246 (327) BRUKER BIOSPIN User Manual Version 002
STMAS
which are indicated by the (red) filled circles. The open rectangles symbolize the
second pulse (one pulse at the end of each individual t
1
increment). They must al-
ways occur precisely on top of the rotational echo. For each increment the t
2
data
acquisition, which is not shown here, starts after the second pulse.
In the following discussion we will ignore this part completely. It showed up in the
original experiments but can be completely suppressed by a double quantum fil
-
ter. The contribution of the ST “rides” on top of the CT signal like spikelets. Since
MAS efficiently averages the 1st order quadrupole interaction of the ST, the corre
-
sponding MHz broad signal is now narrowed into a huge number of spinning side
bands. These coherency originating from the ST dephase rapidly and refocus into
rotational echoes with each rotor cycle. A pulse precisely on top of such a rota
-
tional echo can transfer the SQ coherency from the ST to SQ coherency of the
CT, the signal from which can then be acquired under standard MAS conditions.
The evolution in the indirect dimension is achieved in such a way that the delay
between the two pulses, which is the evolution period t
1
, is incremented by integer
multiples of the rotor period.
Two extremely important points must be considered for the experimental realiza-
tion of the STMAS experiment. Firstly, the spinning frequency must be kept abso-
lutely constant. The duration of the rotational echoes in the STMAS experiment is
determined by the width of the satellite transition, giving a length of e.g. 1 µs for a
satellite transition of 1 MHz width. If the rotor period varies from that specified in
the parameters, the calculated delay in the pulse program is incorrect and the
pulse misses the echo top, so less or no signal intensity is obtained.
Table 19.1.
summarizes the time deviation that occurs when the spinning frequency fluctuates
by ± 1 Hz and ± 10 Hz at various desired spinning frequencies. One can see that
when the t
1
increment accumulates to as much as 100 rotor periods it is possible
to miss an echo completely. For example, if the duration of the rotational echo is 1
µs it will be missed when the deviation is larger, which is the case for a 1 Hz devi
-
ation at 10 kHz, but requires a fluctuation of 10 Hz at 30 kHz spinning.
Typically the spinning frequency must be stable within ≤1 Hz throughout the entire
2D data acquisition. Secondly, the accuracy of the magic angle setting is extreme
-
ly important. The sidebands resulting from the first order broadening are narrowed
from the full first order line width by a factor of (3cos
2
θ
-1), hence for a deviation of
d
θ
from the magic angle the broadening is 3cos
θ
sin
θ
d
θ
, which close to the magic
angle is
√
2d
θ
. The magnitude of the interaction that must be narrowed in the pres-
Table 19.1. Time deviation of the rotor period for spinning frequency variations of
±
1 and
±
10 Hz for
various spinning frequencies.
Fluctuation of … Hz
@ desired spinning
frequency
Deviation from precise rotor
period after 1 rotor period
Deviation from precise rotor period
after 100 rotor periods
10 Hz @ 30 kHz 11 ns 1.1 µs
1 Hz @ 30 kHz 1.1 ns 110 ns
10 Hz @ 20 kHz 25 ns 2.5 µs
1 Hz @ 20 kHz 2.5 ns 250 ns
10 Hz @ 10 kHz 100 ns 10 µs
1 Hz @ 10 kHz 10 ns 1 µs
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