Pulse NMR spectrometers

The self-diffusion coefficients in supercritical ethylene were measured using the pulsed NMR spectrometer described elsewhere (9,10), automated for the measurement of diffusion coefficients by the Hahn spin echo method (11). The measurements were made at the proton resonance frequency of 60 MHz using a 1 1.2 kG electromagnet. 

NMR measurements were carried out on the c-axis oriented polycrystalline powder of high quality Tl2Ba2Cu06+s (Tc = 85 K) in the external field along the c-axis. The 205TI spin echo signals were obtained by a pulse NMR spectrometer under the field cooling condition (FCC) in a constant field. The spectra was obtained by convolution of the respective fourier transform spectra of the spin echo signals measured with an increment of 50 kHz. 

A pulsed NMR spectrometer, with a variable frequency and variable temperature facility is best suited for the study of these disordered systems.26 27 For dipolar glasses and relaxor systems, a spin echo Fourier transformation NMR spectra of the system have been measured in a wide bore superconducting magnet ( typically at 9.2 T). 

The NMR measurements were made at 57.5 MHz on a pulsed NMR spectrometer that included a 12-inch Varian electromagnet and a Nicolet NMR-80 data system interfaced with a Biomation 805 wave form recorder. Unattenuated 90° pulse widths were 4 to 5 microsec using a 10 watt ENI power amplifier. At best, recovery time for the home built receiver was 10 microsec. Nitrogen gas boiled from a liquid nitrogen dewar was used to cool the sample. The temperature was measured to within 2 deg with a diode thermometer. 

All DOR experiments were performed at room temperature on pulsed NMR spectrometers operating at either 104.23 MHz or 130.29 MHz, using home-built DOR probes that have been described elsewhere [15]. DOR spinning speeds of 5 kHz for the inner rotor and 800 Hz for the outer rotor were obtained routinely. Spectra were obtained using a 1-s delay between 45 pulses with 1000-2000 acquisitions. All isotropic Al shifts have been referenced to A1(N03)3 in aqueous solution. 

H. T. Stokes, "Tuned limiter for receiver amplifier in a fast-recovery pulsed NMR spectrometer," Rev. Sci. Instrum. 49, 1011-1012 (1978). 

In purchasing a pulse NMR spectrometer and magnet, all of these general considerations as well as those discussed in V.B.4. about dealing with manufacturers are applicable. It is wise to buy as flexible an instrument as one can afford. The field of pulse NMR spectroscopy is changing rapidly and it would be tragic to be limited by some needless instrumental characteristics. In addition, it seems to be a fact of life that we never can think of all of the uses of device capabilities until we actually use them. A serious alternative to purchasing a complete spectrometer is to construct one out of commercial components and the reader is referred to section V.C. for further discussions. 

The use of a low-resolution pulsed NMR spectrometer as applied in Section 6.2.5 for the determination of the solid fat content is considered by Paquot (1979b) as an auxiliary method. The dimensions and location of the measuring tubes allow... 

A suitable low-resolution pulse NMR spectrometer is the Brucker minispec P20i. Briefly, the principle of the technique is as follows (Van den Enden et aL, 1978, 1982) the intensities of the relaxation signal caused by the nuclei of the protons of the triacylglycerols, which are solid at the measuring temperature, and that due to the nuclei of the protons of the solid and the liquid triacylglycerols at the same temperature are deduced from the response given by the pulse NMR spectrometer 10 and 70jus respectively after the pulse. 

Special calibration samples consist of a plastic material immersed in liquid paraffin to give percentages of solids of approximately zero, 35 and 70. These must be standardized precisely before being used to calibrate the pulse NMR spectrometer. The pulse NMR spectrometer must first be checked by means of the calibration samples. If the values are found to deviate by more than 0.3% the instrument should be adjusted. The standard deviation of the pulse NMR spectrometer should always be smaller than 0.3% solids. 

After 30 35 min, the sample tube is quickly transferred to the sample holder of the pulse NMR spectrometer ensuring that the tube touches the bottom of the holder the solids content is read on the meter. The procedure is repeated till all samples have been measured. 

After 60-65 min the sample tube is transferred to the sample holder of the pulse NMR spectrometer. The solids content is then read. 

Preferably, the pulse NMR spectrometer should be equipped with a computer performing automatically three successive measurements at 10 jus and 70 jus and the calculation of the mean solid content which is directly read on the digital volt meter. The measuring time is then equal to 6 s. 

The SFC is a critical parameter for the fats and oils industry. The official American Oil Chemists Society (AOCS) wet method is dilatometery. Alternative wet methods are differential thermal analysis and differential scanning calorimetry (DSC). LR NMR was proved to be an alternative method for SFC determination in late 1950s. The early continuous wave LR NMR spectrometers rapidly found their way into the fats and oils industry, the method being accepted by the Instrumental Techniques Committee of the AOCS as early as in 1972. Presently the technical choice is radio frequency (RF) pulsed LR NMR. Pulse NMR spectrometers are more compact, very efficient, and relatively cheap. They have the advantage of exciting the protons in the whole sample at once. 

Figure 8 shows the time course of the laboratory and rotating frame spin-lattice relaxation of protons in lyophilized y-globulin formulations containing dextran, determined at 55°C using a pulsed NMR spectrometer [26]. 

The longitudinal relaxation time of Na has been determined in aqueous PAA solutions as a function of the degree of neutralization a and as a function of polymer concentration. These results have been collected in Figure 1 and Figure 2. All relaxation times were obtained with a Bruker pulsed NMR spectrometer operated at 16 MHz and using (n, nil, n) pulse sequences. For a few solutions it was verified that the narrowing limit (Ti = was valid. 

Heparin was exposed to various doses ofy-rays of a Co-soiuee. The sulphation degree and sulphation pattern were analysed by C-NMR-spectroscopy (Pulse NMR-Spectrometer AC 300, Braker, Karlsruhe, Germany). 

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