I received seven responses to my recent inquiry about running VT
experiments on vendor equipment.
*** If there are any Varian users with Varian or Nalorac probes ***
*** that have experience (positive or negative) running low temp ***
*** experiments, I would really appreciate hearing from you. ***
*** I was hoping to get more feedback from Varian users. ***
We run very cold (-150C) all the time on two of our Bruker systems,
and have some confidence that new Bruker systems can also meet our
needs for these experiments. We have less confidence as to whether
Varian or Nalorac probes can hold up to running this low over long
periods of time. Everything worked fine during our demo, but this
lasted only 6h for VT; that's a lot different than running regularly
for a few years.
[comments in brackets are mine]
SUMMARY:____________________________________________________________
From: Jane Strouse, UCLA mjs@argon.chem.ucla.edu
[following is summary of phone conversation]
Runs regularly to -150C on various Bruker systems; warns that should
have an adapter on rt shims to allow ambient gas flow to keep magnet
o-rings from freezing. [We have both gas flow through shims and
heating tape on shims to help magnet stay close to ambient.]
Jane has had to repair RT shims that have broken at the bottom of the
magnet at the connection to the RT shim tube a couple times only on
systems running cold VT. [This has happened twice here at UW also.]
_____________________________________________________________________
From: Charles L. Mayne, University of Utah, Dept. of Chemistry
B103 HEB, Salt Lake City, UT 84112, Voice: (801)581-7413
FAX: (801)581-4763 or 8433, E-mail: mayne@chemistry.utah.edu
http://www.chem.utah.edu/atlas/facilities/nmr.html
We occasionally run our 10 year old Varian broadband probes below -100 C
without problems with the probe. We sometimes run for a day or more at
these temperatures. We have had some problems with the plastic parts of
the plumbing that eventually get very cold and crack on long runs. Exact
temperatures weren't an issue for us so I don't have very good data
about that. We also had problems with the tachometer which stopped
working at low temperatures even though the sample was spinning OK, so
we could not use feedback to control the spinning rate. We had to just
control the air pressure to get proper spinning like in the old days.
All this was on a Varian VXR-500S. We haven't had occasion to do this
type of experiment on our newer Unity Plus systems yet.
_____________________________________________________________________
From: Jerry Dallas, Berlex Biosciences
The major problem with Bruker probes has been low flow rates for
VT gas. This has been the case for 15 years and I don't see any sign
of change. At extreem temperatures a temperature gradient exists over
the length of the sample. The only way to minimize the gradient involves
increasing the flow rate. On every Bruker system I have ever seen this
can not be accomplished because the spinner turbine suffers from a piston
effect and the sample/turbine lifts (even when the flow rate is as slow
as 15 SCFH). When I worked for GENMR Instruments we used holes in the
glass sleeve to permit 50 SCFH flow rates of VT gas. This was adequate
to minimize a gradient. Unfortunately the holes had a negative effect on
lineshape and were removed in 1990. The last time I checked, Bruker was
still using a pressurized canister to deliver cold nitrogen gas to the
probe. The main drawback to this approach involves stopping the entire
run to refill the canister. If you can imagine that better VT performance
is achieved by tripling the VT flow rate...just think about the impact on
the life expectancy of a full N2 canister.
To you, the other important issue involves isolation of the magnet from
the cold exhaust gas. At Edmonton Alberta (Chem Dept), low temperature
operation caused freezing of a Bruker magnet O-ring and a quench from loss
of vacuum. Varian is exhausting VT gas upward in the room temperature bore.
Today I don't know where the Bruker exhaust is routed.
The VT controllers have all been modified in recent years to deal with
precise regulation for biological samples near ambient temperatures. This
has been done to improve the quality of 2D & 3D spectra. I wouldn't be
surprised to learn that these alterations have diminished the performance
at extreem temperatures. The main issue ivolves the engineering concept of
a transfer function.
_____________________________________________________________________
From: dave scott, iowa state university, 515-294-4057
We've had good luck with our WM/AC spectrometer too, somebody
from Angelici's group was always running at 125K & we didn't have
any problems.
We haven't pushed the new equipment yet and since the gradient
probes on the DRX are only rated to -50C, we probably never will.
You may have to by a non gradient probe for low temp work.
[Bruker, and I believe Varian and Nalorac are now quoting single axis
gradient probes going down to -150C.]
_____________________________________________________________________
From: Jan Meine Ernsting, University of Amstedam, Inorganic Chemistry
NMR Department, Nw. Achtergracht 166, 1018 WV AMSTERDAM
tel: 020-5256441
We run low temperature (below 120C) on our bruker AC100 (10 years old) and on
our bruker AMX300 (5 years old)
On our ac100 I use a 5 mm BB probe and I have repaired the probe twice, a
electric connection was broken.
On our amx I use a 5mm QNP probe up till now without any problems.
The only problem we had with low temperature was that tunning and matching the
probe, some times it was impossible to tune the probe at very low temperatures.
If we keep tunning the probe while cooling down we can overcome these problems.
_____________________________________________________________________
From: Jim Breeyear, Manager, Inst center, Chem Dept, Univ of Vt.,
Burlington, Vt.
We have done quite a lot of low temp 100 kelvin on a wm250 and arx500
with acceptable results. It is a narrow bore magnet. Standard Inverse
probe and standard proton probe. If you want further infor let me know.
_____________________________________________________________________
From: Dr. Roy Hoffman, Manager, NMR Laboratory, Department of Organic Chemistry
The Hebrew University of Jerusalem, Jerusalem, Israel 91904.
Tel. (972) 2 6585327, Fax (972) 2 6585345, Email roy@shum.cc.huji.ac.il
Web page http://drx.ch.huji.ac.il:8080/hoffman.html
We also work at low temperatures, usually down to -105C but occasionally to
-104C. We have a Bruker DRX 400 with a Eurotherm B-VT 2000 controller and
also have inaccuracies with the thermocouples. We have two important
problems that we had to overcome. The Bruker heaters have a ground glass
joint which fits into the ground silica joint of the probe's Dewar. Repeated
usage at extreme temperaatures causes one of these to crack. Our solultion
is to cut off the heater's ground glass joint and replace it with a PTFE
'O' ring (such as from J. Young) held in place by a ring of sticky tape.
This seals much better and we have not had a breakage in a year. The other
problem is that heavy usage causes the thermoswitch in the liquid nitrogen
heater to fail every few months. I recommend that you keep some spare at
least once the guarrantee runs out.
Please let me know how you calibrate temperatures below the freezing point
of methanol.
[We use a thermocouple in oil (does freeze but still provides good
thermal contact) placed in the bottom of an NMR tube.]
----------------------------------------------------------
Charlie Fry Tel: (608)262-3182
Director, MR Facility Fax: (608)262-0381
Chem. Dept., Univ. Wisconsin
Madison, WI 53706 USA email: fry@chem.wisc.edu
----------------------------------------------------------