The Shim System

Another way to shim is to use the FID as a criterion for homogeneity instead of the lock level. The goal is to get a smooth exponential decay curve with the longest time constant (slowest decay) possible for the FID. Bruker uses the command GS to enter an interactive mode where the FID is acquired over and over again, displaying it each time 

If shims are really bad, you may be able to recall a recent shim file from the disk. Ideally, a spectrometer should be shimmed regularly (daily or weekly) by an expert, and these current shims should be saved. Rather than waste a lot of time trying to get home from someone else s / /-dimensional wanderings away from the optimum, you can just read the latest shim file and start from there. Of course, you will still need to adjust Z1 and Z2 because these change a great deal based on the sample volume, position, and polarity of the solvent. 

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NMR microscopy systems are built as add-ons to NMR spectrometers. The general technology of an NMR spectrometer is not the subject of this chapter, only the parts that are of particular importance in NMR microscopy applications, such as the magnets, the shim systems, the gradient systems, the NMR microscopy probes and the gradient amplifiers, will be described in some detail. 

The whole NMR imaging sensor system usually consists of a magnet, a shim system mounted inside the room-temperature bore of the magnet, a gradient system mounted inside the shim system and the rf coil mounted inside the gradient system. In the case of a saddle coil or a birdcage resonator, open access can be realized from the bottom to the top of the entire system with the coil diameter. 

As chemists performing spectroscopy, we are usually looking for the highest spectral resolution possible. Generally, one of the factors that limit the spectral resolution is the uniformity of B0 (the homogeneity of the static field) over the volume of the sample. If the field strength differs from point to point within the sample, there will be a similar point-to-point variation of Larmor frequency, and the width of the resonance line will reflect that distribution in frequency. The shim system (Section 3.2), which is used in a spectrometer to improve B0 homogeneity, consists of a set of electrical coils within the... 

Multiple receive systems with multi-coil arrays have become widely distributed in medical MRI. The benefit is the enhancement of the signal-to-noise ratio per time or a reduction of the acquisition time. This technique is not used in NMR microscopy for objects of intermediate size in standard bore (52-mm id) and wide bore (89-mm id) magnets, which are the most widely distributed magnet types for NMR microscopy. The main reason is the restricted space in such magnets for the shim... 

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. 

If the spectrum involves only one resonance (or if linewidths do not allow for the separation of several resonances), a single experiment can be run with acquisition of the amplitude of each echo along the pulse train (for sensitivity enhancement, accumulations can be carried out). This experiment is especially valuable for determining the relative proportions of two species which differ by their transverse relaxation time, for instance the two types of water (free and bound), if exchange between these two states is sufficiently slow. For this type of measurement, a low resolution spectrometer (without any shim system) proves to be quite sufficient. 

Bruker Smart Magnet control System (BSMS) including the digital deuterium lock system and the BOSSl shim system... 

Glass, nylon, polyester, and cotton fabric or mat are often used as a carrier in tape or film adhesive systems. In addition to being a carrier and a reinforcement, the strands of the fabric offer an internal shim so that the bond line cannot be thinner than the thickness of these strands. Sufficient pressure need only be applied to cause the adhesive to flow so that the shims meet the substrate surfaces to provide a positive stop. Paper, mat, and other carrier materials may also be used for this purpose. 

To prevent damage to the membranes, they should be properly shimmed and the thrust ring correctly installed (see Chapter 4.3.3 and Question 17.3.4). The shims and thrust ring will minimize or prevent movement of the modules during start-up and shut-down of the RO system. 

To solve this, PC card manufacturers have come up with a piece of software called a PC Card "shim" that allows the card to be used as any other expansion card is used. In addition, it takes up less conventional memory. The only downside is that the shim might not be completely compatible with the system. Windows 9xincorporates this software as virtual device drivers, or VXDs. 

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