Electricity: measuring and testing – Particle precession resonance – Using an electron resonance spectrometer system
Patent
1992-04-16
1994-04-12
Arana, Louis
Electricity: measuring and testing
Particle precession resonance
Using an electron resonance spectrometer system
324300, G01R 3308
Patent
active
053028988
DESCRIPTION:
BRIEF SUMMARY
This invention relates to electron spin resonance spectrometers.
Electron spin resonance (ESR) effects are associated with the presence of unpaired electrons, referred to as free radicals, in solids, liquids and gases. Free radicals often occur as transient intermediates during chemical and biochemical interactions, and are chemical species having a characteristic magnetism which can be most sensitively detected by electron spin resonance spectroscopy. This technique involves aligning the magnetism of the free radicals in a strong magnetic field and then observing their resonant interaction with microwave energy. The conditions under which resonance occurs and the intensity of the resonance allow characterisation and quantification of radical species within a test sample.
In addition, a number of synthetic chemical agents have been developed that are stable free radicals or that are capable of reacting under specific conditions to form stable free radicals. The study of naturally occuring free radicals allows insights to be gained into reaction mechanisms and kinetics, and synthetic agents have allowed the scope of such studies to be broadened to include previously inaccessible areas of chemistry, biology and biophysics. Electron spin resonance measurements are now widely applied in chemical, biochemical and biophysical sciences, and their usefulness has gained wide acceptance in toxicology, pharmacology and in various chemical industries.
Electron spin resonance (ESR) spectroscopy techniques have been used for over twenty years to detect free radicals characteristic of a particular substance. In a basic electron spin spectrometer, a magnet is used to align the free radicals of a test sample. A source of microwaves is made to interact with these aligned radicals and control systems are used to control accurately the magnetic field and the microwave frequency applied to the test sample so that the free radical resonance condition of the test sample can be observed.
According to theory, the electron spin resonance (ESR) condition occurs when the microwave frequency and magnetic field strength bear a precise relationship to one another. Specifically, if H is the magnetic field strength and f is the microwave frequency then resonance occurs when the following relationship is obeyed: the Bohr magneton B (i.e. h/B). The factor g (which will be referred as the "g-value") is a characteristic of the free radical species under investigation. It follows that for resonance to occur, the quotient f/H must have the specific value which is characteristic of the radical species under investigation.
Since the resonance frequency of a microwave cavity critically depends on its geometry (and is temperature sensitive), and on the dielectric properties of the sample (which may vary during an experiment), it is not feasible in practical electron spin resonance (ESR) spectrometers to maintain a constant microwave frequency. Because the microwave frequency will vary, it follows that the magnetic field strength needed to maintain the g-value corresponding to the free radical resonant condition must be varied in order to keep the quotient f/H constant.
Some commercial ESR spectrometers are equipped with field/frequency locking devices. In these devices, an independent (ESR) probe is mounted next to the sample cavity. This second probe contains a standard free radical sample and the electronics controlling the probe adjusts the magnetic field strength until the standard sample resonates. This locks the entire ESR spectrometer to the g-value of the standard sample. By providing additional current-carrying pancake coils around the probe, the local magnetic field experienced by the probe sample can be made to deviate from that experienced by the test sample in the microwave cavity. In order to keep the probe in resonance, the magnetic field controller has to compensate for the magnetic field experienced by the probe by an amount equal to the offset generated by the current flow of the pancake coils. Thus the g-value in the sample cavit
REFERENCES:
patent: 4455527 (1984-06-01), Singer
patent: 5142232 (1992-08-01), Konishi et al.
Capstick Myles H.
Gascoyne Peter R. C.
Pethig Ronald
Arana Louis
Gyros Technology Limited
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