Spin-Echo Spectrometer

Wideline NMR spectra were collected using a Bruker CXP 200 NMR spectrometer, operating at u>o/2n ( H) = 30.7 MHz. To obtain spectra void of spectrometer artifacts, the solid spin echo pulse sequence, n/2) -T-n/2) -x-echo, was used. Unless otherwise noted, the delay between pulses, X, was set at 30 Js. 

Self-Diffusion of Desmopressin and Monoolein by NMR. The self-diffusion coefficient was measured with the NMR diffusion technique using a Bruker MSL 100 spectrometer. Two magnetic field gradient pulses were applied at either side of the 180-degree pulse in a [90x-T-180y-T-echo] spin echo sequence (7,8) (Figure 2). Due to diffusion, the amplitude of a component in the spin-echo spectrum is attenuated according to (7)... 

Fig. 2. Schematic representation of a neutron spin-echo spectrometer. (Reprinted with permission from [12]. Copyright 1987 Vieweg and Sohn Verlagsgemeinschaft, Wiesbaden)...

Fig. 3. The neutron spin echo spectrometer IN11 at the ILL Grenoble. The two large coils producing the precession fields are clearly visible (Photograph was kindly given to us by H. Biittner, Scientific Coordination Office of the Institute Laue-Langevin Grenoble).

Spin Echo spectrometer facilities at Argonne National Lab. 

The NMR measurements were performed on systems composed of ca. 25 wt. % samples of aPS (M = 6.6x 10 g/mol, PD = 1.1, Pressure Chemical Company) in either reagent grade toluene (Aldrich) or toluene-d0 (Aldrich). The protonated solvent was used for the diffusion measurements and the deuterated solvent for the relaxation studies. At this concentration, the Tg j for the system was determined to be about -65 °C (i). The NMR spectra were run on a JEOL FX-90Q NMR spectrometer operating at 90 and 14 MHz for protons and deuterons, respectively. The T and T2 measurements were made with the standard inversion-recovery and spin-echo (CPMG) sequences, respectively. 

The rehydrated samples were obtained by exposing dehydrated samples to water vapor at least three days over saturated NH4GI solution at room temperature. A duraction of 0.5 s between scans were allowed for nuclear spin to recover to their equilibrium magnetization. The one—dimensional Na NMR spectra were recorded by using the spin—echo technique. The strength of the radio-frequency field for the two dimensional nutation experiments was 80 kHz and 128 ti values were used (0 250 /is). Each two- dimensional experiment took about 12 hours of spectrometer time. 

Figure 3. High resolution proton NMR spectra of cheese, obtained by application of a Hahn spin echo pulse sequence with and without field gradient pulses. Measurements were performed on a Bruker MSL-300 spectrometer, operating at 300 MHz. The field gradient unit used with this spectrometer was home-built and the strength was calibrated to 0.25 T/m, using a 1-octanol sample for which the diffusion coefficient is known at several temperatures.

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. 

The use of NMR spectroscopy as an analytical technique is well established ( 1 8). In order to quantitate our spin-echo height to the number of protons present, we performed an independent calibration using standard solutions of naphthalene in carbon tetrachloride. Concentrations for the standards were chosen to correspond to the anticipated supercritical C02 solubilities, and all calibration measurements were performed using a sample cell of the same dimensions as the solubility sample cell previously described. The response of our spectrometer to the standard solutions was linear over the concentration range. The reproducibility for independent measurements of the calibration curve was 3 . Throughout the experiment, all spectrometer conditions (pulse lengths, phases, receiver amplifier gain, etc.) were closely monitored, and frequent checks on the calibration of the spectrometer were performed. In this way we were able to obtain the molar solubility of solid naphthalene in supercritical carbon dioxide to an estimated experimental accuracy of 6%. 

The electron spin echo (ESE) method is a kind of time-domain ESR using pulsed microwave radiation and it directly observes the relaxation behavior of electron spins. Since the construction of an ESE spectrometer was first reported by Kaplan early in 1962 [8], the ESE method has become more and more popular slowly but steadily. Owing to the progress in microwave technology and to the continual effort of pioneering workers such as Tsvetkov, Mims, Kevan and others [9, 10]. the ESE method has been extensively developed and improved. It is now considered to be requisite for studying the paramagnetic relaxation of radical species. 

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). 

Samples are chopped into tiny pieces to avoid skin-depth problems at the Larmor frequency. Spin echo (7i/2-t-ti/2, 90° phase shifted) or free induction decay (FID) (n-z-n/2) sequences are used for Ti measurement depending on the sample and the sensitivity of the spectrometer. 

The faithful representation of the shape of lines broadened greatly by dipolar and, especially, quadrupolar interactions often requires special experimental techniques. Because the FID lasts for only a very short time, a significant portion may be distorted as the spectrometer recovers from the short, powerful rf pulse. We saw in Section 2.9 that in liquids a 90°, t, 180° pulse sequence essentially recreates the FID in a spin echo, which is removed by 2r from the pulse. As we saw, such a pulse sequence refocuses the dephasing that results from magnetic field inhomogeneity but it does not refocus dephasing from natural relaxation processes such as dipolar interactions. However, a somewhat different pulse sequence can be used to create an echo in a solid—a dipolar echo or a quadrupolar echo—and this method is widely employed in obtaining solid state line shapes (for example, that in Fig. 7.10).The formation of these echoes cannot readily be explained in terms of the vector picture, but we use the formation of a dipolar echo as an example of the use of the product operator formalism in Section 11.6. 

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