244 (327) BRUKER BIOSPIN User Manual Version 002
MQ-MAS: Sensitivity Enhancement
FNMODE: Even though this parameter is not evaluated by the pulse program it
will be used by the processing AU program xfshear.
D4: A very short delay is used here, just to allow for amplitude and phase switch-
ing.
l4: This loop counter is internally used for checking if the echo or anti-echo is cur-
rently being acquired.
l5: In the acquisition of echo-anti-echo 2D spectra signals from the echo and anti-
echo pathways are stored into consecutive FID’s in the serial file. In MQMAS ex
-
periments these echos and anti-echos behave differently. For t
1
= 0 both signals
have their echo top immediately after the selective 90° pulse. As t
1
is incremented
the top of the echo appears at later point in time whereas the top of the anti-echo
appears at an earlier point in time. It means that the contribution of the anti-echo
becomes less and less until finally the signal fades out completely and only noise
is sampled. It can be advantageous to terminate the acquisition of this noise in or
-
der to increase the overall S/N and save spectrometer time. However, in the pro-
cessing of echo-anti-echo data two consecutive FID’s are linearly combined in the
following way:
(Eq. 18.2)
Where re and im refer to the real and imaginary points of FID’s 1 and 2. Hence,
acquiring a smaller number of anti-echos than echos leads to the usual truncation
effects (wiggles in the spectrum). Furthermore, since both signals contribute to
the phase information care must be taken that the pure absorption line shape of
the 2D peaks is not obscured. Therefore, in case of doubt it is probably the best
idea to set L5 = TD{F1}/2. If less anti-echos are to be accumulated the question is
how many anti-echos to acquire - this depends on the sample. In amorphous or
disordered materials the FID decays rapidly and so does the anti-echo. In such a
case 4 to 8 anti-echos may be sufficient. In the case of crystalline materials it
takes many more t
1
increments before the anti-echos decay. Hence, the number
of anti-echos should be of the order of half the number of echos. It is always better
to acquire more anti-echos than are really needed, because then you can be sure
that you acquire a 2D spectrum with a reliable 2D absorption line shape. Never
risk gaining sensitivity or saving experimental time at the expense of quality of
lineshapes.
ll6: the value of this loop counter is needed to set the phases of the soft pulses
correctly and define what is an echo and what is an anti-echo (which are different
for spin I = 3/2 and all the other spin quantum numbers).
Processing of these spectra is done in analogy to spectra obtained with
mp3qzqf.av. However, phase correction in the acquisition dimension F2 cannot be
determined on the first FID. Therefore, xf2 must be applied first and then F2
phase correction can be determined on either the first slice, in case of nuclei with
spin I = 3/2, or the second slice for all other nuclei. 2D processing is then done
with the AU program xfshear. Alternatively xfshear can be used first, with a sub
-
sequent 2D interactive phase correction.
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