Dear all,
There are few months (I apologize for the delay) I asked to you this question:
« I look for a stable compound at high pH values (pH 13 - 14) for
concentration standard 31P NMR spectroscopy. Indeed, I tried the
Me3PO4. At acid pH this compound is actually stable, but at very
basic pH the degradation is very fast. »
Many thanks for the response of :
- Peter Lundberg (« I would like to recommend a (concentric)
reference capillary with any suitable compound, concentration and pH.
This capillary can be standardized against other concentration
references if required. I guess that it would work best with larger
tubes like 10 mm or so. »),
- Roger M. Sheets (« I have used Triphenyl phosphine sulfide for most
of the internals standards. Reference this against trimethyl
phosphite to get an accurate shift. I do not know that that high of a
pH if this will work, but I have found this compound to be very
stable. »),
- Robert J. Webb (« I have used PF6- anion in the past. »)
- and Robert A. Kleps (« Depending on the particular sample, you
could use Methylene diphosphonic acid H2O3PCH2PO3H2.It is very
soluble and stable and gives one signal at about 15 ppm downfield of
85% H3PO4. For biological samples we has put it into a sealed short
capillary tube, quantitate the amount against a PO4 solution and run
the sample with the capillary. »)
I did not test all these possibilities, and I retained the idea of
Robert J. Webb (just for reasons of facilities related to my problem).
Indeed, NaPF6 (be careful : hygroscopic!) is stable in water at pH
13-14, and for at least 3 days. For a solution of 0.2 M in water and
at pH 13-14 the signal is at -145.0 ppm. The only one problem is the
coupling with fluor that gives a massif of nearly 26.3 ppm, and that
the (99.9 %) commercial sample is not completely pure (there are some
extra picks near 5.1, 3.7 and -1.7 ppm, but may be it depend of the
vendor!).
(chemical shifts are given from classical H3PO4 reference).
Therefore, thanks to you I succeeded to do it!
Thanks again.
Eric.
*************
Dr Eric Condamine
Institut Fédératif de Recherches Multidisciplinaires sur les Peptides
Université de Rouen
UMR CNRS 6014
IRCOF - ECOBS - RMN
Rue Tesnière
76821 Mont-Saint-Aignan
Eric.Condamine_at_univ-rouen.fr
Tel : (33 2) / (02) 35 52 29 39
Fax : (33 2) / (02) 35 52 24 41
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Received on Tue May 06 2003 - 15:14:03 MST