Solenoidal Coil Geometries

This equation indicates that the coil sensitivity varies inversely with the diameter of the coil (for a fixed length-to-diameter ratio). For diameters below 3mm, the AC resistance of the coil itself acts as the major noise source, even for lossy biological samples. The resistance depends on both the winding geometry (including wire diameter, number of turns, and turn spacing) and the resistivity of the conductor. 

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The following formulas (Landee, et al., 1957 also Radio Amateurs Handbook) help to estimate how many turns are required for a given solenoidal coil geometry. If a" is the diameter and b the length of the coil in inches, the inductance L in microhenries is given by... 

FIGURE 7.1 RF coil geometries and their orientation with respect to the magnetic field, Bq (a) a saddle or Hehnoltz RF coil and (b) a solenoidal coil, shown here wrapped directly around a separation capillary. 

Figure 7.3.1.1 Schematics of two different RF coil geometries showing the coil and the sample tube position relative to the Bq field (a) saddle-type (b) solenoidal...

Advances in micro-coil probes offer additional benefits in sensitivity for NMR detection of metabolites and are especially useful for mass limited samples (49-55). Apart from the small coil volume, their solenoidal geometry... 

A cylindrical structure fitting the geometry of the magnet bore is the saddle coil. It can provide homogeneous B fields at moderate NMR frequencies (25 MHz) for larger volumes (up to 30 cm diameter). However, its sensitivity is lower by a factor of /3 compared to a solenoid. But a factor of >/2 can be recovered, if two saddle coils are arranged in quadrature for independent detection of the jc- and y-components of the induced signal. 

The performance of the spectrometer is impacted by changes in the dimensions of the radiofrequency coil. The signal-to-noise ratio (S/N) of an NM R experiment is increased by decreasing the size of the radiofrequency coil. Decreasing the size of the receiver coil results in increasing the S/N ratio since this ratio is proportional to the inverse of the coil diameter (for saddle and solenoid geometries). This functionality holds until the diameter of the coil is approximately 100 pm and... 

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