Bruker BioSpin Solid State NMR, User Manual

Page: 87 / 328

Basic Setup Procedures
User Manual Version 002 BRUKER BIOSPIN 87 (327)
Field Setting and Shift Calibration 4.9
Note: It is essential that, if spectra taken at different times and/or taken with differ-
ent probes need to be compared, the shift calibration is executed correctly. If the
magnetic field was not the same, the spectra will have different values of Spec-
trum Reference (
sr ).
To reduce the requirement of readjustment and to make referencing more reli-
able, the field should be set to be the same for all spectra.
To keep the field the same, 2 parameters must be taken into account:
1. The drift of the magnetic field
2. The difference in shims and field between different probes.
The drift of the magnet should be measured in the following way:
Insert a sample of D
2
O and run a proton spectrum just like it was done on ada-
mantane in
"Calibrating 1H Pulses on Adamantane" on page 65
.
Without changing any parameter, rerun the spectrum every 10 minutes for a full
day. This can be done with
popt, storing the data as a 2D experiment, or use the
pulse program zg in a 2D data set, with appropriate settings for d1 and td1 so that
the drift of the magnet can be followed by the changes in peak positions of the
D
2
O protons. The pattern of changes in the peak position will reveal whether the
changes are solely due to magnet drift, or whether there are additional disturbanc-
es to the magnet field. A magnet drift will always be constant towards lower fre-
quency (note.: A freshly charged magnet may also drift to higher frequency, but
this will change, so it makes no sense to account for this initial drift). A nonlinear
drift pattern indicates temperature changes, abrupt changes reveal magnetic dis-
turbances (elevator, cars or trucks trains, or the keys in your pocket- only if your
magnet is not shielded).
From these data, filter out the linear magnet drift. Determine the number of digits
the field value in the
bsmsdisp menu must be changed to set the field back to the
exact same value as in the first spectrum. Recalculate this number to a drift time
of exactly 1h/24h. Note these values as your magnet drift rate.
This drift rate will only reach a stable and constant value some time after charg-
ing, so the drift rate measurement should be repeated until the drift value is con-
stant. This may take some weeks to months (for high field magnets).
The magnet drift value will allow to calculate the time dependant component of the
field. The probe dependant component can be established by shimming every
probe and setting the field on the same sample, following the same procedure for
every probe after shimming. When then the shim file is written, the current (cali-
brated) field position is written to disk with the shim file.
Executing the AU program (or command, TopSpin version 3.0 and later) probe
field will now set the field to the appropriate value according to drift and time (tak-
en from the date of the shim file, so the computer clock should be correct) and
probe shims (from the current probe setting, so the appropriate probe must be se-
lected in
edhead ). However: this will only work if all shim files contain the precisely
determined field value for the same reference compound.