Electron paramagnetic resonance (EPR) spectroscopy is a technique that detects species with unpaired electrons. A large number of materials have unpaired electrons, including
free radicals, many transition metal ions and defects in materials.
Free radicals are often short-lived but play crucial roles in significant
processes such as photosynthesis, oxidation, catalysis and polymerization. Therefore, the application of EPR spans one of the widest
ranges of any major analytical technique, from molecular research
to quality control in chemistry, quantum physics, structural biology,
materials science, medical research and much more.
A number of chemicals exist within food, medicines and the
environment that have the ability to form free radicals. For example,
oxidative stress occurs when an oxygen molecule splits into free
radicals, and these may persist in the environment and enter biological systems. Inside the body, free radicals can attack biomolecules,
damaging cells, proteins and DNA and potentially causing disease.
Monitoring free radicals and other species with unpaired electrons in the environment is therefore of critical importance. As well
as short-lived free radicals, long-lived species also exist, known as
environmentally persistent free radicals (EPFRs). EPFRs can remain
in the environment almost indefinitely, particularly when associated
with the surfaces of fine particles.
EPR for air pollution
Outdoor air pollution is a major environmental hazard that affects
human health worldwide. The link between inhalation of ambient
particulate matter (PM) and various adverse health effects is documented extensively by epidemiological and toxicological studies. Transition
metals have been identified as crucial PM components, triggering
hydroxyl radical (•OH) generation. This occurs via Fenton-like reactions, which result in the formation of •OH radicals from hydrogen
peroxide and a transition metal catalyst. Short-lived reactive oxygen
species (ROS) and reactive nitrogen species (RNS) are produced from
polycyclic aromatic hydrocarbons (PAHs), which are frequently found
in ambient PM and are known to have toxic and mutagenic effects in
the body, as do their oxygenated derivatives. EPFRs have also been
observed in PM and have the ability to generate ROS in the body.
The results from a study on haze events in Beijing show that
EPR detects EPFRs identified as semiquinone radicals in PM with
different particle size, and
that EPFRs are mainly
persistent in the PM fraction
of dae < 1 µm, which are
the most hazardous (Figure
1). The daily monitoring of
the EPFRs (spins/g) shows
environmental changes that
impact long-term effects on
human health. Such moni-
toring can be used to enact
counter measures to reduce
health risks to the public.
Analyzing soil with EPR
Soil pollution can
originate from a range of
sources, and impacts many
environmental and agricultural processes. Toxins
can be assimilated by plants and leach into groundwater, thereby
distributing across landscapes and potentially entering food systems.
Common soil pollutants originate from industrial waste and heavy
metal by-products, and from agricultural practices such as pesticide,
insecticide, herbicide and fertilizer use.
All these pollutants are toxic and often participate in processes resulting in the formation of surface-stabilized EPFRs. EPFRs
play a role in the further generation of toxic compounds and are
additionally involved in radical processes that impact the formation
of humic substances, and carbon sequestration. Most common
pollutants can be detected with EPR, which can help in the development of measures to control their distribution and to aid in clean-up
Detailed research is required to understand the impact of pollution from industrial and agricultural sources on the soil environment. Understanding the mechanisms and roles of the inorganic, organic and biological components of soil leads to effective strategies
to neutralize toxic compounds. EPR works well in soil analysis as
scientists can identify, quantify and monitor long-lived EPFRs in soil
Tracking Free Radicals
in the Environment with Electron
How EPR enables scientists to detect species with unpaired
electrons, such as free radicals, in soil, air and water.
by Kalina Ranguelova, Ph.D., EPR Applications Scientist, Bruker BioSpin Corp
Figure 1: Electron Paramagnetic
Resonance (EPR) study on airborne particulate matter (PM) in Beijing during
haze events (EMX-plus X-band EPR
spectrometer, Bruker BioSpin).