I have gotten overwhelming responses for my question. Here is a summary
after much delay. I wish to thank everyone.
Most said that they do not have problems with silicon compounds in
borosilicate glass tubes. It was mentioned that, if studying silicate
compounds in basic solution, the glass may become brittle. This turns out
not to be the case. Our group here is making polymers derived from silanes.
It was also mentioned that cholorsilane is aggressive stuff. I have to
investigate that here to determine whether that contributed the breaking.
Many messages concern the use of teflon tubes. In that regard, both NEW-ERA
and Norell wrote me about their teflon cells and liners, glass accessaries,
and the instruction in their catalogues how to use them. Other
manufacturors may also have this information in their catalogues.
I am posting all the messages in the order of arrival below after my
original question.
Hsin
-----
Hsin Wang
NMR Facility Manager 718-982-3809
Department of Chemistry 718-982-3910(fax)
College of Staten Island email: wang_at_mail.csi.cuny.edu
2800 Victory Blvd
Staten Island, NY 10314
-----Original Message-----
We have a group doing silicon chemistry here. I have seen their tubes break
off at the edge or even splinter at the bottom and made a spill in the
probe. Initially I questioned the quality of the tube. But perhaps the
silicon chemistry is responsible? What is the general advice to do this
kind of work. I have seen advertisement on teflon sleaves. Can one put a
teflon tube directly on the spinner?
-------AMMRL replies--------------
Hmmm.... Might depend on the specific type of silicon chemistry. Bob
West's group here does a _lot_ of silicon chemistry, and we've had
little to no problems with tubes. They do not use teflon sleeves. But
some of their compounds are air sensitive, and these they glass seal
under inert atmosphere. I believe most of these also have to stay cold.
-----------------
It is not quite clear what you mean by "silicon chemistry".
If it is silicate chemistry, using very basic solutions which stay in
the tube for long time, one could imagine that the glass is thinned and
turned more brittle. If it is silicone polymers or organosilicon
compounds I do not see any reason (except, perhaps, after repetitive
tube cleaning to remove the leftovers from chlorosilanes and similar
aggressive stuff). We have not observed anything like that in 40 years
doing NMR of silicon compounds.
-----------------
This is not totally related to your question, but....
When I was in George Olah's group doing super acid chemistry, we used quartz
tubes. In our case, it was impractical to use standard nmr tubes as they
react with the HF.
-----------------
New-Era Enterprises sell a PTFE tube with a plastic & glass support
system - have a look at page 18 of their current catalogue at
http://newera-spectro.com/. It fits into a normal spinner with only
the teflon section in the active volume.
I don't know why tubes would splinter at the bottom, but breaking off
the top edge of the tube is usually mishandling by students forcing off
the tube cap (easily solved by warming the cap gently with a hair-drier
prior to removal).
---------------------
We use the teflon inserts at times. I typically put the teflon inserts in a
5mm glass NMR tube, parafilm the Teflon tube/cap to the NMR tube and put it
in a 5mm spinner. The quality of the teflon liner is quite important. We
noticed that one teflon insert produces normal looking spectra, while the
next we cannot shim and there is essentially no resolution of any of the
resonances.
---------------------
Make them buy and bring their own probe or make it very clear they are
responsible for repairs. I have seen it once too many times.
---------------------
Are they running SiF compounds? These might etch the glass and do this type
of damage. Because NMR tubes are largely borosilicates, maybe they should
be using polyethylene tubes. Have them consult Wilmad. Wilmad had tubes
for this situation, I think?
I don't know enough Si chemistry to tell you. SiF can release F- ion, which
at low enough pH can etch glass. If they are looking at the Si-29 NMR
signal, they have a background in the NMR tube and in the Quartz insert in
the probe. Could they be "bouncing" their NMR tubes against the desktop?
I've had a sample recently, which blew out the bottom of the tube, when it
dropped into the probe. My only guess based on the fracture pattern of the
tube, was that someone had dropped it on the countertop once too often. If
the group wants to use glass tubes, I would suggest using the cheap ones
that are thicker glass and are cheaper because of it. My favorites in this
class are Norell XR-55s. I used them extensively doing synthesis in
graduate school, with no problems.
--------------------
One easy step would be to use thick-wall tubes.
Another precaution would be to use a gentler way of putting samples in the
magnet. The way most air lifts ramp down leaves the sample in free-fall
for a foot or so. Conceptually the simplest is to lower the sample on a
string. In practice it's better to use a wire with a hook so you can
remove the wire to spin the sample. (Retrieve the sample from the magnet
using the airlift). A surprisingly good method is to drop the sample into
the magnet without the air on, then immediately seal the top of the upper
stack with your hand.
However, given that you seen tubes break off at the edge, I would also
suggest you get an observation of this animal in its native habitat -- some
otherwise bright students just don't instinctively have a gentle
touch. Again, thick (or regular) walled tubes; we've forgotten that 'thin
wall' tubes was once an optional tradeoff to squeeze a little more
sensitivity out.
---------------------
>From Norell
Place sample into a Teflon NMR tube liner (8" long), and place liner into a
3-4” long glass NMR tube that is open on both ends. Place a rubber o-ring
around the top open end portion of the liner to prevent it from slipping
through the glass NMR tube. Position the glass NMR tube into a spinner
turbine, and place into magnet. The rf coil will only “see” the Teflon liner
and sample, since the glass tube portion is too far from the coil to be
detected. Recommend is the Norell Teflon NMR tube liner part # TL-5-7 with
3-4” long open-ended glass NMR tube (we can make this for you).
---------------------
About 10 years ago I worked with a student who was running 23Na in
telfon tubes (10 mm). This was done without spinning and posed no
problems (this was high-resolution work).
---------------------
I have done Si-29 nmr for years from RT to -100C and never had the problems
you described. What in the solution, is it generating an increased
pressure in the tube?
---------------------
We routinely use Teflon Tube liners as our NMR tubes. Our tube design is
outlined in A Lee Smith's "Analytical Chemistry of Silicon".
In short we take a 15mm Teflon tube liner and wrap it with Teflon tape at
two locations towards the top and middle of the tube liner. We then slide a
16mm OD glass sleeve (about 4" long) over this to make a tight fitting. The
turbine now fits nicely on the glass sleeve, and we have a silicon free tube
at the probe coil. This should be readily adaptable to most tube sizes.
--------------------
I do a great deal of 29Si NMR, and have never had a problem with NMR tubes
breaking. I doubt that the silicon chemistry itself would have that kind
of effect on the NMR tube. I have used Teflon inserts, but that is
primarily to reduce the 29Si NMR background signal from the NMR tube glass.
The Teflon insert cannot be put directly into the spinner. The Teflon
inserts are designed to fit inside of an NMR tube. So what I did, is cut
off about two inches of a regular NMR tube, put my sample in the Teflon
insert, then place the insert into the cut-off NMR tube. I use tape or
Teflon tape to secure the insert to the NMR tube. Then the tube assembly
fits nicely into the NMR spinner and only the bottom of the Teflon insert
actually goes into the NMR probe.
One additional issue is the 29Si NMR signal from the probe itself. These
background signals (from the probe and NMR tube) are very broad, and can
make phasing and integration difficult.
In addition to using the Teflon insert, I have also collected blank spectra
(only solvent in the NMR tube), that I can subtract from my sample
spectrum.
One possible solution to 29Si background signals is to use a 29Si INEPT
experiment, but as of yet, I have not had time to play around with the
parameters to set that up.
----------------------
You might try contacting tube manufacturers directly for suggestions, too.
---------------------
You can put a little electrical tape around the Teflon tube then put it
in the spinner, but you must never spin a Teflon tube. They are not very
straight and spinning them could break your probe!
This is the way we do almost all of our B11 and Si29 work.
----------------------
We use 10mm Quartz tubes from Wilmad (513-7pp-7QTZ) for Si29 NMR to minimize
the glass signal, and have not seen any tube breakage in the probe, though I
was warned that these tubes are more brittle than the normal thin wall glass
tubes. Maybe their samples are highly basic, then it could corrode the
glass, but it should be a very slow reaction.
I've heard about people using teflon sleaves for Si29NMR work. I think you
put a teflon sleave through an open end glass tube, so the RF coils will
only "see" the teflon portion with the sample on the bottom. I've never
used it though.
--------------------
I have run a lot of Si-29 in JEOL and in Varian-400 NB
and have never had any problem at all . We use Wilmad
528 or 535 tubes. Sounds like they have tubes that have
some of their stuff on the outside and then the spinner
breaks the tube when inserting or removing from the
spinner. Is there stuff heating and cooling and thus
expanding in the tube? I had a tube crack that way with
a heavy duty polymer going to 160 and down, but the tube
actually held.
But I have had no problems with Si-29 NMR as a source.
--------------------
The teflon tube liners that are obtained from Wilmad will not directly fit
into the spinner. If you happen to find some that do please let me know.
I found sometimes running silicon there is a background signal from the
glass of the NMR tube, so I usually cut off the bottom of the tube and then
insert the teflon liner through it so that it is in the probe coil. Though
you may still see some background signal from the glass insert in the probe.
--------------------
We have used teflon sleeves for radioactive samples. The sleeve fits into
the tube. It cannot go directly into the spinner, since it is somewhat
flexible, so you need something rigid around it. There is of course some
loss of resolution and sensitivity.
--------------------
>From NEW-ERA
Your e-mail was forwarded on to me. In our NMR catalog we show non-glass
sample cells which have been used for silicon studies.
In this case the Teflon cell slips into a glass sleeve where most of the
length is suspended below the sleeve. Only the Teflon is in the coil.
The standard spinner fit the glass sleeve. This system will spin.
Information can be found on page 18 of our NMR catalog.
A less effective and less costly approach is to use a Teflon liner (page 22)
which needs to be supported in the spinner with tape or an o-ring.
The NMR catalog , with pricing, is found at
http://www.newera-spectro.com
Maybe the caps are on the tube too tightly and too difficult to remove.
You might try some of our 10mm tubes, NE-L10-7. $9.25 each.
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Received on Fri May 13 2005 - 13:38:17 MST