Probes

In an NMR spectrometer the probe is the piece of equipment that holds the sample and detects the signals from it. This post will give an overview of probe design and the various types of probes available.

NMR probes consist mainly of a long tube that is inserted into the center of the magnet. The bottom of the tube contains a box with connections for cables to transmit radio frequency pulses and regulate temperature, for hoses to regulate air flow, and knobs to adjust the tuning and matching. When fitted, the top of the probe reaches the center of the magnet and this is where the sample, in an NMR tube, is placed. A nice series of photos of the inner workings of an NMR probe can be found here.

The figure above shows a schematic of an NMR probe on the left, with an expansion of the top section of the probe on the right. The sample fits inside the top of the probe inside the radio frequency (RF) coils. Normally there are two detection coils, an inner one and an outer. These coils deliver the RF pulses and detect the signals they generate. The RF coils are very delicate and are typically supported on glass tubes. This is why it is important to properly align the depth of your sample before putting it in the magnet. If too much of the NMR tube projects below the spinner you can damage the probe. If the NMR tube is too high in the spinner then the sample will not be in range of the detection coils.

Outside the RF coils sit gradient coils for altering the magnetic field around the sample in a position dependent manner. Below the RF coils lie the tuning circuit and variable capacitors, shown as a black box in the figure. The variable capacitors are connected to long rods that project from the bottom of the probe. These are the tuning and matching rods that are used to optimise the wobble curve for each sample.

The SSPPS NMR Facility has four probes, three of which are shown in the photo below. We have two Bruker triple resonance cryoprobes (CP-TCI), a Bruker broad band inverse detection probe (BBI), and a Protasis inverse detection flow probe (ICG). One of the cryoprobes takes 5mm tubes and the other takes 1.7mm tubes. All four probes have single axis z-gradients.

All of our probes use inverse detection. This means that the inner, most sensitive coil is used to transmit and detect ¹H signals and the outer coil detects the other nuclei. In the early days of NMR the inner coil was used for the less sensitive, heavier nuclei, but with the development of polarisation transfer pulse sequences the inverse arrangement, ¹H on the inner coil, was found in most cases to be more useful.

The triple resonance cryoprobes are designed for biomolecular experiments and are optimised to detect ¹H, ¹³C and ¹⁵N. These are the only nuclei these probes can detect, however, the probes are optimised for performance on these nuclei. Cryoprobes use gaseous helium to cool the the detection coils to 17 K and the preamplifier to 77 K. Operating at such low temperatures minimises thermal noise without reducing the signal, thereby giving a 3-4 fold boost in signal to noise. Cryoprobes are constructed with the preamplifiers built in, instead of being separate units on the floor, which is why the box at the bottom of the cryoprobe is larger than in the other probes.

The broad band probe operates at room temperature so it is not as sensitive as the cryoprobes, but it can be tuned to detect a variety of different nuclei, from ³¹P through ¹³C, ¹¹³Cd and ¹⁹⁵Pt, down to ¹⁵N. It comes with a list of frequencies, making tuning to different nuclei quite easy.

The flow probe is designed for working with mass limited samples. It can be connected to a HPLC so that your sample can be pumped directly into the magnet. This means it requires just 5 μl of sample. However, it operates at room temperature, so is not as sensitive as the cryoprobes, can only detect ¹H and ¹³C, and is more difficult to set up and calibrate than a normal probe.

None of our probes is capable of detecting ¹⁹F. The frequency of ¹⁹F is quite high, it's close to that of ¹H, so it is out of the range of the BBI probe. In the past, the ¹H channel on many probes was capable of being detuned so that it could detect ¹⁹F, but our probes are so optimised that ¹⁹F is out of reach.

Since all of our probes use inverse detection, directly detected ¹³C experiments are not very sensitive. The most sensitive probe for ¹³C would use the inner coil for ¹³C detection and be cryocooled to minimise noise. This is exactly the arrangement used by the Xsens probe available on the vx500 Varian NMR in Chemistry. If you need a 1D ¹³C spectrum on a minute amount of material this is the instrument to use.