Bruker BioSpin Solid State NMR, User Manual

Page: 250 / 328

250 (327) BRUKER BIOSPIN User Manual Version 002
STMAS
As for MQMAS the setup must be done in two steps; in the first step a central tran-
sition selective pulse that merely excites the central transition must be calibrated.
This pulse must be weak enough so that only the central transition is affected and
it must be short enough so that the central transitions of all sites in the spectral
range are excited. These conditions are typically fulfilled by a 20 µs pulse. For the
calibration of this pulse a power level around 30 dB with 500 W and 1 kW amplifi-
ers and around 20 dB with 300 W amplifiers should be expected. The pulse pro-
gram
zg (which uses
p1
and
pl1
) or zgsel.av (which uses
P3
and
PL21
) can be
used. For more details please refer to chapter 16.
Once the central transition selective 90° pulse is calibrated the STMAS pulse pro-
gram can be loaded. Available pulse programs are
stmasdqfz.av and stmas-
dqfe.av . Both are double quantum filtered 4-pulse sequences, the first with a z-
filter, the second with a shifted echo. If this second sequence is to be used a prop-
er setting of the timing for the shifted echo is required, to allow collection of the full
echo signal. This is explained in chapter 17, where a shifted echo can be used in
DFS enhanced MQMAS experiments.
In
Table 19.3.
and
Table 19.4.
the starting parameters for the setup of the two se-
quences are given. Typical values for the pulses are entered so one should see
some signal for further optimization. Parameters like
O1
,
TD
,
SWH
,
RG
, should al-
ready be set in the standard 1D spectrum. Since the experiment is not as depen-
dent on the pulse lengths or the applied RF field amplitude as MQMAS, pulse
lengths between 1 and 2 µs, which can be achieved with every probe with 4mm or
smaller rotor diameter, are sufficient.
For
PL11
an initial value that corresponds to 150 to 300 W can be used. Optimiza-
tion will be done on the first increment of the 2D sequence, which is calculated
within the pulse program according to „D0=(1s*
L0
/
CNST31
)-
P1
/2-
P4
-0.3µ-
P2
/2“,
Table 19.3. Initial Parameters for the Set-up of stmasdfqz.av.
Parameter Value Comments
pulprog stmasdqfz.av Pulse program.
NS 16*n For set-up the full phase cycle is not so critical.
D0 see text Calculated in pulse program.
D1 5 * T
1
Recycle delay, use dummy scans if shorter.
D4 20 µs Z-filter delay.
P1 1.5 µs Excitation pulse at pl11.
P2 1.5 µs Conversion pulse at pl11.
P3 20 µs 90° selective pulse at pl21 taken from previous pulse calibration.
PL1 =120 dB Not used
PL11 start with
≈
150
to 300 W
Power level for excitation and conversion pulses.
PL21 Power level for selective pulse, approx. pl11+30 dB taken from
previous pulse calibration.