NMR spectrometers, field

Key Words NMR spectrometer, Field-programmable gate array, Customization... 

The stabilization of the field-frequency ratio, which is required for HDMR experiments in the INDOR mode, can be obtained by locking the NMR spectrometer field-frequency ratio to the resonance condition of a strong and sharp signal in the sample under investigation. (14) Additional stabilization can be achieved by locking together the main frequency of the spectrometer and master oscillator of the synthesizer. (15, 16) In this case, the drift in the frequency of the synthesizer is balanced by the change in the main frequency of the spectrometer. 

Figure Bl.l 1.2. Simplified representation of an NMR spectrometer with pulsed RF and superconducting magnet. The main magnetic field Bq is vertical and centred on the sample.

In practice, NMR spectrometers vary the magnetic field strength and measure the value at which a constant radio frequency is absorbed. 

Replacing one of these protons by chlorine as a test group gives (/ )-2-chloro-l-propanol replacing the other gives (.S)-2-chloro-l-propanol. Enantiotopic protons have the sane chemical shift, regardless of the field strength of the NMR spectrometer. 

In the method of NMR spectrometer operation described in Section 13.2 the rf frequency is held constant while the strength of the magnetic field i... 

Fig. 28. Room temperature 2H NMR spectra of the smectic liquid crystalline polymer (m = 6), oriented in its nematic phase by the magnetic field (8.5 T) of the NMR spectrometer with director ii parallel (left) and perpendicular (right) to the magnetic field...

Many atomic nuclei behave like small bar magnets, with energies that depend on their orientation in a magnetic field. An NMR spectrometer detects transitions between these energy levels. The nucleus most widely used for NMR is the proton, and we shall concentrate on it. Two other very common nuclei, those of carbon-12 and oxygen-16, are nonmagnetic, so they are invisible in NMR. 

Magnetic resonance imaging is a noninvasive structural technique for complex systems of molecules, such as people. In its simplest form, MRI portrays the concentration of protons in a sample. If the sample—which may be a living human body—is exposed to a uniform magnetic field in an NMR spectrometer and if we work at a resolution that does not show any chemical shifts or fine structure, then the protons... 

Flexible superconducting tapes provide promise of uses for superconductors in motors, generators, and even electric transmission lines. Meanwhile, superconducting magnets cooled to the temperature of liquid helium already are in use. High-field nuclear magnetic resonance (NMR) spectrometers have become standard instruments in chemical research laboratories, and the same type of machine (called an MRI spectrometer) is used for medical diagnosis in hospitals worldwide. 

NMR spectrometers have improved significantly, particularly in the present decade, with the development of very stable superconducting magnets and of minicomputers that allow measurements over long time periods under homogeneous field conditions. Repetitive scanning and signal accumulation allow H-NMR spectra to be obtained with very small sample quantities. 

The significant improvements in sensitivity achieved during the last 5 years have been because of improved probe design and radiofrequency circuits. Since the probe needs to be located very close to the sample, it must be made of a material with a low magnetic susceptibility, for otherwise it would cause distortions of the static magnetic field thereby adversely affecting line shape and resolution. Much research has therefore been undertaken by NMR spectrometer manufacturers to develop materials that... 

The recent development of high-field NMR spectrometers with increased resolution, dynamic range and... 

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