I thank my fellow scientists for offering their expert advice on my
question (July 8, 2011) of how to measure diffusion constant of water
(90%H2O/10%D2O); below is my summary of the replies that I received.
The majority of the advice points to reducing the sample volume:
-Could you use a narrow/capillary NMR tube containing the H20/D20 (or a
narrow co-axial tube within a standard tube) to reduce the absolute sample
volume and hence signal strength?
-I recommend that you purchase 5mm/3mm or 5mm/2.5mm NMR tubes. I purchase
these from NewEra Enterprises as NE-H5/3 or NE-H5/2.5. By greatly reducing
the filling factor, and keeping the sample further from the coil, the effect
is greatly reduced. We have had good luck measuring 100% H2O diffusion (no
lock at all) using the 2.5mm probes using an indirect probe at 500MHz
without any problem from radiation damping. Using these reduced-diameter
micro-tubes also greatly minimizes temperature gradients over the sample;
thereby, minimizing problems from convection currents.
http://www.newera-spectro.com
-RD has to be minimized. This can be done by using a very small amount of
water, which can be achieved through a micro-tube insert. The added
advantage is that convection due to temperature gradient should be reduced
too.
-When I work with water, to get around radiation damping I fill a 20 uL
capillary and center it in a 5 mm NMR tube.
-Try a smaller OD tube like 4mm or even 3mm. A few tubes of either size
costs next to nothing, but most people do not consider them for these
special experiments due to the shortage of a suitable spinner turbine. If
you were to note the correct normal sample depth, note the contact points
on a 5mm tube, and add some Teflon tape to a 4mm tube at the same points,
you can continue to use your 5mm spinner turbine and a smaller OD tube.
Once you have found the optimum and the experiments become routine, then
you source a spinner for sample size X. (Wilmad have a good selection.)
Others pointed out the detuning is not effective in overcoming radiation
damping, and to use high gradient strength and eddy current corrected
pulse programs
-Detuning is not very effective, and attenuating will not help at all with
radiation damping. There are two simple things that work: (1) use a
capillary sample, and/or (2) use a STE pulse sequence with a low first
pulse flip angle (see Anal. Bioanal. Chem. 378, 1568-1573 (2004)).(2) is
available as standard with the Varian Doneshot pulse sequence, just
change the paraneter "startflip" from 90 to 5.
-use stronger gradient and one of the bipolar eddy current corrected pulse
programs
-Run your gradient series from a relatively high gradient strength
starting point, where the signal is already quite a bit attenuated because
of the relatively fast diffusion. You can throw away 95% of your signal up
front, attenuate it across a big enough range to get an accurate
measurement, and still have high enough S/N at the weakest point. At 25 C,
it should be something around 1.8 e-5 in cgs units.
-Just a quick shot: Use a (very) short pulse for excitation. Just go
shorter until the radiation dumping disappears. Be aware that the 180
need to be calibrated ones (in order to get the needed echos in the
sequence). When you try a short excitation pulse, better do NOT
detune the probe! This will inherently lead to longer pulses. For the
very short excitation pulse (could be as short as 1 us or even shorter)
this doesn't seem to matter, but since you need to use good calibrated
180s, these would become quite long. In order to minimize effects of
B1-inhomogeneity and keep a good excitation profile you should go for
the shortest 180 possible.
For the use with a cryoprobe, one user recommends the use of high attenuation:
-When we measure the T1 of water, we put 60 dB attenuator in the receiver
path. The gradient pulses in the diffusion experiment should be able to
minimize radiation damping.
Thank you again, and I will try these out. Please contact me if you would
like more information.
Kellie Hom
University of Maryland, School of Pharmacy
NMR facility
1-410-706-3118
Received on Tue Jul 19 2011 - 09:52:58 MST