Spurious Peaks in 13C
Jim Van Epp (James_van_Epp_Jr@brown.edu)
Thu, 02 Dec 1999 12:21:19 -0500
It sounds like spurious peaks have plagued many out there. I have a couple of
experiences that are more obvious, although one of them I never was able to
trace to the source. We had a 400 and 250 in the same room in close proximity.
The lock frequency of the 400 was close to the 13C observe of the 250 so that
a spike ("birdie") would appear in a 13C spectrum taken on the 250. This was
easily tested as lowering the lock power on the 400 removed the spike. We also
had a spike in 13C spectra done on the 400 which was at ~110 ppm and usually
showed in D2O as a solvent. This spike would disappear if the sample was run
unlocked (lock power minimized and cable disconnected). Also, if O1 (transmitter
offset) was changed by, say, 100Hz, the spike would disappear. It seemed that it
was a result of interaction of the lock and observe frequencies, and that
changing O1 would move or remove the spike from the sweep width. Your
description
of positive and negative peaks seems more than a spike. I would try running the
13C undecoupled and then try running unlocked with 2H power minimized and
the cable disconnected from the probe. Maybe the 1H stop in your probe needs
tuning
or you could get rid of this with different or better bandpass/bandstop filters
in your observe channel. Certainly, external RF sources, as has been widely
documented, could also be your problem. Possibly running spectra within
different time periods could help to trace this. Good luck, and please don't
send any spikes this way!
P.S.: These were AM400 and WM250 consoles.
Jim
James E. Van Epp, Jr.,Ph.D.
Department of Chemistry
Brown University
324 Brook St.
Providence, R.I. 02912
401-863-3069
401-863-2594 FAX
e-mail: James_van_Epp_Jr@brown.edu