![]() In VT experiments, some users may set the VT gas flow rate too high. The sample is not all the way in the magnet. If the sample is not centered on the coil, the spectral lineshape will be bad and the lock may fail.ģ. It is important to position the sample symmetric to the center of the coil. In depth gauge, the probe RF coil position is marked by dashed line, while the center is marked (CL) on the long horizontal line, as shown in Fig. The sample does not sit symmetrically to the center of the coil. Please add enough solvent to meet the length requirement (approximately 600 micro liters in a standard 5 mm diameter NMR tube and 250 micro liters in a standard 3 mm diameter NMR tube).Ģ. In the worst case, the lock will also fail. If the sample is too short, the bottom of the tube and the surface between the liquid and air will disturb the homogeneity of the magnetic field around the sample, and will reduce the effectiveness of the gradient shimming, resulting in a bad lineshape. This will fill the coil with sufficient excess to maintain magnetic field homogeneity. ![]() The minimum sample length is 3.6 cm for 5 mm diameter tube and 2.8 cm for 3 mm diameter tube in the Agilent NMR machines. Lock failure and a bad line shape can be found under many different conditions. ![]() ![]() If the lock level is smaller than 10 after shimming, the lock may fail during the acquisition and lineshape may be bad. Lock and shimming are very important for getting a good line shape. ![]()
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