Solenoidal receiver coils

The development of microcoil techniques has been reviewed by Minard and Wind [24, 25] and by Webb [26]. In a more recent publication Seeber et al. reported the design and testing of solenoidal microcoils with dimensions of tens to hundreds of microns [27]. For the smallest receiver coils these workers achieved a sensitivity that was sufficient to observe proton NMR with an SNR of unity in a single scan of 10 pm3 (10 fL) of water, containing 7 x 1011 proton spins. Reducing the diameter of the coil from millimeters to hundreds of microns thus increases its sensitivity greatly, allowing analysis of pL to pL sample volumes. 

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... 

Fig, 2. Different transmitter/receiver coil arrangements in probes of NMR spectrometers (a) iron core electromagnets (b) cryomagnets with Helmholtz coils (c) cryomagnets with solenoids. Arrangement (c) has been used in the studies reported in this chapter. 

Figure 12.4. Block diagram of a modem NMR spectrometer. These systems use superconducting magnets that are based on a solenoid of a suitable alloy (e.g., niobium/titanium or niobium/tin) immersed in a dewar of liquid helium. The extremely low temperature of the magnet itself (4.2 K) is well insulated from the sample chamber in the center of the magnet bore. The probe in which the sample is housed usually incorporates accurate temperature control over the range typically of 4 to 40°C for biological samples. The rf coil in the probe is connected in turn to a preamplifier, receiver circuitry, analog-to-digital converter (ADC), and a computer for data collection.

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