The problem you described is probably manifestation of probe
ringdown. The magnetoacoustic resonance set up in the receiver coil decays
more slowly as the NMR signal frequency decreases, so for low gamma nuclei,
the ringdown signal masks the first few data points of the FID. This
phenomenon also becomes more pronounced at low temperature, where the coil
is less resistive.
There are several solutions which you might try. First, if you can
sacrifice some sensitivity and accept somewhat longer pulse lengths, try
detuning the probe slightly away from the exact resonance frequency.
Second, you can make the probe coil more resistive by adding some
resistance in series with the receiver coil. For example, you could use a
narrower guage wire for connecting the tuning capacitor to one end of the
coil. Further, if the ringdown problem is not severe and if the FID is
"well behaved", you could try using backward linear prediction to
reconstruct the first few data points of the FID, replacing the points
which were corrupted by ringdown. Finally, you could use a quadrupole echo
sequence (e.g. 90X -tau-90Y) to generate an echo whose peak comes long
after the ringdown from the second pulse. The digitization of the echo
begins several data points before the echo peak, and the data is
left-shifted to the peak and Fourier transformed to give an undistorted
spectrum, assuming a suitable distribution of crystallite T2's and a
suitable choice of tau.
Best regards,
Ken Fishbein
Facility Manager, NMR Unit
NIH/NIA/GRC
Ph. (410) 558-8512
FAX (410) 558-8173
E-mail: fishbein@vax.grc.nia.nih.gov
Address: Ken Fishbein
NIH/NIA/GRC
4940 Eastern Avenue
Baltimore, MD 21224 USA