Basic MQ-MAS
User Manual Version 002 BRUKER BIOSPIN 215 (327)
sponding to an RF field amplitude of a few kHz. Delays d0 and d4 are the incre-
mented delay for t1 evolution and 20 µs for z-filter, respectively. Delay d10 initially
is 0 and can be incremented proportional to d0 (in10=in0*7/9), if the observe nu
-
cleus has spin I=3/2. Phase lists are as follows, for phase sensitive detection in F1
the phase of the first pulse must be incremented by 30° in States or States-TPPI
mode:
ph1 = 0 60 120 180 240 300
ph2 = 0*24 90*24 180*24 270*24
ph3 = 0
ph4 = 0*6 90*6 180*6 270*6
receiver = {0 180}*3 {90 270}*3 {180 0}*3 {270 90}*3 {180 0}*3 {270 90}*3{0
180}*3 {90 270}*3.
Of course, the sequence with more pulses has slightly inferior sensitivity; howev-
er, it is the basic sequence to improve sensitivity by FAM or DFS. The 3-pulse se-
quence itself can be used directly to enhance sensitivity by soft-pulse added
mixing (pulse program mp3qspam.av). In
"MQ-MAS: Sensitivity Enhancement"
on page 231 some of the sensitivity enhancement techniques will be described.
Note that pulse programs suitable for AV and AVII spectrometers have the exten-
sion.av, pulse programs for the AVIII have no extension.
Data Acquisition 17.3
Before the 2D experiment on your sample of interest can be started, two set-up
steps must be done as described in detail below. All set-up steps should be done
on a sample with a) a known MAS spectrum, b) with sufficiently good sensitivity to
facilitate the set-up and c) a 2
nd
order quadrupole interaction in the order of the
one expected for your sample of interest. In the first step a low power selective
pulse must be calibrated in a single pulse experiment. With this the MQMAS ex
-
periment can be optimized using the 2D pulse sequence for t
1
=0.
Setting Up the Experiment 17.3.1
Sample: There are a large number of crystalline compounds which can be used to
set up the experiment. Please refer to table 1 to select a suitable sample. For the
general procedure described here the spin I of the nucleus is not important, of
course obtained pulse widths will depend on the spin I, and the Larmor frequency.
You can use any arbitrary sample showing a considerable broadening by the 2nd
order quadrupole interaction to adjust the experiment, however, reasonable 1D
MAS spectra should be obtained quickly for sensitivity reasons.
The set-up must be done in two steps; in the first step a central transition selective
pulse that merely excites the central transition must be calibrated. This pulse must
be weak enough so that only this transition is affected and it must be short enough
so that the central transitions of all sites in the spectral range are excited. As an
example, the sinc shape excitation profile of a 20 µs pulse has its zero-crossings
at 1/20 µs = ± 25 kHz which means that the central transition signals must not ex
-
tend beyond this range, otherwise severe line shape distortions will be observed.
On the other hand the corresponding RF field amplitude of a 20 µs 90° pulse will
be 1/(80 µs*(I+1/2))=12.5 kHz/(I+1/2). This means that w
RF
<<w
Q
as a prerequi-
To Next Page
Loading...