Field homogeneity

The niobium-zirconium wire used remains superconducting at 4° K. even in the strong field of the solenoid itself. The unique feature of the new apparatus is the very high field homogeneity in the sample region (2 cm. diameter sphere) kept at room temperature (34). 

The Giesekus criterion for local flow character, defined as

extensional flow, 0 in simple shear flow and — 1 in solid body rotation [126]. The mapping of J> across the flow domain provides probably the best description of flow field homogeneity current calculations in that direction are being performed in the authors laboratory. 

The resoludon is controlled not only by such factors as field homogeneity and intrinsic nature of the compound (such as the presence of exchange or restricted rotadon), but also by the sampling rate. With a spectral width of 7500 Hz on a 500-MHz instrument, the sampling rate was 1/(2(7500)) s, i.e., 0.066 ms. An acquisition dme of 1.0 s requires 1.0 s/0.066 ms s 15,000 data points of computer memory. 

Micro reaction systems may help to overcome or at least reduce some of the above-mentioned limitations [69]. Electrochemical micro reactors with miniature flow cells where electrodes approach to micrometer distances should have much improved field homogeneity. As a second result of confined space processing, the addition of a conducting salt may be substantially reduced. In addition, benefits from a uniform flow distribution and efficient heat transfer may be utilized. 

The magnetic field homogeneity is the next important requirement for MRI magnets. In many MRI applications, the magnetic field homogeneity of around... 

It is normal in conventional NMR to work with deuterated solvents, which serve both for optimising magnetic field homogeneity (lock, shim) and for avoiding the presence of the unwanted strong signals from protonated solvents. 

The next step is to set the same conditions for the HPLC system which is coupled with the NMR spectrometer. The field homogeneity of the probehead is first optimized (shimmed) using the same separation column and solvent mixture. 

Figure 2.1 Sample depth and magnetic field homogeneity.

Of course, there are always samples that refuse to dissolve completely even after ample exposure to heat and prolonged dunking in an ultrasonic bath. Samples that appear in any way cloudy when held up to the light, simply must be filtered. Any particulate matter held in suspension will severely compromise field homogeneity and thus line shape (Figure 2.2). Suspended material (of whatever origin) is the major cause of substandard line shape in NMR spectroscopy. 

Figure 2.2 Undissolved material causing loss of magnetic field homogeneity.

Shimming The process of adjusting current flowing through the room temperature shim coils in order to achieve optimal magnetic field homogeneity prior to the acquisition of NMR data. The process may be performed manually or under automation. 

Of course, the most precious part of a spectrometer is the magnet Both field stability and field homogeneity are of prime importance and have a huge effect on the quality of spectra. A tittle calculation will show how difficult it is to achieve the required field stabil-... 

TD-NMR is performed on NMR spectrometers that are equipped with lower magnetic field strength magnets with relatively low field homogeneity. Thns, relatively short FlDs on the order of a few milliseconds are obtained and FT of this signal yields broad lines from which no chemical detail can be obtained. However, the data is rich in information regarding the relative amonnts of different phases that are present in a sample, snch as water and oil, liqnid and solid, crystalline and amorphous. The data can be approached in two ways - analysis of the FID or analysis of relaxation times and their relative distributions. 

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