I have experience running these experiments on a Bruker system. In
general, limit the size of your sample with Shigemi tube or some
equivalent means, and be certain it is located in the center of the coil.
Increase the duration and amplitude of the gradient.
Verify that your are getting a linear attenuation of the signal that
reflects the variation in gradient strength. You can do this with 2
experiments, one with a 95 % gradient and one with a 5% gradient being
applied. Signal attenuation should be approximately 20:1
You can get maximum gradient strength by using rectangular gradients
instead of Gaussian or sine shapes. Run the experiment at higher
temperatures, convection can become more of a problem though. You might
have to edit you calculation algorithm to reflect the change in the
gradient, for example a sin shape is approx 0.621 of the area as a
rectangle of the same duration and amplitude.
With the stated expected diffusion coefficients, I would expect to be able
to measure them on a 5mm inverse configuration probe with 30 Gauss/cm.
You might want to measure your actual gradient strength and recovery time
before going to further and check the DC of doped water
Hope this is helpful
Steve Huhn
Novartis Institute for Biomedical Research
Cambridge MA
"Katherine Aubrecht" <KAUBRECH_at_holycross.edu>
12/13/2004 04:19 PM
To: <ammrl_at_chemnmr.colorado.edu>
cc: (bcc: Stephen Huhn/PH/Novartis)
Subject: gradient strengths for DOSY experiments
We have a Varian 400 MHZ Inova instrument with Performa 1 Z axis
gradients- the highest gradient strength is 30 Gauss cm-1. I am interested
in doing PGSE experiments on aqueous solutions of polymeric micelles to
measure their diffusion coefficients. The diffusion coefficients are small
* on the order of 10-9 to 10-12 m2 s-1. I would plan to use the
Dbppste-convection controlled pulse sequence. In order to get significant
attenuation in the signal, I would need to use long delays during the
sequence- 0.5 to 1.5 seconds.
At this point, we can not afford an upgrade to higher strength gradients.
Are there any opinions on the validity of the experiment I am proposing to
do?
thanks
Katherine Aubrecht
Assistant Professor
Department of Chemistry
College of the Holy Cross
kaubrech_at_holycross.edu
508 793 3378
Received on Tue Dec 14 2004 - 13:07:48 MST