Bruker BioSpin Solid State NMR, User Manual

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206 (327) BRUKER BIOSPIN User Manual Version 002
Relaxation Measurements
Once the parameters are set, process the data with
xf2
, to execute a Fourier
transform in the f2 dimension only. The phase can be adjusted from within the re-
laxation analysis tool, but baseline correction should be carried out with
abs2.
Start the relaxation analysis guide with the command
t1guide
. The sequence of
icons guides you through the analysis as follows:
Extract slice : The first spectrum row should be selected for phase correction, as
this contains maximum signal. The spectrum should then be phased to give posi-
tive peaks.
Define ranges : Here you must define integral regions containing the peaks of in-
terest. The fitting routine can either use the integral of the signal, or the intensity,
in which case the maximum signal in each integral region is used. Regions can be
defined via the cursor, or between specified limits via a dialogue box. The integral
regions need to be saved to a special file, by clicking the disk icon towards the left
of the integral window (not the standard save integrals button on the right), and
selecting ‘export regions to relaxation module and.ret’.
Relaxation window : Here the intensity or area values from the first integral re-
gion are displayed. The icons at the top of this window allow you to move between
the integral regions, exclude points from the calculation, display the data on a va-
riety of axes, and start the fit for the displayed region or all regions.
Figure 16.1.
shows the decay of the a-carbon signal of glycine as a function of relaxation de-
lay, along with the fit and calculated relaxation parameters. Note that any peaks
with integrals or intensities too close to zero will be omitted from the analysis by
the software – if you see less points in the relaxation window than were actually
recorded, this may be because they have insufficient intensity.
Fitting function : Here the parameters of the fitting calculation are set. The gener-
al parameters should be determined automatically, but ensure that the limits for
baseline correction are set to cover the whole spectrum. The fitting function de-
pends on the experiment, but in this case the signals decay exponentially, so the
function ‘expdec’ should be chosen. The list filename should be ‘vd’ – this will take
the specified
vdlist
from the data set. Note that when the experiment is run, the
selected
vdlist
is written into the acquisition data directory as the file “vdlist”, so it
is always available, even if the source list is edited. The fitting program can calcu-
late multi exponential fits, but data with very good signal-to-noise is required for
this to be accurate. Unless there is obvious overlap of peaks, the assumption is
usually that each peak corresponds to a single nuclear site, and thus a single T
1
value.