Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1990-12-03
1992-03-31
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324307, G01R 3320
Patent
active
051011581
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to methods for investigating a sample using nuclear magnetic resonance (NMR).
DESCRIPTION OF THE PRIOR ART
It is well known that certain nuclei, for example the hydrogen nucleus, exhibit an intrinsic spin so that when the nucleus is exposed to a magnetic field it will precess around the magnetic field with a frequency .omega. known as the Larmor frequency with the spin preferentially aligned in the direction of the magnetic field.
In general, since the nucleus concerned will be surrounded by electrons associated with the chemical structure of the molecule containing the nucleus, the magnetic field experienced by the nucleus will be modified by the screening effect of those electrons. This will cause each chemically different set of nuclei to precess at a slightly different Larmor frequency. This phenomenon, known as chemical shift, reflects the chemical environment of a nucleus and enables the chemical structure of many molecules to be determined by observing the precession frequencies present within the sample.
The intrinsic spin of a spin-1/2 nucleus is quantised and can take up one of two states, either parallel or anti-parallel with the main magnetic field. In the presence of the main magnetic field only, the spins will preferentially take up the parallel state, to produce a net magnetisation vector in that direction. However, the injection of energy in the form of a radio frequency pulse will cause a proportion of the nuclei in the sample to change their spin state causing a rotation of the net magnetisation vector from alignment with the main field to a direction dependent on the duration of the pulse. The component of the vector which is perpendicular to the direction of the field can then be observed to precess about it due to chemical shift and scalar couplings.
It has been found that the spins of adjacent magnetic nuclei interact (scalar spin-spin coupling) via the electrons of the intervening bonds. In a simple case of two nuclei, the two spin system can exist in four different states depending on the relative orientations of the respective spins to each other and to the main field. In this instance the spin system will give rise to four single-quantum transitions; in the (conventional) single-quantum coherence spectrum the chemically shifted peak of each spin will be split into two lines, the splitting between which is known as the scalar coupling constant. In general, the number of component lines of a multiplet into which the resonance of a particular spin is split is dependent upon the number and type of adjacent spins.
The energy associated with a simple two spin system in which the spins are labelled A, X is given by the formula: nuclear angular momentum vectors associated with each spin. It will be noted that this energy is independent of magnetic field strength.
Knowledge of the scalar coupling constant enables properties of the sample to be derived such as the angle between bonds of adjacent nuclei and the number of intervening bonds. The size of the scalar coupling constant can be obtained by determining the separation between the components of a multiplets within a chemical shift band.
In the past, in order to obtain this information, static magnetic fields of very high homogeneity have been required: to 1 part in 10.sup.9 to resolve scalar couplings, and to 1 part in 10.sup.7 to resolve chemical shifts. Recently, techniques have been developed to obtain the information with lower homogeneity fields and examples include the use of zero-quantum coherence or, alternatively, the N-type peaks produced by the SECSY experiment as described in J. Magn. Reson. 1980, 40, 321. However, neither technique produces the conventional single-quantum spectrum of a spin system.
SUMMARY OF THE INVENTION
In accordance with the present invention, we provide a method of investigating a sample using nuclear magnetic resonance, the method comprising: sample; type of period; after the other of the first and second types of period; type of period, the duratio
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Journal of Magnetic Resonance, vol. 40, 1980 Academic Press, Inc. (N.Y., U.S.), "Experimental Techniques of Two-Dimensional Correlated Spectroscopy", pp. 321-334.
Hall Laurence D.
Norwood Timothy J.
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