AMMRLers,
Thanks to everybody who responded to my posting regarding the use of J
Young tubes in Nalorac and Bruker probes.
As expected, there was a range of opinions. I did receive several
testimonials reporting no problems at all with Nalorac probes and insert
breakage.
Owing to the sensitivity of the issue, I am going to summarize with a
paraphrasing or 'distilling' of the e-mails I received. What I write below
is a mixture of my own experience and what others told me.
(1) Nalorac probes are harder and more expensive to repair.
I should also add from personal experience that Bruker probes are the
easiest to service yourself.
(2) Varian and Bruker typically have quicker turnaround times for repairs,
on the order of 1-2 weeks, whereas the Nalorac turnaround time is typically
around 3 weeks. There are third party vendors that can do the repair more
quickly than Nalorac, e.g., JS Research. I would imagine that MR Resources
can also do this.
(3) Quartz inserts can be purchased from other parties. One simply needs
to specify the dimensions accurately. A caliper will allow this to be done
(need OD, ID, and length). Possible vendors include Wilmad and New Era
(800-821-4667, Frank Bosco). There are surely others.
(4) The insert failure rate I reported was also possibly attributable to
the following:
-an improperly positioned thermocouple (not my problem because we've
already addressed that issue),
-an improperly set up depth gauge (again, not my problem - we've superglued
ours in the 5mm setting),
-loose or greasy spinner (we clean the spinner often, so unlikely),
-the alignment of the upper bore tube and the RT shim set (maybe, but I'm
very reluctant to take the RT shims out of the magnet for the purposes of
alignment).
(5) People with older Bruker probes seemed to report fewer problems, but I
didn't get detailed information on the vintage of everything with which
people had experience. My own personal experience is that my old AC250
with Bruker probe didn't have the problems that I have with my newer (1999
Avance/DPX) console and probe. Part of the increased breakage may be
attributable to the 10-15 delay in between when the lift button is clicked
and when the lift air comes on fully. That is, some of my breakage may be
attributable to people dropping samples in with insufficient (in some cases
zero) lift air. Nothing that a sound thrashing can't fix if the training
doesn't take the first time.
This reminds me of a nearly meaningless personal anecdote (skip ahead if
you just want the facts...). When I was at Texas A&M in the 80s, I took a
graduate course from the esteemed Prof. Bernard Shapiro (old school NMR,
editor of the TAMU NMR Newsletter that evolved into the NMR Newsletter many
of you now know). In one of the homework problems I arrived at the right
answer by trial and error, and I had a page or two of really awful-looking
pontification that went to Prof. Shapiro along with the correct answer. He
gave me zero credit on that problem so I thought I'd ask him why. He
proceeded to tell me a story about a man selling a donkey which went
roughly as follows: The buyer asks if the donkey responds well to voice
commands. The seller gives a command and the donkey does nothing,
whereupon the seller picks up a 2x4 and smacks the donkey in the head, and
then repeats the command which the donkey obeys. The prospective buyer
asked what the 2x4 is all about, and the seller says "You just have to get
his attention." I took this as Prof. Shapiro's way of calling me an ass.
I was more careful thereafter in presenting a clearer train of thought in
my solutions to the homework problems.
Back to the present...
Another possible cause of broken inserts is that the newer Bruker probes
seem (I haven't measured the inserts) to have a thinner-walled piece of
quartz. I do know that the part number and nomenclature has changed (they
are not called quartz inserts anymore). Recall that the closer around the
sample you put the coil, the better your filling factor (greek letter eta
in most of the literature) and hence the stronger your NMR signal (all
other things being held equal). I did hear one comment that the Nalorac
inserts are even thinner than those used by Bruker in their newer probes.
(6) Spinning J Young tubes was also reported to give problems including
scoring of inserts and outright breakage. Some forbid the spinning of
these tubes, while others operate at reduced spin rates (10-12 Hz).
(7) Finally, a number of people suggested lowering heavy tubes manually. I
have done this before with 10mm tubes full of 15 atm of Xe gas (I
definitely didn't want to bust one of those open!) by taping a small loop
of string to the top of the tube. The tube/spinner can then be lowered
with a second string or monofilament which can be removed. Lift/eject as
usual, although additional pressure may be required to lift heavier
samples. One person also reported using the manual lift button and slowly
reducing the lift/eject air. This is not going to be a viable option for
the grad students in my lab based on how I see some of them tune the
probes. I was thinking that T'ing a regulator into the eject air line
going into the upper bore tube might allow one to gently raise and lower
samples...this may end up being my solution.
Here is what I've gotten out of this: if you have a small number of users
that you trust, then you can go ahead and get a Nalorac probe with its
better sensitivity, but if you have a probe that is going to be used by
persons immature, unaccountable, incorrigible, uncaring, or inconsiderate
(i.e., an academic NMR lab with lots of users), then you might want to opt
for something that is easier to repair yourself or is less prone to
breakage in the first place.
My personal experience with modern Bruker probes versus Varian probes is
that the Varians hold up better, but when they break it's gonna cost you
$4k to have the coil/insert assembly replaced compared with the $80 (or
less) you'll be paying for Bruker quartz inserts. Turnaround times for
Varian repairs are 1-2 weeks on average.
I'd finally like to end with a placating gesture to all of the vendors out
there that I've undoubtedly pissed off. This may be obvious to many, but I
will state it explicitly for the sake of completeness (charitable view) or
ass-covering (cynical view). Each user and each lab has different needs,
and all the vendors are collectively able to cover the needs of the users.
What works well in one lab might not be so good for another lab. Make your
own decision.
Jeff
Dr. Jeffrey H. Simpson, Instrumentation Facility Director
Department of Chemistry, Massachusetts Institute of Technology
77 Massachusetts Avenue, Cambridge, MA 02139
617-253-1812 ofc, 617-253-1806 lab, 617-253-0873 fax
e-mail: jsimpson_at_mit.edu
http://web.mit.edu/speclab/www/
Received on Thu Feb 14 2002 - 13:37:25 MST