Ernie,
Never tried PFG shimming in automation. But we do it (both 1H
and 2H) all the time in manual use on our Varian spectrometers,
and it works quite well. Sometimes leaves a slight Z1/Z2 split,
but that is easily corrected. This observation might suggest
problems in an automated environment, but I think we see this
only when using an inferior map.
Although your equipment is different, I don't believe there
should be any substantial differences in experiences between
Varian and Bruker users; this statement assumes quite a bit
about the software, but I believe both companies are taking
similar approaches, and therefore experiences on one system
should translate. You can see a general email I wrote to the
AMMRL group on this subject at:
http://wwitch.unl.edu/ammrl/archives/June-99/0066.html
Comments more specific, perhaps, to your situation:
1. I recommend redoing maps regularly. And DMSO works best for
making a good map that you can keep for a while. Definitely
would not recommend a CDCl3 map; they can work, but have the
worst S/N. Redoing maps regularly (at least once a week) may
be difficult if you are doing 3D maps, but they are fast and
easy with 1D maps. Once a week may not be really necessary,
but if you let natural curiosity work, you will learn what
works to give the best map after a few months of playing.
2. I don't think there's any reason to limit the shims to just
Z1-Z3. The total number of shims is perhaps dependent on
the shim set, but I would include at least Z4, even in the
initial iteration. The map fitting is improved if you
include more shims, assuming the data in the map and in your
run are good.
3. Note my comments in the email link above about relaxation
properties of 2H causing significant modification of
parameters (d3 array in Varian's psg) depending on solvent.
I believe both companies are providing a single set of parameters,
but my experience is changes depending on solvent helps.
It may be difficult to allow for this in automation? I would
definitely modify these parameters to see if the ones you are
using are optimum over the range of solvents being run. If
the dephasing delays are not set right, then you either will
have insufficient signal in the 2nd experiment (d3 too long),
or insufficient dephasing (d3 too short): either condition
will produce a noisy map.
4. PFG shimming cannot correct for scratched tubes or unfiltered
samples. In these cases nothing can produce good shims. But
I've seen PFG shimming push the shims so far that they could
not get back afterwards. If you can read in a standard
shimset for each sample, it would help to prevent this
situation.
Certainly would like to hear how this all turns out. Contact
me further if you have additional questions. I hope we'll have
a discussion at the AMMRL meeting about PFG shimming; hope to
see you there if you're at the meeting.
Good luck,
Charlie
At 10:11 AM 2/14/00 -0500, you wrote:
>BUM'ers,
>
>First, I'm new to BUM, so let me introduce myself....My name is
>Ernie Schubert and I've been involved with NMR for over 11
>years. I've spent the last 5.5 years supporting synthetic
>chemists in the pharmaceutical industry via automation and
>structure elucidation.
>
>Here at BMS in Wallingford, CT, we have 5 Bruker NMR's ranging in
>field from 300-500MHz. I've been exploring the utility of 2H
>gradient shimming under automation on our new Avance 500 equipped
>with a Z-gradient QNP probe (1H, 13C, 31P, 19F). My experience
>thus far hasn't been too gratifying. I'm using a map generated
>by a Bruker applications chemist on a 600uL sample of CDCl3. I'm
>also using a 2 step method: first shimming on Z1Z2, then
>Z1Z2Z3. The gradient shimming seems to take longer and doesn't
>do a very good job at shimming to boot.
>
>I'm hoping to draw on the experiences of the members of this mail
>list. What kind of shimming do you utilize under automation? Do
>you have any experiences/opinions with 2H gradient shimming under
>automation? Any suggestions for me?
>
>Thanks in advance for all of your suggestions,
>
>Ernie
>
>--
>Ernest M. Schubert
>Research Scientist I
>Bristol Myers Squibb
>5 Research Parkway
>Wallingford, CT 06492
>(203) 677-7953
>(203) 677-7702 fax
>ernest.schubert@bms.com
>
----------------------------------------------------------
Charles G. Fry, Ph.D. Tel: (608)262-3182
Director, MR Facility Fax: (608)262-0381
Chem. Dept., Univ. Wisconsin
Madison, WI 53706 USA email: fry@chem.wisc.edu
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