Dear all,
I firstly note that I am much less conservative than the responses thus far,
but since I have never had a tube break doing VT work I stand by my approach.
However, everyone needs to evaluate the risks for themselves and I absolve
myself of any responsibility.
I would, though, like to reiterate some important points already mentioned:
1. Spinners should be used within their specified temperature range, although
they can easily be taken safely above and below by a fair margin (10s of degrees).
The big danger is getting caught up in the work, running a spectrum at RT and
then thinking that, for example, well 100 C would do nicely to make a good
spectrum and forgetting about either the solvent or the spinner. But mostly
it seems it is the spinner that is forgotten about yet, no problems have
been encountered thus far, which is why we know about the tolerance of the
spinners. I guess the reason for their tolerance probably is the small contact
area of the spinner with the heated gas, especially when the sample is spun.
2. It is absolutely recommended to check the real temperature inside the probe,
so use of a Pt thermocouple is recommended, but if highly precise knowledge of
the temperature is not important and you just need to be assured you are not in
a dangerous state, then the NMR thermometers (methanol, ethylene glycol, etc.)
suffice. In my experience, one system may underestimate a high temperature
(so actually less than indicated) and overestimate a low temperature (so actually
higher than indicated) as you move away from room temperature, but this is
obviously safe. Another system may do the opposite, and this is obviously a
problem with high temperature work so it is definitely good to know how your
particular system and setup functions.
3. I have never worried about using new tubes, but this seems like a good
practice to avoid tubes with unsighted cracks. But I would advise against
using economy tubes and use only Pyrex/Duran (ASTM Type 1 Class A) for VT
work.
4. Checking tubes outside of the NMR at the intended temperature (or even
somewhat above) for high temperature work, especially sealed tubes or even
valved tubes where you are exceeding the boiling point by a large margin, is
strongly recommended.
5. I recommend changing the temperature in steps of 30 or 40 C maximum for
room temperature probes and waiting several minutes at each step when getting
there (can always write a simple macro to do this for you) – both going up
and down in temperature and returning afterwards.
Other points:
1. I see no problem with extended VT runs and have performed them many times,
even over the course of days. Setting up/changing back I consider more of a risk.
2. Teflon inserts (redundant ?) can cause problems due to the high thermal
expansion coefficient of Teflon, while low temperature will shrink the Teflon
and the insert falls to the bottom this will not cause damage, but at high
temperature the Teflon can expand sufficiently to crack the tube.
3. Low temperature work. Melting point suppression due to the presence of a
solute means that going to the melting point, or even some degrees below it,
means that the solution will not freeze. Actually, I am not sure what the
concern is with low temperature work, solvents never seem to break the tubes
when frozen. The exception of course is water. Long-term storage of aqueous
solutions in NMR tubes may result in 5–10% of the tubes breaking in my
experience. But the crystallization process is slow so even if an aqueous
solution freezes, it will not necessarily break the tube immediately.
4. High temperature work. It is possible to even go several degrees above the
boiling point without anything happening even for a normal capped tube. There
are three possible reasons for this, 1. Boiling point elevation due to the
presence of a solute, 2. If the cap is pressed down tightly, the pressure
from vaporized solvent raises the boiling point, and 3. The NMR tube is not
fully heated, only that below the spinner, so the part that is above the
end of the spinner acts like a condenser reducing the pressure. This especially
the case for 7” tubes and longer.
Regards,
K. Klika
________________________________________
> From: main_at_ammrl.groups.io on behalf of Charles Fry via groups.io
> Sent: 20 October 2025 19:20:26
> To: main_at_ammrl.groups.io
> Subject: Re: [AMMRL] Facility policies for Variable Temperature Experiments?
Hi Ken,
Great set of questions. I have always worried about VT because it would be
relatively easy to significantly damage the spectrometer (with one significant
probe damage and one big danger to the magnet happening during my first few
years managing a lab of chemists). Look forward to reading replies.
*
Do you require a specific grade of NMR tube?
I never set a rule about this, other than no chipped tubes (not supposed to
be used even when not doing VT).
*
New NMR tubes?
Didn't set this rule. Work done at pressures above 1 bar need thicker wall
tubes, and new tubes are correct.
*
What buffer do you allow between the solvent freezing/boiling point and the
target temperature? 10, 15, 20 degrees?
10 deg is reasonable when the researcher is checking the temp directly prior
to doing the VT expts using a VT standard or thermocouple. Careful
consideration/understanding of the gas flow is critical.
*
Anything else I'm missing?
Anything sealed should be tested outside the spectrometer first (including
when I do experiments for ppl). We had one sealed sample explode many years
ago, taking out the probe coils (expensive!).
No blue spinners for any VT work. I've found the white kel-f spinners to be
adequate for all other temps we've worked at.
The big danger to the magnet was me being not smart. A Varian probe was alowed
to run very cold (-100C) overnight during a humid June day. 1" of ice
on the top of the magnet next morning (and me apoplectic).... I had a cap
and tube made that fit the top of the magnet bore tube that ran the VT exhaust
gas away from the magnet after this.
Insist that researchers be trained in detail prior to approval for them to
do VT over larger ranges of temps.
Charlie
________________________________
> From: main_at_ammrl.groups.io on behalf of Kenneth Sharp-Knott via groups.io
> Sent: Monday, October 20, 2025 10:46 AM
> To: main_at_ammrl.groups.io <main_at_ammrl.groups.io>
> Subject: [AMMRL] Facility policies for Variable Temperature Experiments?
We're revamping and/or putting in place various policies/procedural documents
now that we have moved on from the Era of the Flood and into the Era of
Shiny New NMRs.
I'm curious what kind of specific guidelines you have in place for variable
temperature NMR?
*
Do you require a specific grade of NMR tube?
*
New NMR tubes?
*
What buffer do you allow between the solvent freezing/boiling point and the
target temperature? 10, 15, 20 degrees?
*
Anything else I'm missing?
Thanks in advance!
Ken Sharp-Knott
Manager of Analytical Services and the NMR Facility
Department of Chemistry
Virginia Tech
(540)267-6502 (Cell)
(540)231-0885 (Office)
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