Hello fellow Bruker Users.
After looking at the archives, I realized that I should have joined a long
time ago, since I got my first WM250 in 1978. Since 1995, I have been using
a MSL300 built in 1989, and since repaired with various parts from used
consoles. It still works amazingly well, including the ASPECT 3000, and I
mostly do C-13 solids on soils (yes, that's dirt), decomposing plants and
charcoal from forest fires - I'm in a forestry lab.
I would love to get some information/advice on doing dipolar dephasing at
moderately high speeds; i.e., around 12,000 Hz MAS at 75 MHz for C-13.
Around 5000 Hz, no problems with DD or DD+TOSS, but at 12,000 for charred
samples, the intensities vary quite erratically with dephasing time. I
havn't found anything in the soils literature - most work is done at lower
fields and MAS. I are very interested in knowing (at least qualitatively)
how condensed the aromatic C is. I was wondering if I should try the
Harbison variant, which has a 180 refocusing pulse for C, and takes at least
two rotor periods. (G.S. Harbison et al., 1985. High-resolution carbon-13
NMR of retinal derivatives in the solid state. JACS 107:4809-4816). This
sequence is applied to soil organic matter samples by Smernik and Oades
(Eur. J. Soil Sci. 52:103-120, 2001) for 5,000 Hz MAS at 50 MHz for C-13.
There is also an Alemany sequence with refocusing pulses on both channels,
but Smernik and Oades claim that the Harbison one is better. If I could
really understand what is going on, I could even try some more quantitative
approaches.
So, would one of these variants help, or this there something else I should
know about at higher MAS?
Second question; I'm pretty much on my own here, and there aren't too many
people still using DISMSL on the ASPECT 3000! I'm a little uncertain how to
modify the Bruker NQS program with the 180 C-13 refocusing pulse. I have
played with it a bit, but really, I'm a bit out of my depth. Are there any
Harbison pulse program users out there, or anyone who still knows how to
write these DISMSL programs? I did check the various pulse program libraries
you get to from the Bruker site, but they are mainly for much more
complicated things. I'm still stuck in the first dimension, and plan to stay
there.
(Since I don't have RAMP, for high speeds I've started setting the CP on the
-1 sideband of glycine at the same speed, and resetting/checking after every
sample. A good test of amplifier stability!)
regards, Caroline
Caroline M. Preston
Research Scientist/Chercheuse scientifique
Canadian Forest Service/Service canadien des forets
Natural Resources/Ressources naturelles Canada
Pacific Forestry Centre/Centre forestier du Pacifique
506 West Burnside Road/506 chemin Burnside ouest
Victoria BC/Victoria (C.-B)
Canada V8Z 1M5
250-363-0720 fax/téléc. 250-363-6005
cpreston@pfc.cfs.nrcan.gc.ca
nihil umquam facile est
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