Room temperature shims

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. 

The gas flow rates are adjusted to ensure a constant pressure inside the cell and to minimize degradation of the spectral resolution by the gas bubbles. Typical line-widths at half height are <4 Hz on NMR (200 MHz) and <2 Hz on (80.3 MHz). The probe fits into the room temperature shims of a wide-bore NMR magnet and the pressure and temperature range of the high pressure NMR flow are 0.1 to 20 MPa and -40 to 175 °C, respectively. 

The fields with m = 0 are independent of the angle zonal fields. The others are called tesseral fields. Some shim coils for production of tesseral and zonal fields are sketched in Fig. 2.3.2 [Krel, Mori], Room temperature shims may provide correction fields up to = 5, while cryoshims frequently provide corrections only up to n = 1. In this way homogeneities of 2 ppm can be reached over 30 cm and 5 ppm over 50 cm diameter volumes. 

The gradient coils are inserted inside the room temperature shim coils in the magnet (Fig. 2.3.1). Depending on the type of spectrometer, they can be part of the magnet assembly or part of the removable probe assembly. The former are typical for medical... 

Computer control of the room temperature shim currents that control the field homogeneity, of sample spinning rates, autosamplers, pulse sequences, and many other aspects of the instrument operation is a necessity. 

Figure 6.22 MAS NMR spectra (14.1 T, Vr = 6.0 kHz and dl = 120 s) of a synthetic sample of triclinic (Tl) tricalcium silicate (CajSiOs) obtained with (a) a standard set of room-temperature shim parameters and (b) after shimming of the probe for H on a sample of H20 D20 (1 10 volume/volume). Six hundred scans were accumulated for the spectrum in (b). (c) Same as (b) but for an array of five subspectra (each 120 scans), taking into account a constant drift in the magnetic field before summation (see text). The numbers indicate the assignment of the individual resonances to the nine inequivalent Si sites in triclinic CajSiOj. (From Skibsted J. et al., Chem. Phys. Lett. 172, 279-283, 1990.)...

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