Thank you to all those who gave advice on the suppression of boron from the
boron nitride in the MAS stator block. Suggestions included:
(1) Introduce a long enough preacquisition delay to remove the
extraneous signal
(2) Use the DEPTH sequence
(3) Use a Hahn echo sequence timed to the rotor period
(4) Just use a Hahn echo
Well, yesterday I managed to squeeze a day for myself on the MSL 500
instrument in between my graduate students and was able to forget about
tedious things like admin and teaching!
Here is what I found.
The DEPTH sequence (90 - 180 - 180) comes out tops with excellent
suppression of the stator block signal and decent signal to noise provided
that the 90 and 180 are accurately calibrated (to within 0.2 microsec)
(note that the 180 is not necessarily exactly twice the 90). I got an
excellent 11B spectrum on an alumino boro silicate with only 0.5% boron
present in under one hour with virtually no background and a marginally
acceptable spectrum overnight on a similar sample with only .01% boron, but
here the unsuppressed background was the dominant feature.
As expected introducing longish preacquisition delays leads to phasing
problems and so I rejected that idea.
The Hahn echo is a nice idea in principal but I couldn't readily get it to
work in practice. It probably needs some adjustment of the echo interval
to optimise it and I wanted to press ahead.
In three weeks time I will attempt the same set of experiments in a a probe
made by DOTY which has a stator block containing no boron and so can
compare the results. This will let me check the possibility of lineshape
distortions etc introduced by DEPTH.
Thanks to all for your help particularly Anthony Bielecki whose pulse
program worked first time after I had modified it for our instrument and
had introduced 1H decoupling during acquisition.
And the sun is shining in St Andrews - again!
Frank Riddell
Frank Riddell, School of Chemistry, University of St Andrews,
St Andrews, Fife, KY16 9ST, Scotland, UK.
Tel: + 44 (0) 1334 - 463815 Fax: + 44 (0) 1334 - 463808
E-mail: fgr@st-andrews.ac.uk
WWW: http://www.st-and.ac.uk/~www_sc/personal/fgr