RF receive coils

Restriction of the sample volume can reduce substantially the overall inhomogeneity across the sample. In most instruments the effective volume of the sample i.e., that within the rf receiver coil) is restricted to about 0.1 ml, when the sample is placed in a 5-mm-diameter cylindical tube—a size that is regarded as the standard NMR tube. However, larger diameter tubes (8—15 mm) are sometimes preferred for studying dilute solutions or for observing nuclei of low sensitivity, even with some loss of homogeneity across the sample. (As we see in Section 3.11, much smaller diameter tubes are also valuable when the total amount of sample is limited.)... 

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