Tuesday, October 08, 2002
Summary of Responses
to my question posted to AMMRL on October 02, 2002:
Now that cryo-probes
are becoming more ubiquitous, I have been asked a specific question, to
which I don't know the answer. Is it practical to have a cryo-probe
equipped spectrometer that is used in an "open-access" lab (for use by
Of course, the sensitivity
benefits are well-known for large-molecule work, and for trace analysis
of natural products; however, as far as I know most (or all) facilities
with these probes only allow the NMR experts to use them. If my impression
is wrong (i.e. if there are labs in which the average user has access to
cryo-probe-equipped spectrometers), I'd like to know about it.
Please reply directly
to me if you can shed light on this issue. More and more people are
asking me about the practicality of these probes in various settings, and
I really don't have any answers. I will, of course, post a summary
of the informative responses that I receive.
Thanks in advance,
Responses From AMMRL
(Neil Jacobsen [firstname.lastname@example.org])
We are considering a cryo-probe on a new 600, primarily for protein work
but it will be "open access" to fill all the available time. I don't
think there is any reason that the probe should be limited to experts (although
beginners should be banned so we don't get depth gauges in the probe, etc.).
I just did a demo trip in Billerica
and Clemens Anklin
brags that the probe is "just like any other probe" except for the ensitivity.
of the fixed expenses ($12,500 / year for mandatory maintenance contract)
we will charge 50% more for time on this instrument. I think that
alone will weed out the people who are looking for time on "any NMR".
Hey Rich...the only information I can give you is with regard to their
installation and removal. Basically, it is not recommended that they
be un-installed (or warmed up) once they are installed. Too many
free/thaw cycles are not good on the RTSC, I guess. And that would
affect how you might use one in an open access facility if you don't have
a magnet to spare for one, a magnet dedicted to one, and have to change
probes to accommodate other uses. The probes are apparently not "tuneable"
in the classic sense (the Varian probes aren't yet, perhaps Bruker's are
different) so in my facility we would have trouble dedicating one multi-purpose
instrument to their use. The Varian probes will accommodate either
a flow module or standard paddles which is a neat touch. Big gain,
I guess, for organics (not lossy) and reasonable gains even for salty samples.
Good luck with this. I think that a flow system could be devised
that would revolutionize how we do "walk-up" nmr in support of organic
labs. Maybe all the pieces aren't there yet and a new system with
cryo-flow would be expensive ($750 to $850K for 500) but perhaps in a year
(Rainer Wechselberger [email@example.com])
We do not really have open access to our cryo-probe system (mainly because
we don't have open access to any of our systems), but in general the same
users who are using the other instruments are also using the cryo-probe
system. Assuming that your 'open-access'-users are using 'standard'-experiments,
which are setup by you anyway, I don't see that much of a problem. The
main thing you really have to take care of with those systems is that the
heating of the sample/probe keeps limited. Dangerous in this respect are
TOCSYs (especially heteronuclear) or heteronuclear decoupling where the
duration or the power is 'excessive'. We trained our people to check the
temperature response of the probe first thing, when they start a new experiment.
That worked fine so far. But even if something goes wrong with that, this
is only a medium catastrophy, leading to an automatic warm-up cycle of
the cryo-system (and to half a day of down-time...). As far as I understood,
usually no damage on the system should to be expected.
) meanwhile we have two of these
cryoprobes, one for protein NMR and one for routine analysis. Both
of them are by no means uncritical in use. For routine use there are three
main problems. a) the Q factor of these probes is very high. Due
to this fact the influence of the solvent of the Q factor is much larger
than in the case of conventional probes. As a consequence the 90 degree
pulse lengths varies strongly. In the case of conventional probes you have
a variation of about 20% and in the case of cryoprobes the variations are
larger than 100% between CDCl3, Acetone and DMSO. This is valid for the
case of optimal tuning and matching. b) Tuning and matching in routine
work is a pain. Even slight changes of the sample detunes the probe. Until
now there is no ATM unit available for cryoprobes. This detuning additionally
modifies the 90 degree pulse length. c) Due to the high Q factor
the probes are much more sensitive against high power output especially
of the proton transmitter. Usually you get arcing starting from 8 W output
from the proton transmitter. The main problem for the protein NMR
is the high Q factor again. Water suppression is much more complicated
in comparison to conventional probes. And a last remark: you have
to use nitrogen as the flow gas for the probe. Oherwise forget a
high shim quality. That's comparative easy to achieve. B u
t : at no time the gas flow may stop!!! Your cryoprobe
is dead after ten minutes without gas flow! We ensure this using two ony
way valves one for nitrogen gas with a pressure of 1.6 bar and one for
pressurized air of 1.3 bar. If the nitrogen fails, pressurized air ensures
the life of the probe, but of course the measurement cannot be used.
Please excuse my bad english.
(Frank Koehn [KOEHNF@wyeth.com])
We have 2 cryoprbes in our facility which are used predominanty for small
molecule work. The first of them a 5mm triple resonance Bruker 500
MHz probe, we have had operational a little over 2 years. The sensitivity
aspect of these is a tremendous asset for limited mass samples and allows
one to solve problems using experiments unatainable with conventional
probes. However, they are not well suited for wide open access for a number
first is that they must be precisely tuned and matched to each particular
sample in order to fully realize the sensitivity gain and pulsewidth reproducibility,
even within the same solvent. In most open acces environments one sees
a wide variety of samples and users. Some samples are mass limited but
others are many mg and in a variety of solvents. Cryoprobes are more difficult
to tune than a conventional probe, and cannot be tuned in automation. (At
least not ones we have) This makes them unsuitable for open access
environments where multiple users , usually not spectroscopists and usually
in somewhat of a hurry, walkup and use the spectrometer. It is actually
possible to have too much sample, so users must be instructed to limit
the amount of material, otherwise one runs into receiver gain issues, etc.
Ironically, it is also difficult to work at low sample temperature.
there is quite a bit of additional delicate hardware, transfer lines, extra
cables, etc. around the magnet which makes it less convenient to
insert and retrieve samples. This might not be much of an issue in a spectroscopy
lab but in open access one must remember that any inconvenience, trip hazard
etc. is multiplied many times.
open access probes need to be robust and rugged. The cost ($200K plus extra
infrastructure) , difficulty of repair, and mechanical sensitivity of these
probes outweighs their impact in an open access environment. They have
high infrastructure requirements.
exception to this would be limited open access, say a natural products
or biological NMR group, where a small number of OA users usually with
mass limited samples, could be trained in the cryoprobes nuances and get
the full benefit of the hardware.
(rolf tschudin [firstname.lastname@example.org])
We have 3 of these. Used by small groups, 3 to 5 people each, after a little
training. With one person responsible for the system. They are fairly routine
systems most of the time.
(Peter Demou [email@example.com])
We have a cryoprobe on our DMX-500, but it is NOT used for open access.
The reason I caution against this is that the vacuum line and the helium
gas line are obstacles that come out the front of the magnet and should
not be bumped at any time. Sure everything is supported and the fittings
are tight, but there some unexplained messages that pop up in the log file
such as (pressure loss in system). This one indicated that it lost
some helium that it made up from the "bottle". You also have to expect
to warm it up every 6 - 8 or so weeks so it can purge itself. I hope
you know that the annual service contract on the cryo platform is $15400.
One might think that a time and materials deal would be better, but the
annual service that is required plus time and materials ( a new cold head
for the platform and adsorber unit for the helium compressor) come out
to the same, but your probe isn't covered. A warm up and cool down
cycle takes about 5 hours if there are no problems. The probe is
so heavy that it takes two people to remove and install it. Forget
about cleaning all the juice from broken samples easily. Sure you
can get a factor of 3 in S/N but you can end up with a warm up / cool down
cycle necessary at the wrong time. Yes, there is warning in the log
that indicates the impending need for this, and you can plan for it, but
it can happen by itself unexpectedly. Also, the helium compressor cannot
be on a UPS unit. The lockes rotor current is 110A! In reality
the circuit that runs it is three phase 60A @ 208VAC. The cryo platfrom
can be on a UPS with the console. So, you can end up in a situationwhere
the compressor can shutdown. The system then needs to eject the sample
to keep it from possibly freezing in the probe. If you get one it will
keep you on your toes.
(Industrial Lab: anonymous by request,
but someone who's opinion I respect!)
For a factor of four in signal to noise is it worth it to spend ten times
more money for the initial probe and an equal amount for a backup??? With
walkup use, how long before someone breaks a sample in the probe? Or how
long until a foreign object falls down into the probe? I'm not accusing
the everyday user of being careless, in fact quite the contrary. However,
in my experience of managing walkup operations, Murphy?s law applies perfectly.
If it can happen, it will.
Most of the results I have seen using cryoprobes exhibit nice s/n improvements
for organic samples where temperature is not important. Otherwise they
still seem to be having problems controlling sample temperature. This usually
becomes apparent in spectra of sucrose where the water lineshape is very
noticeable. Some of the better results I have seen involve 3mm samples
in a 5mm tube.
(Stephanie Mabry [firstname.lastname@example.org])
I should be able to answer your question this spring. We have a cryoprobe
for our department 500 MHz instrument, but installation has been delayed
(yet again). I am hoping we will be up and running by the end of
this instrument is open for "advanced" users, i.e. gradate students and
post-docs who are willing to take the extra time to train on the Bruker
software. These users need higher sensitivity and resolution or are
doing 2D experiments. Note that the majority of our NMR users are
happy with the 300 MHz QE/Tecmag instrument since they want a spectrum
in 15 minutes or less.
(George Sukenick [email@example.com])
OK we don't have one, but I have been thinking about it. You might think
that for organic synthesis such a probe would be overkill, but quite often
C13 (and other X, and 2D's) are required for small quantities. When the
next step of a synthesis is hanging on an NMR result ("as the sample decomposes
before my eyes..." :-) time is literally money and it is essential to get
people on and off the instrument as efficiently as possible.
the other consideration : $$ of the probe, and it needs to be sent in for
repair anytime someone breaks a tube (or too many dirty tubes are inserted...)
the people who use that probe should be well selected.
(Sudha Veeraraghavan [Sudha.Veeraraghavan@uth.tmc.edu])
Dear Rich, We have just bought a cryoprobe. Given that it costs $220k
and maintenance is likely to cost another 12k per year anyway, I would
only have it be used for biomolecular NMR applications and only by those
grad students and postdocs, of NMR labs, who get direct supervision &
training in NMR use.
Nunlist [firstname.lastname@example.org]) Hi
Rich, from what I gather, this pretty much requires a dedicated instrument.
Given that, if some qualified, properly trained users can help out the
others, it should work. Perhaps a sample changer would be useful?
Of course, the only sure way is to get one and find out yourself ;=)
(John Harwood [email@example.com] )
FWIW, here at UIC we are planning on the next year or two to try to acquire
a 5mm C/H cryoprobe to go in a new 500 MHz Bruker system. This
will be part of our departmental facility and will have the same open access
as our other machines. I may be incredibly naive, but I have not
seen or been told anything about croprobes (at least the Bruker setup)
that suggests to me they wouldn't be OK to use in an open facility.
What I like about the cryoprobe is that we can get this extra sensitivity
while still using standard 5mm samples that are consistent with our other