Summary posting about diffusion experiments using probe with 30 G cm-1
maximum gradients
Initial query:
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?
Responses/Suggestions:
The effective applied gradient strength can be increased by applying a
series of small gradient pulses- Latour ,JMR B, 101, 72 (1993)
Measure attenuation of a singlet so that J coupling is not a concern.
Choose a peak with a long T1.
Limit the sample size and center it in the coil.
Verify that attenuation is linear.
Use rectangular gradient pulses, not shaped ones, to maximize the
power.
If the T1s are long enough, the experiment should work.
Don't use delays longer than 500ms because of loss of signal due to
relaxation.
The attenuation is acceptable if the last data point is 1/e of the
first data point.
Pay attention to attenuation due to exchange.
The convection suppression sequence can display dependencies of the
slope on the delay (big delta), which is not a problem in the BPPLED and
BPPSTE sequences.
thanks everybody for your responses
Katherine Aubrecht
Assistant Professor
Department of Chemistry
College of the Holy Cross
kaubrech_at_holycross.edu
1 College Street
Worcester, MA 01610
508 793 3378
Received on Mon Dec 20 2004 - 11:54:36 MST