Hi AMMRL,
Here are the responses to my question on using Vespel caps for LT-MAS. No clear ‘winner’ on whether they are safe or not, in my opinion.
Thanks,
Sameer
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We have a new Bruker AV-600 widebore NMR spectrometer with a standard 3.2-mm MAS probe with temperature specifications of 173 K to 425 K (i.e., -100 C to +150 C). While I, too, used a zirconia cap initially, Bruker's application scientist recently visited here and demonstrated the use of a Vespel cap using lead nitrate as the NMR thermometer over this entire variable temperature range, both low and high temperatures. Since then I have run a sample of interest, using a Vespel cap, over this same temperature range. Thus far, we have had no issues whatsovever in using a Vespel cap over the entire temperature range.
When doing VT, I typically spin at 10 KHz with this probe, which is the spin rate I used for the above measurements. In order to use the Vespel cap over this full temperature range (which does extend beyond the stated temperature range for Vespel caps in the MAS control window), it is necessary to go into the MAS control window (under configuration, if I remember correctly) to deselect, i.e., uncheck the box, which tells the software to limit the temperature range to that selected for the cap. I should also mention that I am using dry nitrogen gas for the spinning. In short, the Vespel caps work for variable temperature for me.
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I’ve had some experience with using the vespel caps. Generally speaking, you can get them to work, but the costs of probe repairs in the case of a crash in my mind outweigh the cost savings of not buying the zirconia drive caps. Every new user generally goes through a few of the zirconia caps before learning to handle them gently, but once you get the hang of it, they really shouldn’t break very often if at all. Once I’ve ejected a sample, I’ll often remove the cap and reuse on a different rotor, meaning that I can have far fewer caps in circulation than I have rotors.
At my facility, we had a slightly different procedure that the MagLab’s for testing the fit: we would hold the rotor, or specifically the side with the drive cap, up to the open nozzle of a liquid nitrogen tank under a gentle spray of LN2 until the rotor got cold, then see if the cap could be easily removed by hand. I’m inclined to think that the method where you dunk the whole rotor in liquid nitrogen for a minute is probably more reliable, though harsher on the sample. Since there is no “matched cap” system for 3.2 standard wall rotors, some caps were definitely way too loose; others were too tight whereby they couldn’t be inserted correctly at room temperature. Once we had selected the right cap for the rotor, we often used a sharpie to paint around the stalk of the drive cap, with the sharpie ink acting as a sort of glue to provide additional adhesion. I’ve done the same thing here at Bruker a few times as well, even with thin wall rotors, and never had a crash this way. A couple of caveats however: I do think the MagLab’s precaution of not spinning above 8kHz unless necessary is wise, as faster MAS makes the cap much more likely to pop out. With that said, I’ve spun at 12.5 kHz for a week _at_ ~100K with rotors sealed in this manner without problems. For the sake of caution, I have always spun these rotors up slowly to the target MAS frequency, in steps of 1kHz or less beyond the initial spin-up procedure to 5 kHz, to avoid placing additional stress on the cap. Furthermore, if using the sharpie method, the rotor can sometimes spin great the first time, but the cap will pop off on subsequent insertions. My theory here is that the ink “glue” cracks with heating/cooling, so it’s best to reapply the sharpie every time the sample is warmed. Also, with relatively tight-fitting caps, it’s quite easy to damage/destroy them in the process of removing them, which can be a real pain if you then have to find another well-fitting cap every time.
In summary, I’d say you can get away with using the vespel caps most of the time, but at the cost of a lot of headache and worry with each sample that something could go wrong.
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I was able to use Vespel caps on Chemagnetics/Varian Pencil probes because the VT gas was Isolated; the bearing and drive were RT gas. Unless the Bruker DNP is a similar design, cold gas will make the cap shrink and lose its grip on the rotor; if you're lucky you'll have a soft crash, but it is possible for a crash to be sufficiently violent to destroy the probe.
A Vespel cap that grips the rotor well at l-N2 temperatures is likely to crack the rotor at RT - I wouldn't operate that way.
If you would like a source for less expensive caps, contact the folks a Revolution NMR - they usually want less than half the vendors' prices and their quality is top notch. They have vast experience and would be happy to give an unbiased recommendation for your application.
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You can use the VESPEL caps for sapphire rotors and DNP. We find that they work most of the time, but occasionally the cap would pop out inside the probe and cause spinning problems. We find that using a sharpie and draw a layer around the cap where it interfaces with the rotor seems to provide an extra micron that prevents the cap from popping out.
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----- original email below -----
Hello AMMRL,
Does anyone have any success or horror stories using Vespel rotor caps for low-temperature MAS? Our specific application is a Bruker DNP probe with 3.2 mm sapphire rotors at a temperature of 100 K and MAS speed of 8 kHz. To date we have been using the rather pricey zirconia caps.
I did see the following document which has a sensible ‘dunk it in liquid nitrogen’ test for Vespel caps.
https://nationalmaglab.org/images/users/nmr_mri/searchable_docs/instruments/600mhz_dnp_changing_spinning_sample.pdf
Any thoughts/comments would be greatly appreciated and I will post the usual summary with names/places removed.
--
Sameer Al-Abdul-Wahid, Ph.D.
Manager, Nuclear Magnetic Resonance Centre
University of Guelph
50 Stone Road East, Guelph, ON N1G 2W1, Canada
Tel: 519-824-4120 x58914
Web: http://nmr.uoguelph.ca/
From: Sameer Al-Abdul-Wahid <s.wahid_at_uoguelph.ca>
Date: Monday, July 31, 2017 at 15:16
To: "ammrl_at_ammrl.org" <ammrl_at_ammrl.org>
Subject: Vespel rotor caps and low-temperature MAS
Hello AMMRL,
Does anyone have any success or horror stories using Vespel rotor caps for low-temperature MAS? Our specific application is a Bruker DNP probe with 3.2 mm sapphire rotors at a temperature of 100 K and MAS speed of 8 kHz. To date we have been using the rather pricey zirconia caps.
I did see the following document which has a sensible ‘dunk it in liquid nitrogen’ test for Vespel caps.
https://nationalmaglab.org/images/users/nmr_mri/searchable_docs/instruments/600mhz_dnp_changing_spinning_sample.pdf
Any thoughts/comments would be greatly appreciated and I will post the usual summary with names/places removed.
Best regards,
Sameer
--
Sameer Al-Abdul-Wahid, Ph.D.
Manager, Nuclear Magnetic Resonance Centre
University of Guelph
50 Stone Road East, Guelph, ON N1G 2W1, Canada
Tel: 519-824-4120 x58914
Web: http://nmr.uoguelph.ca/
Received on Thu Aug 10 2017 - 03:15:48 MST