Hi Mike,
Thank you very much for the comment. Over the years we have been using manual
insert, BACS, and SampleJet without problem but will be soon getting a new
system with SampleCase so I take your advice on board and will be extra
vhigilant going forward.
Cheers,
Karel
________________________________________
> From: main_at_ammrl.groups.io on behalf of Michael Groves via groups.io
> Sent: 21 October 2025 17:00:48
> To: main_at_ammrl.groups.io
> Subject: Re: [AMMRL] Facility policies for Variable Temperature Experiments?
Hi all,
I have one note regarding what Karel said. Regarding point 1--"they can easily
be taken safely above and below by a fair margin..." This depends somewhat on
the configuration of your system. If you have a samplecase sample changer, the
tolerances are tight enough that if you heat the blue spinners much over their
limit (80C) they will deform enough to render them unusable with the samplecase.
If you're inserting samples manually they'll likely be okay but we have seen this
cause problems with that sample changer.
Cheers,
Mike
On Monday, October 20, 2025 at 04:02:58 PM MDT, Karel Klika via groups.io wrote:
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 allowed
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
> 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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