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Department of Chemistry,
University of Alberta October
2009
NMR News 2009-03
News and tips from the NMR support group for users of the Varian NMR systems in the
Department
Editor: Albin.Otter@ualberta.ca
http://nmr.chem.ualberta.ca
There are no fixed publishing dates for this newsletter; its appearance solely depends on whether there is a need to present information to the users of the spectrometers or not.
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Contents
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Many questions have been asked regarding all the
changes taking place in the NMR labs. Here are, hopefully, all the answers you
need to understand what is going on.
Please share this information with your colleagues and your supervisor. The more
people know about it the better.
EB-44 lab renovation and new
equipment
The EB-44 renovation was long and intrusive (yes, a Royal pain!) but now
finally over. The reasons for this massive undertaking: (1) to accommodate a new 700
MHz spectrometer and (2) install a Xsens (C13-optimized) cryoprobe on the existing
u500. Both pieces of new equipment are here and installed but neither of them
has reached its final stage.
What everybody noticed: the EB-44 lab has now an intense bird-like noise. This is the first of
two "birds". Once the 700 has its own cryoprobe, there will be two of them
"whistling" at the same time! What it really is: the cold-head in the cryobay that cools the helium
gas to 10 K with the help of gas expansion which is controlled by valves. The
valves are the reason for the noise. The machine shop is working on a cover to
reduce the noise level. It will likely not go away completely but much reduced -
at least this is the hope.
Why cryoprobes and are they
expensive?
The goal in every NMR experiment is to reach a sufficient signal-to-noise ratio
(s/n) so that signals from the sample can be distinguished from noise. How much
s/n is needed depends on what needs to be extracted from a spectrum. For
example, if a spectrum is acquired, 1D or 2D, to find minor isomer peaks or
impurities then a high s/n is required. There is, without a doubt, also a "gray
zone" re. how much s/n is enough.
NMR, compared to e.g. mass spectroscopy, is an insensitive
spectroscopic method, i.e. it takes fairly large amounts of material to get a spectrum in a
reasonable amount of time. The intensity of the signal is pretty much directly
correlated to the concentration of the solution in the NMR tube. The noise, the
second component in the fight for good s/n, is in simple
terms the sum of two contributions:
the sample itself (sample noise) and
so-called thermal noise from the receiver coil(s) and the pre-amplifier(s).
The complexity of cryoprobe design should by no means be understated here, but in a very simplified way it works like this: instead of working with a coil and pre-amp at room temperature (300 K), both are cooled down dramatically to reduce their thermal noise. This is achieved by pumping He gas at 10 K (from the noisy cryobay!) to the probe. Starting out at 10 K makes it possible to keep the receiver coil and pre-amp at 18 K. 300 K vs. 18 K dramatically reduces the thermal noise, and while keeping the signal the same (given by the sample amount), improves s/n dramatically. Measurements in our own lab have shown that whether the coil/pre-amp are at 18 K or 20 K there is a measurable difference in s/n (with the 18 K winning, of course!).
A comparison
to illustrate the benefit of the Xsens probe
Editor's note dated 2009-12-07: what is said below is
still true. However, after trying for 2 months without success to make the Xsens
a probe that can be used by non-experts, the decision was made to return it to
the manufacturer. At this time it is not clear if it will return for a second
chance or another type of cryo probe will be installed eventually. Until
decided, u500 is again available for walk-up access with the old room
temperature probe.
The previous probe on the u500 (for all practical reasons identical to the
one on ibd5) provides a C13 s/n of about 250:1 measured with a standard
sample. The exact same standard sample provides a s/n of 2500:1 on the
Xsens probe. Although these numbers are somewhat simplified, they do show in
essence a 10-fold increase in sensitivity (the precise number is closer to
90-fold).
This translates into reaching a certain s/n for a given sample
concentration 100 times faster. It is hard to imagine, but it means, for
example, that instead of 5 hours (300 minutes) the s/n is reached in 3 minutes
with the Xsens probe!
The other and probably more important way to look at it: 10 times less material
will now produce in 5 hours on the Xsens what otherwise would need 500 hours
with the conventional probe. A
500-hour experiment (3 weeks non-stop!) is clearly way beyond what is possible.
Conclusion: with the Xsens probe C13 spectra can be obtained in a reasonable
time with samples 10x less concentrated than previously possible.
The benefits are not limited to C13. The H1 coil and H1 preamp are cryo-cooled as well and provide very high sensitivity for H1 (3200:1). In fact only the v700, once it has its own cryo-probe (see below), will be better. This has to be put in perspective by looking at our best system so far, i600 with a proton sensitivity of 1200:1.
Our
Xsens probe is only the 3rd in the entire world, the first in Canada. There are
at least two reasons why cryoprobes are not just found everywhere.
(1) The probe and associated equipment is worth approximately $300'000. That is
pretty much what an entire 400 MHz spectrometer with magnet would cost.
Universities and private companies alike, have to decide whether or not they
have the funds for this kind of expensive equipment. In addition there are
maintenance cost in the order of $20'000/year.
(2) Cryoprobes are not as user-friendly as room temperature probes (RT probes).
A simple look at all the equipment in EB-44 that was put in place shows that.
Unfortunately, the handling of the probe itself is not trivial. Furthermore,
some initial problems have to be expected for the equipment to reach its full
potential. This is what we are currently experiencing. The probe meets 3 out of
4 performance criteria. Efforts are under way by the manufacturer to reach the
fourth. Only then the probe becomes our property (at the moment it is not) and
we can start using it.
Once
the probe is accepted and we had a chance to thoroughly test it, every group in
the Department can submit the names of two students/PDFs to be trained in the
use of the Xsens probe. The two "group Xsens experts" are allowed to measure
samples of other group members. When the selected people are capable of using this piece of
equipment safely and know the peculiarities of data processing (baseline
corrections are a must for C13 data), they will get permission to use it.
u500 users will also have to deal with a different version of
VNMRJ. The new hardware is not compatible with the older version.
The on-line
reservation and the login procedures will only work for people that have gone
through the training and are approved. No doubt unpopular but necessary in view
of the value of the probe and its limited user-friendliness. The access policy
will be reviewed once sufficient experience has been obtained with the probe
and two-users-per-group test phase.
v700: access and usage
The decision was made last week by the NMR Committee (Drs. Bundle, Cowie,
Wasylishen and Otter) that there will be no walk-up access to v700.
24 hours per week are available to the NMR service for running samples and
maintaining the spectrometer: Tuesday 08:00 - Wednesday 08:00.
Anybody in the Department can submit samples to be run on that system during
that time. The remaining 6 days per week are assigned to the Carbohydrate Center
(groups of Drs. Bundle, Cairo and Lowary). The Carbo Center purchased the instrument,
paid for the lab renovation and also pays for the cryogens hence their almost
full time use of the system. Other groups in the Department who have legitimate
requests for this instrument can get time on it. What is needed is to contact
the Carbohydrate Center or the NMR service supervisor to place a request.
Ideally, this is done well before the time the instrument is needed.
Once installation is complete, the v700 will have a cryoprobe like u500 but in this case optimized for H1. It is currently in use with a RT probe and provides 1500:1 in sensitivity. The cryoprobe will increase this to 7000:1. These measurements are made, like for C13, with standard H1 test samples.
Changes to the
i600
The 600 MHz spectrometer (i600) will undergo an upgrade within the next 6
months. It will get ProTune and as such become much more user-friendly than it is
now. The first week per month will be assigned to solid state NMR (groups of
Drs. Wasylishen and Szymanski in Bio Science). The rest of the time will be made
more accessible to Dept users although full walk-up access is still under review.