Electron paramagnetic resonance (EPR) spectroscopy, also known as electron spin resonance (ESR) spectroscopy, is a magnetic resonance technique that is used to monitor and probe chemical species that contain unpaired electrons. EPR has applications in fields including pharmaceuticals, food science, chemical analysis, and material science.
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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 firstname.lastname@example.org to discuss your analysis needs.
Suitable sample matrices
- Ferromagnetic materials
- Irradiated substances
Ideal uses of EPR spectroscopy
- Analysis of free radicals
- Monitoring chemical reactions
- Analyzing biological macromolecules
- In-vitro pharmaceutical testing
- Measuring levels of radiation
Frequently asked questions
EPR can be used to detect the presence of paramagnetic species, even when they are only present briefly during a chemical reaction. This gives it applications in food science (e.g. identifying foodstuffs exposed to radiation) and medical research (e.g. detecting free radicals formed as a result of drug delivery). Other fields where EPR spectroscopy may be used include physics, chemistry, biology, and archaeology.
Electron paramagnetic resonance spectroscopy is a non-destructive and highly sensitive analysis technique that can be used to analyze larger molecules than is possible with nuclear magnetic resonance spectroscopy.
As a downside, the unpaired electrons required for analysis are not often present in organic samples, making many sample types unsuitable for EPR. Sample preparation may also be relatively tricky, as the experimental setup often requires very low temperatures to be reached.
Electron paramagnetic resonance (EPR) and electron spin resonance (ESR) are both terms used to describe the same analysis method. There is, therefore, no difference.
Measurlabs offers a variety of laboratory analyses for product developers and quality managers. We perform some of the analyses in our own lab, but mostly we outsource them to carefully selected partner laboratories. This way we can send each sample to the lab that is best suited for the purpose, and offer high-quality analyses with more than a thousand different methods to our clients.
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