Obtaining NMR data at high or low Temperatures

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.

What does it take to change a cryoprobe?

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.

NMR tubes

We have recently seen some researchers producing poor data by using cheap, unlabeled NMR tubes. I am therefore reposting our instructions regarding the use of NMR tubes here:

The only way to obtain useful data is to have well prepared samples in high quality nmr tubes. Do not buy nmr tubes in bulk packets that do not have labels specifying what they are. You will not know the quality of these tubes. At the minimum you will get poor results and the worst they will damage the nmr instruments There are many grades of nmr tubes. Below is a list of nmr tubes that we found to work well for our instruments Quality nmr tubes from other vendors might work as well; we are happy to help testing these tubes.

 For instruments of 500 MHz and less.

Norell (nmrtubes.com)  507-HP-7 (these are sold in the VWR stockroom)

Wilmad (willmad-labglass.com) 527-PP-7

New Era (https://newera-spectro.com) NE-HP5

For instruments of 700 MHz and 800 MHz you can use the tubes above but you will get better results from tubes that are specified for those field strengths.

when to wear gloves?

I see a lot of people wearing laboratory gloves while typing on the keyboard operating the NMR spectrometer.

This appears to be a good thing – why not be super-careful and wear gloves routinely in a lab environment or when touching something which comes out of a lab environement?

But keep in mind – while you protect yourself you might also be transferring toxic residues to places, which are touched by other people, for instance a computer keyboard.

Therefore, pls do not wear gloves while operating the NMR console. If you feel that you have to wear gloves (for instance due to an allergy), please change to a fresh pair prior to typing on the keyboard.

Topspin command “serial”

I have often run into the scenario that I am comparing a series of NMR spectra, which all need to be processed exactly the same way. Changing one processing parameter – for instance the line broadening “lb” – will then require reprocessing of all spectra.

This tedious task can easily be automated using the little known topspin-command “serial”. This command opens a menu, where you specify the list of spectra of interest (i) and define a command or au program for the processing (ii).

Welcome to Professor McShan

Welcome to Andrew McShan, new Assistant Professor in the School of Chemistry and Biochemistry at Georgia Tech. Andrew is expert in protein NMR and they have already started setting up a library of NMR experiments for the characterization of proteins on our 800 MHz instrument. This library will become available to all of our users.

Pls check Andrew’s new website at: http://mcshanlab.com/