A more intense signal indicates a narrower peak and a more homogenous magnetic field. In order to optimize the magnetic field, the intensity of the 2H NMR signal is measured. Inhomogenities in the magnetic field lead to broader signals. During the process of shimming, these coils are adjusted to obtain the most homogeneous field possible around the magnet. In addition to the large magnet, there are many individual electromagnetic shim coils. Once this signal is obtained (called locking) the instrument can be shimmed. The lock signal is a spectrum of the 2H from the NMR solvent. The next step is then to obtain a lock signal. Spinning generally serves to reduce the effects of slight differences in the magnetic field in different parts of the sample. Once the new sample is floating on the air cushion, ij will turn off the air and allow the sample to drop slowly into the center of the magnetic field.
On a Bruker instrument, ej will turn on the air to eject the current sample. A cushion of air is used to remove samples from the magnet. The sample is now inserted into the magnet. If you have a very limited amount of sample, you should center your sample so that it is centered in the receiver coil box. The rectangle is the location of the receiver coils, and this area must have sample. Ideally, there should be the same amount of sample above and below the rectangle labelled as the 5 mm sample coil. This is a good time to check to make sure that you have the proper sample height. It is very important that the sample be at the proper height in the spinner. Inserting the Sample, Spinning, Locking and ShimmingĪfter the sample is prepared, it is inserted into a spinner and a depth gauge will be used to determine the proper positioning of the sample in the tube.
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