Month: October 2022

The design of the NMR experiment does not lend itself for measurements at exactly determined temperatures. During experiments the NMR tube sits in an rf-coil and the proximity of any parts made out of metal – including a thermocouple – will interfere with the experiment. Therefore, the thermocouple used to measure and regulate the temperature of an air-stream surrounding the sample is sited at a distance from the sample. In addition it also does not help that it is virtually impossible to achieve a homogenous Temperature distribution along the NMR tube, which will lead to convections.

The only type of thermometer, which can be used to directly measure temperatures during the conduct of magnetic resonance experiments are fluoroptic thermometers. These thermometers detect the color change of specific dyes with temperature. Fluoroptic thermometers have been used in MRI studies, however for an NMR experiment one would have to deal with the signal from the thermometer (i.e. the dye and its solvent).

A direct way to directly measure the temperature in the NMR tube is through the temperature-dependent chemical shift of some solvents. For instance the chemical shift difference between the two peaks of methanol is temperature dependent and it can be used to calibrate the actual temperature in the NMR tube with respect to the temperature measured by the thermocouple.

If things are bad in solution they are worse for solid-state MAS experiments. Here the temperature is regulated for the stator, which is the ceramic housing surrounding the spinning MAS rotor. In between the spinning rotor and the temperature-regulated stator is a stream of ambient air, the bearing. This air stream is used to float the rotor within the stator. Again, it is possible to regulate the temperature of the sample within the MAS rotor through the chemical shift of a reference sample. In this case it is lead-nitrate, where the temperature dependence of 207Pb-NMR chemical shift is well characterized.

Rey, our 700 MHz instrument has a dual configuration. It can be used as solution NMR instrument with a cryoprobe or it can be used as a solid-state NMR instrument. Changing the cryoprobe to conduct solid-state NMR experiments involves the following:

Pushing the “warm-up” button. This will start the automatic warm up procedure, which will take several hours. After this a lot of cables and hoses need to be disassembled. Pulling out the heavy cryoprobe takes two persons. One has to take great care to only hold the probe at its base to avoid damage. The cryoprobe has to be safely tucked away. Then the solid-state MAS probe can be installed, which involves deactivating the sample-changer, hooking tubes for the MAS pneumatic and connecting a bunch of cables. Once this is done a few calibration have to be performed or at least checked: the shim, the magic angle, power levels. A few glitches are to be expected – on Monday it took us ~4 hours to get the instrument ready, this is not counting the time it took the probe to warm up.

Reinstalling the cryoprobe is a similar effort. It is even worse – some of the connectors are really finnicky. They are difficult to reach and difficult to see for someone who wears glasses with bifocal lenses.

Frequent removal of the cryoprobe also puts wear on the system. Please let us know well in advance if you want to conduct solid-state NMR experiments on our 700 MHz instrument.