Coil NMR Probes

The s/n ratio in an NMR experiment, which is an expression of sensitivity, has been described for a cryogenic NMR probe38,39 and can be represented by the equation  

The most important factors in determining the s/n ratio or sensitivity of an NMR experiment in a cryogenic NMR probe, insofar as the hardware itself is concerned, are the temperature of the coil, 7/ the resistance of the coil, Rc the temperature of the sample, Ts and the resistance added to the coil by the presence of the sample or the sample resistance, Rs. The temperature of the rf coil in a cryogenic NMR probe is typically in the range of 15-30K commercial examples of the Varian Cold-probe operate at 25 K. The preamplifier noise temperature is generally in the range of 10-15 K and the coil resistance is small compared to a conventional room temperature NMR probe. The first two terms in the expression below provide the basis for the vast improvement in the performance of a cryogenic relative to a conventional NMR probe. The sample temperature, Ts, and sample resistance, Rs, 

Another expression that illustrates how Rs impacts on cryogenic NMR probe performance is  

In the example of a 3 mm tube being run coaxially in a 5 mm cryogenic NMR probe, the filling factor, r, is defined by the following equation, where  

Occasionally, in the author s laboratory, we have found that when a sample is inserted it just does not shim well for no apparent reason. In those instances, spinning the sample for a few seconds, insuring that it is coaxially centered in the probe has generally quickly led to acceptable shim settings. 

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When modern NMR was in its infanq, few if any scientists realized that they would ever require a two-coil NMR probe in which the coil lies inside the broadband coU. It was not until the demonstration that experiment time can be drastically reduced for the acquisition of 2-D heteronuclear correlation NMR data sets by employing H detected methods and a two-coil NMR probe with the coil configurations inverted (relative to the normal coil configuration described above) that the inverse probe really caught on. Now the inverse probe (a probe with the coil closer to the sample than the broadband coil) is a staple in nearly every modern NMR laboratory. 

Figure 7.8. (A) Dual isolated-coil NMR probe, and representative C spectra of (B) CCL, and (C) CHsOH obtained simultaneously using this probe. (Adapted with permission from J. Magn. Reson., 138 160. Copyright Academic Press, 1999).

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