MQ-MAS: Sensitivity Enhancement
User Manual Version 002 BRUKER BIOSPIN 235 (327)
Three pulse sequence and coherence transfer pathway for the 3Q MAS experi-
ment with z-filter (mp3qdfs.av). Excitation pulse p1 is the same as for mp3qzfil.av,
P4 is a central transition selective 180° pulse (usually 2*p3). Delays D0 is the in
-
cremented delay for t
1
evolution. Delays D10 or D11 must be incremented propor-
tional to D0. Power level and duration of the sweep P2 must be optimized. Phase
lists are as follows, 2D data are acquired in QF mode, which means that no phase
incrementation is required:
ph1 = 0 30 60 90 120 150 180 210 240 270 300 330
ph2 = 0
ph3 = 0*12 90*12 180*12 270*12
receiver = {0 270 180 90}*3 {180 90 0 270}*3
Initial parameter values are listed in Figure 18.1.. A few of these parameters need
further explanation:
D6: Is calculated as (1s*L1/CNST31)-(P4/2)-(P2/2). This ensures that the delay
from the centre of the sweep to the middle of the 180° pulse is an integer multiple
of the rotor period. L1 must be set so that D6 becomes long enough for a full echo
to build up. If we assume the FID has decayed after 3 ms, spinning frequency is
25 kHz, and the 90° pulse is 20 µs, then L1 should be between 70 and 80 so that
D6 is between 2.77 and 3.17 ms.
P2: Is calculated as 1s/(CNST31*L0). The duration of the sweep can be adjusted
depending on T
2
of the sample. The sweep should not be longer than one rotor
period, for many samples you may find that a quarter of a rotor period or even less
is sufficient.
In ased the values of the two parameters D6 and P2 are greyed, because they
cannot be set anymore. They are calculated in the pulse program from the param
-
eters explained below, which must be set accordingly:
L0: Defines the fraction of a rotor period for the duration of the sweep P2, usually
between 1 and 8.
L1: Defines D6 to be an integer number of rotor periods.
The sweep will be defined by further parameters:
CNST1: Start frequency (in kHz) of the sweep; the sweep should start slightly off
resonance, usually 30 to 50 kHz from the resonance of the central transition.
CNST2: End frequency (in kHz) of the sweep; the sweep should cover the satellite
transition, but this is often broader than the band width of the probe of approxi
-
mately 1 MHz. Therefore, it does not make sense to have this value bigger than
1000.
To Next Page
Loading...