Using at least one instrument for both solids and solution
is a fairly common occurrence. At least 24 of our list members
admit to doing it regularly. The range of switching times
is large, 4 minutes to 4 months. Unsuccessful transformations
usually just involve head scratching, but breaking back-ordered
boards is also possible. The average time to switch is 41 min if you
ignore two outlying points, 133 min if you include them.
Solids to liquids averages 32 min and liquids to solids 44 min.
For four groups the switch to solids was at least half an hour
slower than switching to liquids. But three groups reversed this.
There didn't seem to be significant differences
between vendors. Older systems take longer than newer systems.
Systems that require large changes in shim current can take time to
arrive at thermal equilibrium. Shimming for HRMAS can be time consuming.
Higher fields are slightly slower, but the data is noisy.
1-750 (60min), 5-600 (22 min, 196 with outlying point), 4-500 (42 min),
5-400 (41 min), 4-300 (24 min), and 1-200 (5 min).
The systems that convert in less than half an hour don't
require shimming, transmitter calibrations, magic angle calibration,
Hartman-Hahn match, or shift referencing. This could either be a
life style choice or arise from system "personality". Its hard to control
all the variables, even the magnets are not created equal. Good 1H spectra
can be collected on one of our systems during Helium fills while another
is unstable 15 min after Nitrogen fills.
The good stuff:
unfortunately, generally people switched at maximum N-1 things
(from N), so there is easily another 1 hr. for finding out a) there's
something wrong and b) exactly WHAT is it.
Overall, I'm quite happy with the dual-mode operation,
especially since the demand for solids is rather low (would never
justify a dedicated instrument).
I have taken care to ensure that nobody will accidentally load solids
parameters with a liquids probe...BZZZzzzt! (that was the sound of my
probe getting fried 8-). I have a separate login account for solids,
for gradient shimming on deuterated solvents, then it only takes
15 minutes these days to get to "near" spec.
With modern shim designs and gas cooling of the
shim stack this may well be less of a problem than it once was , but I
would still expect there to be a delay of a couple of hours (at least)
before you could get good, >>reproducible<< solution shims on a 600.
I would still advise to change between setups not more than
1-2 a week. Changing hardware setups often might cause some wear
it is imperative to have painfully obvious markings on the
magnet, upper barrel and probes to make sure everything is lined up
properly.
For changing to liquids, I have found that a half an hour is a
good minimum time. This assumes that the user is experienced at changing
the system and that a quick lineshape test (after the change to liquids
configuration) is acceptable.
The shims are never an issue, even after running unlocked for a week.
Switching from 5 mm hi-res to flow-injection automation
(a switch that we make at least a couple of times per week) can take
anywhere from 10 minutes to 3 hours, depending on what goes wrong or is left
undiscovered.
(Solids to liquids) takes no longer than a standard liquids probe change.
we have two shim tubes, an 18 gradient BOSS-I shim
tube which stays permanently in the magnet and a 28 gradient BOSS-II
which is indexed into the BOSS-I when we switch to liquids.
the physical reconnection of the shim A and B cables must ALWAYS be
done with the BSMS off
when switching to solids the sweep function on the keypad MUST be
turned off!
there is always the risk of not getting the correct spectrometer mode
when switching to liquids and possibly sending 1KW of rf into the
liquids probe.
The vt stuff with the BVT 3000 need to be reconfigured as the
thermocouples are different between the solids and liquids probes.
We were quite skeptical about this arrangement at
first but it really does work in practice.
From solids to solution I find that we can get spectra at specs
in about 30 to 45 minutes with a stable air source. This is consistent.
it usually takes an hour maximum to do the switch either way.
I almost never have a problem getting the original liquids line shape specs.
Liquids probe shims come back if you are careful about marking
up the various alignments with the magnet bore. Typically it takes me 5
minutes to switch, both ways,
99+% of the time on the solids side of things your referencing
and power level settings will be identical when you load in the last solids
experiment and set it up. KBr magic angle adjustments are a rarity. Once
every few years it may need tweaking. This is based on switching probes
regularly - 1-2 times per week, over a 13 year period. The trick is
consistency, however, having grad students and the like making the
switches is probably not the best idea to keep it quick and painless.
Switching back from HRMAS to solution requires a night, since the
shims-currents for HRMAS are very high, and the whole system needs some
time to adjust back to the relatively weak solution state shim currents
However, whenever I have written NSF proposals (a few years ago)
suggesting such shared usage of an NMR, the reviewers always respond
that it is impractical to do so, no matter how much I have tried to
argue that it is feasible. I don't know how NSF comes up with sets
of reviewers who are convinced that this is the case, but that has
been my experience. I suspect that the reviewers have been liquids
people who still harbor the old ideas that doing solids on an NMR
renders it unusable for hi-res liquids work. Maybe this mindset will
be changing soon, or already is - hopefully.
_ _
( \ / )
__\ Y /,-')
(__ .-'
| (
[___]
|oo |
,' \ |
<___/ |
| |
| |
| |
| |
_,-/_._ \,_
_.-"^` // \ `^"-.,__
\ ,// \ /
`\,-":; ; \-.,_/'
|| | ;
|| ; |
:\ / ;
\`----' /
`._____.-'
| | |
__| | |__
jgs / | \
`""""`""""`
The winner of the Brass Figligee With Bronze Oak-Leaf Palm
Device is Rich Shoemaker. He had 5 minutes to switch to solution and
got done with a minute to spare. In his own words "The user following
me had no idea anything unusual was being done ahead of him".
Rich can display it on his desktop, or sell it for millions.
Just remember that its GPL stuff, so if anyone asks for a copy
you have to give it to them :).
Honorable mentions go to Gerd, John, Glenn, Jerry, Dave, Alan, Jerry,
Hanno, Valerie, Marc, Stan, Todd, Guillermo, Patrick, Tim, Sue, John,
Greg, Walt, John, and Julia.
Thanks for your time and thoughts.
dave scott
iowa state university
Sometimes I wonder whether the world is being run by smart
people who are putting us on or by imbeciles who really mean it.
--Mark Twain
Received on Tue Feb 04 2003 - 12:02:15 MST