Image Credit:Physikalisch-Technische Bundesanstalt
ESR/EPR Spectrometers measure the spectrum produced by the magnetic energy level transitions of electrons having a net spin and orbital angular momentum. The spectrum obtained can be used to characterize the material being analyzed.
A very informative video can be found here.
Electron Spin Resonance (ESR) is also referred to as Electron Paramagenetic Resonance (EPR) is a direct method for measuring the absorption of microwave radiation corresponding to the energy splitting of an unpaired electron when placed in magnetic field. One type of species that contain unpaired electrons is free radicals. Free radicals are reactive molecules with unpaired electrons. They cause the oxidative breakdown of oils, food products, and play a role in the biochemisty of disease.
The ESR spectrum of a free radical is the simplest of all forms of spectroscopy. If an external magnetic field is no present, the two electron spin states (spin up and spin down) are degenerate. The degeneracy of the electron spin states characterized by the quantum number, ms =+/- 1/2, is lifted by the use of a magnetic field. Transitions between the electron spin levels are induced by radiation at the matching frequency. When a magnetic field is induced, atoms with unpaired electrons spin either in the same direction (spin up) or in the opposite direction (spin down) of the applied field. These two possible alignments have different energies are no longer degenerate. The alignments are directly proportional to the applied magnetic field strength. This is called the Zeeman Effect. An un-paired electron interacts with its environment, and the details of ESR spectra depend on the nature of those interactions. The readings can provide information on structural and dynamic information, even from the chemical or physical process, without influencing the process itself.
An ESR/EPR spectrometer contains a microwave source, an attenuator, a circulate, sample cavity, and detector diode.
The microwave source is generally a klystron tube or a Gunn diode.
Klyston tube. Image Credit: wikipedia
The attenuator adjusts the power level.
The circulator reroutes the microwaves as they travel through the system.
The sample cavity holds the sample. The sample can be gases, single crystals, solutions, powders, and frozen solutions.
Design Tip: Solutions should not use solvents with high dielectric constants because they will absorb the microwaves and frozen solutions should use solvents that will for a glass when frozen (low symmetry and no hydrogen bonds).
The diode detector is mounted along the E-vector of the plane=polarized microwave and produced a current proportional to the microwave power reflected from the cavity.
ESR/EPR Spectrometer. Image Credit: NM State.
The generated microwaves are sent through the attenuator, through the circulator, and onto the sample. Microwaves reflected back from the sample cavity are routed in the circulator to the detector diode. The signal comes out as a decrease in current at the detector analogous to absorption of microwaves by the sample.
ESR/EPR spectra can be very complicated and often require computer analysis.