Reducing Spinning Side Bands
Probes designed for high proton sensitivity tend to show spinning side bands which
are not due to magnetic field inhomogeneity, but rather due to the spinning sample
modulating the tuning of the probe circuits. This means that, no matter how carefully
the magnet's off-axis shims (meaning shims other then Z1-Z5 on our systems) are adjusted,
spinning side bands are still present. Additionally, radiation damping broadens the
real signal more than the side bands, making matters worse as the sample
concentration increases.
While there is no easy fix for this, the side bands can be reduced by using a high
spin rate, and varying the spin rate during the acquisition and taking many scans.
This averages the side bands to much smaller intensity.
On the AM and AMX spectrometers, the 'ro' command can be used to set the spin rate and
activate/disable the variable rate feature. Using a variable spin rate of 27 on the AM-500
has worked well for case where small peaks from in isomer were initially swamped
by spinning side bands.
If you are going to use variable rate, please remember to use the 'ro' command again
to restore the regular spin mode, as most users expect the spinning rate to be stable.
NMR Facility Home Page
the College of Chemistry Home Page.
Questions, comments to: Rudi Nunlist
rnunlist@bloch.cchem.berkeley.edu
Last Update: 1/19/96. RN