If a beam of electromagnetic radiation with frequency ω irradiates the spin nucleus, when ω = ω0, the spin nucleus will absorb its radiation energy and produce *** vibration, which is called nuclear magnetic *** vibration. The magnitude of the absorbed energy depends on the number of nuclei. This fact, in addition to providing a way to measure γ, also provides a basis for quantitative analysis. This is achieved by attaching a variable magnetic field to a fixed magnetic field H0. The superposition of the two results in an effective magnetic field that varies over a range, i.e., H0 is variable over a range. Another energy and frequency stabilized RF source, its electromagnetic radiation irradiation in the magnetic field in the sample, and RF receiver to measure the sample absorbed by the RF radiation energy. When there is no absorption in the sample, the received energy is a certain value; if there is absorption, it gives an energy absorption signal. But the condition of absorption must be the frequency of the radio frequency ω = ω0. The frequency of the radio frequency is fixed, in order to have different γ value of the different nuclei can absorb the radiant energy, it is only to change the H0, so that the different spin nuclei in the corresponding to a particular H0 has the same and with the frequency of the radio frequency equal to the frequency of the incoming frequency, i.e., ω = ω0. In this way, different spin nuclei can be a feature of the magnetic field strength of the absorption of the radio frequency radiation energy and produce nuclear magnetic **** vibrations. Thus, scanning by varying the magnetic field strength, the receiver can give a series of absorption signals characterized by the magnetic field strength (actually by the spin-to-magnetic ratio). The curve plotted with the magnetic field strength as the horizontal coordinate and the absorption energy as the vertical coordinate is the NM*** vibration spectrum (b in the figure). The horizontal coordinate is the parameter on which the qualitative analysis is based, and the vertical coordinate corresponding to the peak area of different H0 is the quantitative analysis parameter.
Figure c is the schematic diagram of the NMR***Vibration Spectrometer. It is mainly composed of five parts. ① magnet: its role is to provide a stable high-intensity magnetic field, i.e., H0. ② scanning generator: a set of magnetic field scanning coils wound on a pair of magnetic poles, used to generate an additional variable magnetic field, superimposed on the fixed magnetic field, so that the effective magnetic field strength is variable, in order to realize the scanning of magnetic field strength. (iii) RF oscillator: it provides a fixed frequency beam of electromagnetic radiation to irradiate the sample. ④ Absorption signal detector and recorder: the receiver coil of the detector is wound around the sample tube. A signal is generated when a certain nucleus has an incoming frequency that matches the RF frequency and absorbs RF energy to produce a nuclear magnetic **** oscillation. The recorder automatically traces the spectrum, i.e. the nuclear magnetic *** vibration spectrum. ⑤ Specimen tube: a glass tube with a diameter of several millimeters in which the sample is contained and fixed at a definite position in the magnetic field. The entire specimen probe is rotated rapidly to minimize the effects of magnetic field inhomogeneities.
Nuclear magnetic **** vibration spectrometer **** vibration frequency is based on the frequency of 1H to name, 1H **** vibration frequency = 42.57708 × Ho (MHz), where Ho is the magnetic field strength, the unit is T (Tesla). For example, when the magnetic field strength is 4.7T, the **** vibration frequency is 200MHz.
At present, a low-resolution, low magnetic field strength (2-65MHz ), simple structure of the small nuclear magnetic **** vibration spectrometer, usually through the measurement of protons with different nuclear magnetic **** vibration parameters, the measured sample composition or performance analysis; commercial high-field spectrometer for 200- 950 MHz; 1G (i.e., 1000 MHz) spectrometers have been developed but are not yet commercially available.
The world's major nuclear magnetic **** vibration spectrometer manufacturers are Germany's Bruker, the United States of America's Varian and Japan's JEOL company, three companies have their own strengths.