Thanks to all for the suggestions. The possible causes provided included
dirt, alcohol in the probe, exchangeable protons, inherent defects in probe
design or materials, and water in the VT air.
We are inclined to believe the hump at 3.5 ppm is due to water vapor in the
compressed air. Further work showed that the signal was present even
without a sample tube in the probe. We tried switching the two
spectrometers (Varian 200 and Bruker 500, located on different floors, and
on different compressed air systems) from the house compressed air to
nitrogen gas. The hump disappeared on the Varian spectrometer and shifted
to 4.5 ppm on the Bruker, which effectively solves our problem. I already
knew the Varian spectrometer had wet air, but the second spectrometer was a
surprise.
Stephanie
-----Original Message-----
> From: Stephanie Mabry [mailto:mabry_at_umbc.edu]
> Sent: Thursday, October 04, 2001 11:21 AM
> To: AMMRL
> Subject: contaminant at 3.5 ppm
Hello.
A student here is measuring proton spectrum of very dilute samples in D2O.
He needs to be able to accurately integrate a small peak at around 3.5 ppm,
but there is a broad peak or hump located there.
We are unsure of the cause of this hump. We have tried looking at different
solvents, different samples, different sample tubes, and different
spectrometers, and the peak is visible in both D2O and CDCl3 spectra. It
doesn't appear to be present in acetone-d6 or DMSO spectra. The hump is
independent of spectrometer frequency, sweep width, and spinning.
I would appreciate any ideas of what this contaminant is, and (more
importantly) how to remove it.
Thanks!
Stephanie Mabry
--------------------------------------------
Stephanie Mabry, Ph.D., Instrument Manager
University of Maryland, Baltimore County
Dept. of Chemistry and Biochemistry
1000 Hilltop Circle
Baltimore, MD 21250
office (410)455-1031
fax (410)455-2608
mabry_at_umbc.edu
Received on Mon Oct 08 2001 - 16:37:57 MST