What is a Probe?
The probe is the part of an NMR spectrometer that does much of the work, in terms of exciting the nuclear spins, and detecting the NMR signal. The probe goes into the center of the magnetic field, and the sample is inserted into the probe to perform the NMR experiment. The probe contains the radiofrequency (Rf) coils, tuned at specific frequencies for specific nuclei in a given magnetic field. The probe also contains the necessary hardware to control the sample temperature (when combined with an external temperature controller).
Often, probes are constructed with two observe coils, one closest to the sample (the inner coil), and another further from the sample (the outer coil). This allows the probe to respond to multiple frequencies, and to allow the excitation/irratiation of multiple nuclei. The nuclei that use the inner coil are detected with the highest sensitivity. Therefore, a Broadband Observe probe is constructed with the inner-coil tuned to a "broadband" nucleus (i.e. 13C or 31P), for maximum sensitivity for these nuclei. An Inverse or Indirect Detection probe is constructed such that the inner-coil is tuned for 1H (or 1H & 19F). These probes give the highest proton (1H) sensitivity, with much lower sensitivity to directly observe lower frequency nuclei (such as 13C or 31P).
Probes can be designed to accomodate various sizes of NMR tubes. In general, larger volume tubes are best in cases where the sample is solubility (or concentration) limited. Larger volumes allow more sample to be contained in the coil. Smaller volume tubes allow the concentration of the sample to be increased when solubility is not a limiting factor. Small volume probes (i.e. 3mm, Nano, or capillary coil) give the highest sensitivity when very small amounts of highly soluble materials are under study.
Modern NMR probes also include an actively-shielded Pulsed Field Gradient (PFG) coil, which allows the application of field-gradient pulses. Many modern NMR experiments rely on the use of PFG pulses.
Probes for Solid-State NMR also contain the hardware necessary to spin the sample very fast, at a precise angle to the magnetic field (the magic angle). Click on this link for more information about solid-state NMR probes.