Thanks for all responses to my questions about diffusion NMR. Here is a
brief summary of comments, hope other people in our community will get
benefit also:
1. How do I know the coil of the probe is in the center of the field?
-- The sample is at the center of the field if the Z gradient shim does not
cause a signal to change frequency.
-- The standard method for doing this would be
to put a small bulb of water in the probe and slowly move the probe up and
down in the magnet; finding the center of a linear area of position vs
frequency.
2. How to adjust the depth of the sample tube so the center of the sample
> will match the center of the coil?
--I used the center of the gradient coil to judge the depth of the sample.
You can do a z projection across the sample when it is inserted to various
depths and from that you can find out where the center of the gradient
coil is located. From that I then judged the linear region of the gradient
and placed the sample depth accordingly. Then I measured the distance from
the bottom of the tube to the bottom of the part at the top of the Doty
probe which screws in and holds the tube tight. Note that every time I
change the probe I unscrew this part and insert the tube, and measure its
depth with a ruler. This works fine for me.
--Use a very small microsample. The frequency of the sample peak will
change a minimum
amount when the polarity of the gradient feild is reversed.Place the sample
in what
your best guess the middle of the coil is. Then take a FID, tranform it
and measure
the frequency position of the peak. Reverse the polarity or the direction
of the
current in the gradient and measure the peak position again. when the
sample is in
the middle of the gradient coils.
--To check on the gradient coil/sample/main magnet coil alignment, just use
the z-gradient to "image" the detection coil and adjust the position
accordingly. Some of the gradient shimming and micro-imaging capabilities
of the more recent Varian software should be helpful for this also.
--A way to do this experimentally is to use the profile sequence to
generate a profile of your sample. The sample is in the center of the
gradient - if the profile is symmetric about the resonance position.
This should also be the center of the coil.
The sample volumn should be 5mm to 15mm, and the sample for shim ( without
lock) can be H2O/D2O, methanol, Benzine and so on.
Thanks again!
Qing Ning
----------
Qing Ning, Ph.D
NMR Specialist Analytical Service Lab
q-ning@nwu.edu Department of Chemistry
tel. 847-491-7080 Northwestern University
Fax 847-491-7713 2145 Sheridan Road
Evanston, IL 60208