The probe is the heart of the nmr spectrometer and the magnet is its soul. The probe delivers the rf radiation to the sample and receives the (very weak!) signals coming from the sample. Essentially, it is a tuned antenna for both transmitting and receiving rf radiation.

Lets have a look:

This is a 10 mm probe from a Bruker AMX spectrometer. It fits into the room temperature bore in the magnet, inside the room temperature shim coils (see the first diagram in the magnetsection). The part of the probe that sticks out of the bottom of the magnet is where the rf connections are and is where one finds the tuning and matching controls:

This particular probe has two types of tuning controls. The first photo shows tuning rods that are moved in or out to adjust the tuning and matching. This is usually the case on a Bruker broadband multinuclear probe that must be tuned over a wide range of frequencies. The second photo shows the other type of tuning control. The (in this case) yellow rods are turned until the probe is tuned and matched. The "T" stands for ... tuning! And the "M"?

This is a somewhat old probe. Modern probes have extra stuff on them such as computer chips with information about the probe and gradient pulse coils but are basically the same. If we take the sheath off of the probe (If you've never done this before ... don't) we see:

In the above picture we see the tuning and matching rods that lead up to the adjustable capacitors, just below the rf coils:

If you adjust one of the rods too far you will damage the capacitor-rod linkage and it will be a really expensive repair! Take care. This is a delicate piece of equipment and rather easily damaged.

Above the tuning and matching capacitors are the receiver/transmitter coils:

They are very fragile and are supported on glass mounts. Finally, we see a photo of where the sample sits in the probe:

As you can see the sample sits right down in the rf coil area, as close as possible to the coils without actually touching them. Also, you can see why it is important to put your sample into a depth gauge so that it is properly positioned in the probe when it is inserted into the magnet ... too high and the sample will not be close enough to the coils ... too low and the bottom of the sample tube will bump against the bottom of the coil cavity and possible break (another expensive repair!).

Here we see the probe inserted into the bottom of the magnet. This is the view that most users see:

So, what's all this talk about tuning and matching. Well, in terms of high-frequency electronics the probe consists of a couple of capacitors and a coil or inductance. The electrical diagram for this looks like:

There is a theorem in electronics that says that, in order for maximum power transfer between two circuit elements, they must be of equal impedance. This is the primary function of the matching capacitor. It's task is to match the impedance of the coil/sample system to the external electronics. We want maximum power transfer because our pulse calibrations are based on a tuned and matched probe. If the probe were not matched properly not enough power would get to the coil which would mean that the B₁ field was not powerful enough and did not rotate the magnetization as far as we wanted it to. Also, it would mean that the very weak signal coming from the sample would appear to be even weaker than it should be leading to a lower than necessary signal-to-noise ... the cardinal sin in nmr spectroscopy.

You can also see, in electronic theory, that for the parallel circuit of the tuning capacitor and coil (inductance), there is resonant frequency at which electrical energy is built up much more powerfully than it would otherwise be. This is called a tank circuit; it's resonant frequency is determined by the value of the inductance (L) and the value of the capacitor (C). Usually, it is the capacitor whose value is variable. Thus, we tune the circuit to the frequency of interest by adjusting the value of the capacitance so that we can get the strongest signal possible. Remember, nmr is a very powerful technique but not a very sensitive one.

In practice, the two adjustments are often not independent of each other. You may have to readjust the tuning after adjusting the match and vice versa.

The latest development in probe design is the cryoprobe. The idea here is to improve the signal-to-noise (S/N) ratio by cooling the receiver coil and the preamp to approximately 77K. This cuts the thermal noise in the critical electronics before the very small nmr signal is amplified. Current cryoprobes offer S/N values that are about 10 times the value of a normal probe. This is a cryoprobe and its attendant cooling platform: