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User Manual Version 002 BRUKER BIOSPIN 231 (327)
18
MQ-MAS: Sensitivity
Enhancement
18
The MQMAS experiment on half integer quadrupole nuclei is an extremely insen-
sitive experiment. This is due to the low efficiencies of both the excitation of 3Q
coherence and their conversion to observable magnetization. Several approaches
have been taken to enhance the efficiency of the excitation and conversion, main-
ly focusing on the conversion, as this is the least efficient step. Adiabatic pulses
can be used for the conversion instead of a single high power CW pulse, and al-
ternative phase cycling schemes have also yielded improvements. Improving the
efficiency of the MQ excitation pulse has been tried but no generally applicable
scheme exists so far. Before describing the optimization procedures, some exper-
imental approaches used in combination with these enhancement techniques are
introduced.
Split-t
1
Experiments and Shifted Echo Acquisition 18.1
The excitation pulse in the MQMAS experiment creates 3Q coherence that can be
refocused into an observable SQ echo by the conversion pulse. As the t
1
period is
incremented in successive slices of the 2D experiment this echo position relative
to the conversion pulse changes as a function of the duration of the actual t
1
de-
lay. If this observable (SQ) magnetization can be refocused again, by a central
transition selective 180° pulse, a shifted echo acquisition can be implemented.
The delay between the conversion pulse and the 180° pulse or the delay prior to
the start of the acquisition must be incremented proportional to t
1
. This results in a
split-t
1
experiment where the top of the shifted echo appears at a constant posi-
tion after the final pulse throughout the entire 2D experiment. The position of the
echo top depends on the spin
I of the observed nucleus. If the delay before the
selective 180° pulse is long enough, the signal will have decayed and the full build
up and decay of the echo can be recorded. By this method a phase modulated
data set is acquired with a full echo that contains twice the intensity of the simple
MQMAS experiment (if transverse relaxation can be neglected). At the end of the
t
1
period of the split-t
1
experiment, there is no net evolution under the second or-
der quadrupole broadening. This is the case because the evolution of the MQ co-
herence in the first part of t
1
is cancelled out by the evolution of the SQ coherence
in the part of t
1
after the conversion pulse (the lengths of the two periods are relat-
ed by the ratio of the second order broadening of the MQ and SQ coherence).
The resulting 2D spectrum thus requires no shearing transformation to make f2
the isotropic dimension.
Whether this experimental trick is useful for your sample of interest can easily be
determined by running a simple Hahn-echo experiment, where the delay
τ in the
90°- τ -180° sequence is adjusted so that the FID of the signal is decayed before
the 180° pulse is applied. This is shown in
Figure 18.1.
. The FID generated by the
initial 90° pulse is not sampled, but after it is decayed it is refocused with a 180°