Guillermo,
This may be a problem we ran into a couple years ago on our AVANCE, one that shows up only with decoupling protonated solvents during rare-nucleus acquisitions.
On our AVANCE--and two other AVANCE systems I could test--the "asynchronous" decoupling is not truly asynchronous; Bruker fakes asynchronous mode by having the decoupling sequence start at the same spot it ended at at the end of the previous acquisition. This does vary what part of the waltz sequence is in effect at the beginning of each acquisition, but the start of each decoupler pulse is still synchronous with the rest of the sequence. Apparently Bruker decided to not provide a separate clock to enable true asynchronous decoupling.
Because of the partially synchronous nature of the decoupling, significant (~<1%) decoupler sideband noise is observed: with protonated solvents and 13C acquisitions, these sidebands are easily the size of solute peaks. I have observed this problem on three different AVANCE systems. On our system, use of a separate clock with a separate "true" asynchronous decoupler eliminates the sideband noise (reduced into the stochastic noise).
There is a software work-around, however, that works fine for simple 13C acquisition, and I list the sequence below (thanks to Tony Bielecki for it). I know of no software work-around, however, for DEPT or other sequences that require fixed intervals between hard pulses and the start of the decoupler.
Charlie
At 08:18 AM 5/7/2004, Guillermo Moyna wrote:
>Hey gang,
>
>I am wondering if someone has encountered this problem before and got a solution for it. We are taking 13C spectra of carbohydrates disolved in non-deuterated solvents. The solvent peaks are not in the range of the sugar peaks, but since the ratio of sugar peaks vs. solvent peaks is 1:100, the artifacts (spikes) originating from the solvent peaks start showing at regular intervals in the sugar region after extended accumulation. They are small enough not to be a major problem, and we can identify them because they are always there no matter which sugar you are analyzing, but they don't look all that pretty.
>
>Is there any post-processing trick to remove these things? The data is being collected on a Bruker AVANCE 400, but due to the chemical shift range of the solvent we cannot use DQD. My solution so far is to record a spectrum of the neat solvent with the same number of scans and substract this from the spectra of the sugars. This works, but I'd like to avoid doing it if there's an alternative solution.
>
>TIA,
>
>Guillermo
; zgasync
; pulse program for CPD decoupling with a random
; start-up interval, which might give the same
; result as true asynchronous decoupling.
; Written by Tony Bielecki, May 25, 2000.
;
; Set D5 equal to the cycle time of the CPD,
; for instance, D5=96*PCPD2 for WALTZ-16.
;
; Set V9 to 50, which will provide
; random delays from 0.5*D5 to 1.5*D5.
;
; modified cgfry 000601 - allow for normal NOE delay
; with decoupler on
;
; d4=0.01 seems to work fine
ze ; clear digitizer buffer
1 d4 do:f2 pl12:f2 ; recycle delay, decoupler off
3u cpdngs2:f2 ; start CPD timing, decoupler off
d5:r ; random interval
3u cw:f2 ; start CPD
d1 ; allow for NOE
(p1 ph1):f1 ; pulse
go=1 ph31 ; digitize
1u do:f2 ; decoupler off after last scan
wr #0 ; save data
exit ; done
ph1= 0 1 2 3
ph31= 0 1 2 3
; end of pulse program zgasync
~~~~~~~~~~~~~~~~~~~~~~~
Charles G. Fry, Ph.D. Tel: (608)262-3182
Director, MR Facility Fax: (608)262-0381
Chem. Dept., 1101 University Ave, Univ. Wisconsin-Madison
Madison, WI 53706 USA email: fry_at_chem.wisc.edu
Received on Mon May 10 2004 - 17:09:03 MST