Spectrometer Shimming

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 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. 

The sample is introduced into the spectrometer, locked onto the deuter-ated solvent (here CDC13) and the homogeneity optimized by shimming as described by the instrument manufacturer (this can often be done automatically, particularly when a sample changer is used). 

C satellites can actually be quite useful sometimes, as they give a ready-made visual comparator for the quality of spectrometer high-order shimming and for trace impurities that you may be trying to quantify, since we know that each satellite will have an intensity of 0.55 % of the peak it is associated with. 

NMR spectrometers in today s research laboratories are sophisticated pieces of instrumentation that are capable of performing a myriad of experiments to analyze questions ranging from the molecular structure of unknown organic compounds to the different crystal forms contained within a solid. While an NMR spectrometer is quite complex, it is comprised of a few key components. An NMR spectrometer in its most basic form consists of the following (i) a magnet, (ii) shims (iii) a RF generator and a receiver—probe, and (iv) a receiver. 

TD-NMR and HR-NMR spectrometer systems have a majority of components in common. All spectrometers consist of a magnet, magnet temperature sensors, magnet heater power supply, RF frequency synthesizer, pulse programmer, transmitter/amplifier, sample probe, duplexor, preamplifier, receiver, and ADC, all controlled by a computer. In addition to these items a HR-NMR has several other requirements which include an electromagnetic shim set, a shim power supply, and a second RF locking channel tuned to the resonance frequency of Li. The second RF channel is identical to that of the observed H channel. Figures 10.9 and 10.10 show the basic setup of TD-NMR and HR-NMR spectrometers, respectively. 

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. 

However, there is one major drawback implemented by the use of solvent mixtures. The commonly used solvent system of acetonitrile/water displays a strange behaviour. Typically, the line shape of the water and the acetonitrile is different. As normally the water is used in its deuterated form, the spectrometer is also locked on to the D2O component. Trying to shim such a sample by using the lock level, and therefore the line shape of the D2O, provides a good line shape for the water. However, for the acetonitrile component, severe distortions can be observed, and the sample signals show a similar behaviour like the signals of the acetonitrile. 

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. 

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... 

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