Nuclear magnetic resonance spectrometer, field strength

The spin-lattice relaxation time was measured with an ISSH-2-13 coherent nuclear quadrupole resonance spectrometer-relaxometer equipped with a Tesla BS 488 electromagnet (magnetic field strength is 1.6 x 105 A/m) to realize the... 

Nuclear Magnetic Resonance Spectroscopy. Like IR spectroscopy, NMR spectroscopy requires little sample preparation, and provides extremely detailed information on the composition of many resins. The only limitation is that the sample must be soluble in a deuterated solvent (e.g., deuterated chloroform, tetrahydro-furan, dimethylformamide). Commercial pulse Fourier transform NMR spectrometers with superconducting magnets (field strength 4-14 Tesla) allow routine measurement of high-resolution H- and C-NMR spectra. Two-dimensional NMR techniques and other multipulse techniques (e.g., distortionless enhancement of polarization transfer, DEPT) can also be used [10.16]. These methods are employed to analyze complicated structures. C-NMR spectroscopy is particularly suitable for the qualitative analysis of individual resins in binders, quantiative evaluations are more readily obtained by H-NMR spectroscopy. Comprehensive information on NMR measurements and the assignment of the resonance lines are given in the literature, e.g., for branched polyesters [10.17], alkyd resins [10.18], polyacrylates [10.19], polyurethane elastomers [10.20], fatty acids [10.21], cycloaliphatic diisocyanates [10.22], and epoxy resins [10.23]. 

Magnetic Field Strengths of Some Typical Nuclear Magnetic Resonance (NMR) Spectrometers and the Corresponding and NMR Frequencies and CSA (Chemical Shift Anisotropy) Ranges... 

With more recent spectrometers working at very high magnetic fields, the quality of the baseline of 33S FT spectra have partially improved for two main reasons. First, the nuclear resonance frequency increases with magnetic field strength (see 33S resonance frequencies in Table A.l), and second, recent probes give better results because particular attention is paid to the choice of materials and to the optimization of circuit design. 

The spectrometer is a radio receiver, and we change the frequency to tune in each nucleus at its characteristic frequency, just like the stations on your car radio. Because the resonant frequency is proportional to the external magnetic field strength, all of the resonant frequencies above would be increased by the same factor with a stronger magnetic field. The relative sensitivity is a direct result of the strength of the nuclear magnet, and the effective sensitivity is further reduced for those nuclei that occur at low natural abundance. For example, 13C at natural abundance is 5700 times less sensitive (1/(0.011 x 0.016)) than H when both factors are taken into consideration. 

Chemical shift relates the Larmor frequency of a nuclear spin to its chemical environment l 3. The Larmor frequency is the precession frequency v0 of a nuclear spin in a static magnetic field (Fig. 1.1). This frequency is proportional to the flux density Bo of the magnetic field (v0 B0 = const.) 3. It is convenient to reference the chemical shift to a standard such as tetramethylsilane [TMS, (C//j)4Si] rather than to the proton ft. Thus, a frequency difference (Hz) is measured for a proton or a carbon-13 nucleus of a sample from the H or 13C resonance of TMS. This value is divided by the absolute value of the Larmor frequency of the standard (e.g. 400 MHz for the protons and 100 MHz for the carbon-13 nuclei of TMS when using a 400 MHz spectrometer), which itself is proportional to the strength B0 of the magnetic field. The chemical shift is therefore given in parts per million (ppm, 5 scale, SH for protons, 5C for carbon-13 nuclei), because a frequency difference in Hz is divided by a frequency in MHz, these values being in a proportion of 1 106. 

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