Electron Spin Resonance (ESR) - Principle, Instrumentation, Applications

Electron Spin Resonance (ESR) is also known as Electron Paramagnetic Resonance (EPR). It is a non-destructive spectroscopic technique used for studying materials having one or more unpaired electrons. These unpaired electrons are present in paramagnetic substances.

Every electron acts like a small magnet because it has intrinsic angular momentum or spin. When the sample is placed in a strong static magnetic field, the energy level of the unpaired electrons split into two different states. One state remains aligned with the magnetic field and it has lower energy. The other state remains against the magnetic field and it has higher energy.

In ESR, electromagnetic radiation is applied to the sample. Usually microwave radiation is used for this purpose. When the energy of microwave photon becomes exactly equal to the energy difference between the two spin states, the electron absorbs the energy and changes its spin orientation. This condition is referred to as resonance.

The absorption of microwave energy is measured by the instrument and recorded as ESR spectrum. This spectrum acts as a structural fingerprint of the material. From this spectrum, information about molecular structure, local environment and dynamics of the paramagnetic species can be obtained.

ESR is highly sensitive technique. It is used in chemistry, biology, physics and materials science. It is used for detection of free radicals, study of transition metal complexes and defects in solids.

Working Principle of Electron Spin Resonance (ESR)

Electron Spin Resonance is based on the interaction between external magnetic field and magnetic moment of unpaired electron. The electron has intrinsic angular momentum or spin. Due to this spin, electron behaves like a small bar magnet.

In normal condition, the spin energy levels of unpaired electron are same. When the sample is placed in a strong static magnetic field, these energy levels split into two different states. This splitting occurs due to Zeeman effect.

The magnetic moment of electron may align parallel to the magnetic field or anti-parallel to the magnetic field. The parallel state has lower energy and anti-parallel state has higher energy. The energy gap between these two states depends on the strength of applied magnetic field.

During ESR analysis, the sample is irradiated with electromagnetic wave. Usually microwave radiation is used for this purpose. When the energy of incoming microwave photon becomes exactly equal to the energy gap between two split spin states, resonance condition is obtained.

At this point, the unpaired electrons absorb microwave energy and change their spin orientation. The electron is shifted from lower energy state to higher energy state. This process is referred to as spin flip.

The lower energy state contains slightly more electrons than the higher energy state. This is due to Maxwell-Boltzmann distribution. So there is a net absorption of microwave energy.

The absorbed energy is detected by the ESR spectrometer. The detector converts this absorption into ESR spectrum. This spectrum gives a unique magnetic signature of the paramagnetic species present in the sample.

How Does Electron Spin Resonance (ESR) Works?

The working of Electron Spin Resonance (ESR) is as follows-

Presence of unpaired electrons– ESR is used for the materials which have unpaired electrons. These electrons are found in free radicals, transition metal ions and defects present in solids. The substances having unpaired electrons are called paramagnetic substances.
Magnetic nature of electron– Every electron has intrinsic angular momentum, which is called spin. Due to this spin, electron behaves like a small magnet. It has a magnetic moment and it can interact with external magnetic field.
Application of magnetic field– In the absence of magnetic field, the spin states of unpaired electron have same energy. When the sample is placed in a strong external magnetic field, the energy level of unpaired electron splits into two different states. This splitting takes place due to Zeeman effect. One spin state becomes parallel to the magnetic field and it has lower energy. Another spin state becomes antiparallel to the magnetic field and it has higher energy.

Microwave irradiation– The sample is then exposed to electromagnetic radiation. In ESR, microwave radiation is generally used. The frequency range is usually 1 to 400 GHz.
Resonance condition– In most ESR experiments, the microwave frequency is kept constant and the external magnetic field is slowly changed. As the magnetic field strength increases, the energy gap between two spin states also increases. When the energy difference between two spin states becomes equal to the energy of microwave photon, resonance occurs. This is the main condition for ESR absorption.
Absorption of energy and spin flip– At resonance condition, the unpaired electrons absorb microwave energy. After absorption, the electron changes its spin orientation. It moves from lower energy state to higher energy state. This change is called spin flip.

Detection of signal– There are usually more electrons in the lower energy state than the higher energy state. Due to this, there is net absorption of microwave energy. This absorbed energy is detected by the spectrometer.
Formation of ESR spectrum– The spectrometer records the absorption signal. By using magnetic field modulation, this absorption signal is converted into first derivative spectrum. This spectrum gives a characteristic structural fingerprint of the paramagnetic species.

Instrumentation/Parts of Electron Spin Resonance (ESR)

The main parts of Electron Spin Resonance (ESR) spectrometer are as follows-

Microwave source – It is used to produce monochromatic microwave radiation. In earlier ESR instrument, klystron was used. In modern instrument, solid state Gunn diode or high frequency synthesizer is used.
Waveguide – It is a hollow rectangular tube made up of copper or brass and it is plated with silver or gold. It carries the microwave radiation from the source to the resonant cavity.
Attenuator – It is used to control the intensity of microwave power going towards the sample. It prevents power saturation of the spin system.

Circulator or Magic T – It is a non-reciprocal device which directs microwave from the source into the cavity. It also sends the reflected microwave from the cavity towards the detector and prevents backward reflection.
Resonant cavity – It is a metal box in which sample is placed. It stores microwave energy and concentrates the microwave magnetic field at the position of sample. Resonance takes place in this region.
Magnetic field system – It consists of electromagnet which gives a stable and uniform static magnetic field across the sample. The magnetic field is swept linearly during the experiment.

Field modulator – It consists of small modulation coils. These coils apply a small high frequency oscillating magnetic field over the static magnetic field. It is usually 100 kHz and it improves signal to noise ratio.
Detector – It converts the reflected microwave power into electrical signal. Schottky barrier diode, silicon crystal or PIN diode is generally used as detector.
Phase-sensitive detector – It is also called lock-in amplifier. It extracts the signal at the modulation frequency and changes the absorption signal into first derivative line shape.

Amplifier and recorder – It amplifies the final signal. The recorder, computer or oscilloscope displays, stores and analyses the ESR spectrum.

Operating Procedure of Electron Spin Resonance (ESR)

The operating procedure of Electron Spin Resonance (ESR) is as follows-

Sample loading – The material to be analysed is taken in a sample tube. The sample may be solid, liquid or gas. Then the sample tube is placed inside the resonant cavity between the poles of a strong electromagnet.

Microwave irradiation – The sample is continuously exposed to microwave radiation. In CW-ESR, X-band microwave frequency is commonly used and it is about 9.5 GHz.
Cavity tuning – The microwave source is adjusted with the resonant frequency of the cavity. Klystron or solid state Gunn diode may be used as microwave source. This helps the microwave to be stored properly in the cavity and prevents reflection back to the source.
Magnetic field sweep – The microwave frequency is kept constant during the experiment. The external static magnetic field is slowly and linearly changed over a particular range.

Field modulation – A small alternating magnetic field is applied over the sweeping static magnetic field. It is generally oscillating at 100 kHz. This step is used to increase the signal to noise ratio.
Achieving resonance – As the magnetic field strength increases, the energy gap between the spin states of unpaired electrons also increases. When this energy gap becomes equal to the energy of microwave radiation, resonance takes place.
Spin flip – At resonance condition, the unpaired electrons absorb microwave energy. Then the electrons change their spin orientation from lower energy state to higher energy state.

Signal detection – The absorption of microwave energy slightly changes the tuning of cavity. Due to this, some microwave power is reflected back to the detector. Schottky barrier diode is commonly used as detector.
Data recording – The detected signal is processed by lock-in amplifier. It reduces background noise and records the microwave absorption signal. Finally, the signal is displayed as first derivative ESR spectrum on computer for structural analysis.

Applications of Electron Spin Resonance (ESR)

The following are the applications of Electron Spin Resonance (ESR)-

Chemistry and Catalysis – ESR is used in detecting highly reactive free radicals and their characterization. It is also used for following reaction kinetics. In catalytic reactions, it helps to study the active sites and also the intermediate compounds formed during heterogeneous and homogeneous catalysis.
Biology and Medicine – ESR is used for the study of metalloproteins and reactive oxygen species (ROS). Spin labelling method is used to know the dynamics of proteins, lipids and cell membrane. In medical field, ESR is also used for in vivo imaging, oxygen concentration and oxidative stress in tissues.
Materials Science and Physics – ESR is used to identify structural point defects present in crystals and semiconductors. Magnetic materials can be characterized by this method. It is also useful in studying conducting polymers and nanomaterials.

Geochronology and Archaeology – ESR dating is used to determine the age of geological and archaeological materials. Fossilized tooth enamel, quartz present in sediments and corals are studied by this method. It is mainly useful when the samples are older than radiocarbon dating range.
Radiation Dosimetry – ESR is used for measuring absorbed dose of ionizing radiation. Solid state materials such as alanine dosimeters are used for this. It is important in clinical radiotherapy, sterilization of medical devices and also in radiation accident study.
Food and Beverage Quality Control – ESR is used to check oxidative stability of food materials. It is also used to follow shelf life of products. Food treated with ionizing radiation can also be detected by ESR.

Petrochemical Industry – ESR is used to characterize crude oil properties. It helps in real time monitoring of asphaltene and vanadium content. These components can be measured and followed by this method.
Electrochemistry – ESR is used for detecting short lived radical species in redox-flow reactions. It is also applied in water purification process. Battery systems like lithium-ion and lithium-oxygen batteries can be studied by this technique.
Pharmaceuticals and Drug Delivery – ESR is used to measure microviscosity and micropolarity inside drug delivery systems. It is also used for finding paramagnetic impurities. Stability of pharmaceutical formulations can also be checked by this method.

Advantages of ESR spectroscopy

The following are the advantages of Electron Spin Resonance (ESR)-

High sensitivity – ESR is about 1000 times more sensitive than Nuclear Magnetic Resonance (NMR). This is because electron has much larger magnetic moment than atomic nucleus. So free radicals and paramagnetic centres can be detected even in very low concentration, such as 1 micromolar.
High specificity – ESR is specific for unpaired electrons. It does not show response for paired electrons. So it can study only those molecular regions where unpaired electrons are present, without much background interference from diamagnetic substances.

Non-destructive and non-invasive – ESR does not destroy or change the sample during measurement. The sample is not activated also. So same material can be used again for repeated scanning or for other laboratory analysis.
Fast time resolution – ESR works in nanosecond time scale. This is faster than NMR which works in millisecond scale. So it is useful for studying fast molecular movements and short lived transient species in real time.
Phase versatility – ESR can be used for different types of samples. Solid, liquid and gaseous materials can be analysed by this method. So it is a flexible analytical technique.

Simple preparation and fast turnaround – Sample preparation is generally simple in ESR. After the instrument is ready, normal ESR spectrum can be obtained within about 15 to 20 minutes.
Highly stable for dosimetry – In radiation dosimetry, ESR gives linear dose response over a wide range. Alanine dosimeters are commonly used for this. The signal is stable and does not fade with time, so it is useful for long term record keeping.

Limitations of ESR spectroscopy

The following are the limitations of Electron Spin Resonance (ESR)-

Restriction to paramagnetic species – ESR is used only for those substances which contain unpaired electrons. Free radicals, transition metal ions and structural defects can be studied by this method. Diamagnetic substances having only paired electrons cannot be analysed by ESR.
Extreme low-temperature requirements – In many samples, electron spins have very short relaxation time. At room temperature, the ESR line may become very broad and the signal may not be clearly seen. So measurements are often done at very low temperature, sometimes below 10 K, and liquid helium coolant may be required.
Solvent interference – Liquid samples in solvents like water or alcohol may create problem in ESR. These solvents have high dielectric constant and absorb microwave radiation. This decreases the quality factor of the resonant cavity and reduces the sensitivity of the instrument.

Sample size constraints – In higher microwave frequency such as K, Q or W band, better resolution can be obtained. But the sample size must be made very small because the waveguides and resonators are also small in size.
Spectrometer deadtime – In pulsed ESR, after giving high power microwave pulse, there is a very short deadtime. During this time the detector is switched off to protect it. If the sample relaxes very fast, the signal may disappear before the detector starts recording.
Excitation bandwidth limits – In ESR, it is not always possible to excite the whole spectrum by a single microwave pulse. This limits the excitation of the sample. It is a serious limitation when it is compared with Nuclear Magnetic Resonance (NMR).

Complexity and cost – Advanced ESR instruments, mainly pulsed ESR or high field ESR, need superconducting electromagnets and cryogenic systems. The instrument becomes large and expensive. The data obtained may also be complex and mathematical simulation is often needed.
Environmental sensitivities – In some applications like radiation dosimetry, ESR measurement can be affected by humidity and temperature changes. These factors may fade the signal with time or detune the resonant cavity during measurement.

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Electron Spin Resonance (ESR) – Principle, Instrumentation, Applications