Field shimming

The deuterium line of the deuterated solvent is used for this purpose, and, as stated earlier, the intensity of this lock signal is also employed to monitor the shimming process. The deuterium lock prevents any change in the static field or radiofrequency by maintaining a constant ratio between the two. This is achieved via a lock feedback loop (Fig. 1.10), which keeps a constant frequency of the deuterium signal. The deuterium line has a dispersion-mode shape i.e., its amplitude is zero at resonance (at its center), but it is positive and negative on either side (Fig. 1.11). If the receiver reference phase is adjusted correcdy, then the signal will be exactly on resonance. If, however, the field drifts in either direction, the detector will... 

See the NACE Papers Oliver W. Siebert, Correlation of Laboratory Electrochemical Investigations with Field Applications of Anodic Protection, Materials Performance, vol. 20, no. 2, pp. 38-43, February 1981 Anodic Protection, Materials Performance, vol. 28, no. 11, p. 28, November 1989, adapted by NACE from Corrosion Basics— An Introduction. (Houston, Tex. NACE, 1984, pp. 105-107) J. Ian Munro and Winston W. Shim, Anodic Protection— Its Operation and Appheations, vol. 41, no. 5, pp. 22-24, May 2001 and a two-part series, J. Ian Munro, Anodic Protection of White and Green Kraft Liquor Tankage, Part I, Electrochemistry of Kraft Liquors, and Part 11, Anodic Protection Design and System Operation, Materials Performance, vol. 42, no. 2, pp. 22-26, February 2002, and vol. 42, no. 3, pp. 24-28, March 2002. 

A set of shim coils usually represents the field expansion according to spherical harmonics. Depending on the required accuracy, more or fewer orders of the expansion are used. Clinical imaging systems usually use only the first order... 

W. A. Anderson 1961, (Electrical current shims for correcting magnetic fields), Rev. Sci. Instrum. 32, 241—250. 

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. 

If your experiment is short, you don t need to worry about field drift. Modem magnets are quite stable and can be used for at least a few minutes without drifting too far. The disadvantage with this approach is that shimming is normally performed on the deuterium signal and you will need to shim your sample differently if there is no deuterium in the sample. 

Shim coils Coils built into NMR magnets designed to improve the homogeneity of the magnetic field experienced by the sample. Two types of shims are used cryoshims and room temperature shims. Normal shimming involves the use of the room temperature shims. 

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. 

---