Shimming suggestions using various brands of NMR tubes.

There is a great article written back in 1991 by Prof. Gerry Pearson which is still recommended reading for anybody seeking to study in more detail the methods of shimming a supercon NMR. One important aspect of the article to note, which we occasionally receive questions about, is the shimming technique to eliminate spinning side bands. Quite often, Grad Students ask us about this problem, only to find out that they are shimming only the Z and Z nth order shim packs and disregarding the X and Y and combination nth order ones! With out further ado, I would like to give you the link so that you can read this interesting article yourself.

You can find it at the University of Iowa site : http://nmr.chem.uiowa.edu/manuals/Shimming-GAP-NMR-magnet.pdf

Good luck with your shimming.

Scratches in the glass, does it matter to an NMR tube?

We recently had a researcher call us with a problem of tube breakage. He was using a NMR tube washing unit to recycle his NMR tubes, and found that some tubes were breaking very cleanly at specific points along the tubes. After talking to the researcher, he indicated that the NMR tubes were stored in metal racks that had drilled out holes where the tubes were positioned into. At these points of contact, and after being placed into the rack during several cycles of usage, the metal hole portion scratched into the surface of the NMR tube.

Our Norell Technical Service Department informed the customer, that NMR tubes, like any other piece of glassware, become flawed when scratched and will develop a weak spot.

The point to be made, is never store a NMR tube in a metal rack, to prevent scratching on the tubes outer surface, resulting in tube failure.

Round or flat bottomed NMR tubes, what's the difference?

Some researchers in the past have stated that round bottomed NMR tubes are superior to flat bottomed because they shim better, while others couldn't tell the difference. A researcher recently wanted to know what the differences were between the two types of NMR tubes and which type would be better for his experiments.

To answer his question, we have to take a brief look into a past experiment of about 20 years ago, that involved a study in determining the strength of NMR tube bottoms. The experiment involved dropping flat and round bottomed NMR tubes from a height of about 5 feet, to observe any changes in the tubes bottoms. After doing this to 100 tubes of each type, the flat bottomed tubes outperformed the round bottomed ones by not breaking as many bottoms. Round bottom tubes would break more often, because the round bottom tubes had a significantly thinner glass wall than a the flat one's. At the same time, shimming experiments revealed that when the experiment only contained flat bottom NMR tubes, that shimming between the tubes took the same amount of time as with round bottom tubes. There was only a difference in shim times, when round bottom tubes were mixed up with flat bottom ones.

So to answer the researchers question, if you already are using NMR tubes with round bottoms, for the sake of faster shimming times, buy round bottomed tubes. If all your tubes have flat bottoms, continue to use flat bottom tubes. Just don't mix up flat bottomed ones with round bottomed ones, if shimming times between tubes is important to you.

Background Signals in Quartz

It has been well documented that damage to quartz, either by grinding, polishing or spot heating will cause increased ESR background signal. This is notably due to Si-O and Si-H bond breakage and the formation of unpaired electrons that once occupied these orbitals. A second less well known cause is the exposure to x-rays, gamma radiation, and intense UV radiation. It has been found that Quartz sample tubes, upon storage next to high energy radiation sources, have after a prolonged period exhibited strong EPR centers that were significant enough to cause interference of sample data. Should you have EPR sample tubes that are producing higher than average background signals, we recommend that you ship the tubes to Norell Inc. for re-annealing. The annealing temperatures that we use to prepare quartz sample tubes for EPR are high enough to reform any chemical bonds broken by the above conditions. Please contact our Technical Service Department at 609-909-3700 for further details.

Using Hydrofluoric Acid or Ammonium Bifluoride to clean NMR tubes

Some NMR tube cleaning procedures have called for the use of HF and/or (NH4)2F2 before final rinsing with DI water and drying. Since glass is solubilized with these reagents, the NMR tube walls become thinner and also irregular with time, since the dissolution rate is uneven and unpredictable on the wall's surface. Not only will the NMR tube become compromised in its ability to hold sample without breakage, but the quality of the NMR spectrum will become degraded, because of the irregularity of the wall thickness. Our recommendation is to use a high quality glass cleaning surfactant, such as NRS-250, that removes organic contaminants without damage to the glass walls of the NMR tube and extends the life of the NMR tube.

Using EVA and Polyethylene NMR Tube Caps with Organic Solvents

While preparing a sample for NMR, a Laboratory Technician noticed that his sample solution became colored after shaking with CDCl3 while in the NMR tube. The question was directed to our Technical Service Department, as how to prevent this from occurring when making up new samples. Upon questioning, the Technician mentioned that he was using standard EVA NMR tube caps that were supplied with his NMR tubes. He would prepare his samples for NMR by adding solid sample directly to the tubes with Chloroform-d, and then inverting the tubes to facilitate mixing. After further questioning he mentioned that the solvents were in contact with the caps. We informed him that in using halogenated solvent with EVA or Polyethylene, there is a risk of dissolving the cap and/or leaching out monomers, plasticizers, curing agents, and color components from the plastic. Our recommendation was to only use Teflon® caps, TC-5-PTFE or TC-10-PTFE, to avoid any possible contamination of sample solution from leaching effects of the solvent.