Image Credit: MR Resources | University of Colorado
NMR probes are a part of an NMR spectrometer responsible for holding the sample, exciting the nuclear spins, and detecting the NMR signal. Probes can be switched out depending on the specific nuclei to be studied or the type of sample to be analyzed.
Probes incorporate radio frequency coils that are tuned to produce certain magnetic field strengths and detect specific nuclei. Multiple coils (an inner and one or more outer coils) are present in some probes to allow response to multiple frequencies. The inner coil, which is closer to the sample, provides the highest sensitivity.
Probes are designed to accommodate one of various sizes of NMR tubes. Larger volume tubes tend to be best where the sample is solubility-limited, as they allow more sample to be contained in the coil. Smaller tubes allow concentration of the sample to be increased when solubility is not a limiting factor, and they provide the highest sensitivity for small amounts of highly soluble materials.
The key product features of NMR probes include auto-tuning and pulsed field gradients. Auto-tuned probes do not require the user to tune the probe to accommodate variances in impedance between parts of the probe. Modern NMR probes which incorporate pulsed field gradients allow the application of field-gradient pulses, which many modern experiments rely on.