Re: Disappearing peaks with presat

Deane D McIntyre (dmcintyr@ucalgary.ca)
Thu, 7 Jan 99 21:20:29 -0700

In message <36952D64.4231B55D@tufts.edu> camass writes:
>
> Dear NMR Managers,
> One of the faculty observed the following. He
> has a reference sample in CDCl3 sealed in a
> capillary and immersed in H2O. He is attempting
> to optimize the presat.au program on a Bruker
> Am300. As he raises the power from 30L to 10 to
> 1L to get supression of the water the reference
> sample loses a signal at 15.5 ppm but not at
> signals closer to the water peak.

Charles:

I presume that the signal at 15.5 is a NH in something like
DNA (imino protons) or a enol CH which is exchangable with water.
Thus as the power for presat is increased the intensity of the peak will be
reduced and eventually lost due to saturation transfer. Keep
the presat power as low as possible. On an AM it should be possible
to use 20L or so to presat the water; or use a 1-1 or jump-and-return
sequence. I realize that most of the fancy water supression techniques
cannot be done on a AM (gradients etc).

Cheers, Deane D McIntyre
NMR Mananger
Dept Biological Sciences
University of Calgary

dmcintyr@ucalgary.ca