels. The spectrometer measures the reflected microwaves at this point, to determine what was absorbed by the sample. By repeating this process across a range of magnetic field strengths, data about the unpaired electrons within the sample is collected, revealing information about the nature of the sample.

Sample requirements and preparation

EPR analysis is typically performed on powdered solids, liquids, or solutions. Due to the conditions required for testing, liquids, and solutions may have to be frozen with liquid nitrogen. In these cases, an appropriate solvent must be used, ideally with a low dielectric constant to form a glassy solid when frozen. This will ensure that the spectrometer can achieve the highest level of sensitivity and produce high-quality results.

NMR vs. EPR – what are the differences?

Nuclear magnetic resonance (NMR) spectroscopy is a more commonly used analytical technique that is analogous to EPR, with some key differences. Both techniques operate by using a magnetic field to orientate the spins of certain particles and monitor how they change when electromagnetic radiation is applied. In EPR, the spins of unpaired electrons are controlled by applying microwave radiation. In NMR, on the other hand, the spins of various types of nuclei are monitored while radio frequency radiation is applied.

This difference brings a multitude of variations. NMR can be used with most organic and some inorganic species, as it most commonly focuses on hydrogen-1 or carbon-13 atoms. This makes it more accessible than EPR, which requires species to have some unpaired electrons, which is not as common in organic chemistry. This also makes NMR more suitable for full structural analysis of compounds.

On the other hand, EPR analysis is far more sensitive than NMR, especially when analyzing ferromagnetic materials. These kinds of samples can produce broad spectral lines in the NMR, making it difficult to gather accurate and specific results. EPR is, therefore, better suited for many metallic samples.

Need an analysis?

Measurlabs offers a wide selection of laboratory testing services in one place, including both NMR and EPR experiments. For organic samples, small molecules, and polymers, we recommend checking out our 1H NMR and 13C NMR measurements. If EPR is more appropriate for your sample, please contact our testing experts through the form below or at info@measurlabs.com to discuss your analysis needs.