Bruker BioSpin Solid State NMR, User Manual

Page: 221 / 328

Basic MQ-MAS
User Manual Version 002 BRUKER BIOSPIN 221 (327)
Some further comments and explanations on the parameters listed above:
FnMode
must be States or States-TPPI so that the shearing FT can be performed
for processing. If the pulse program mp3qzfil.av is used, no shearing is required in
case of nuclei with spin I=3/2 if
in10
is set correctly in which case a split-t
1
experi-
ment is performed. td determines the number of FID’s to be accumulated in the in-
direct dimension. This value is determined by the line width and resolution that
can be expected in the indirect MQ dimension (F1) and which depend on the pro-
perties of the sample. In crystalline material fairly narrow peaks can be expected
so a maximum acquisition time in F1 of 2 to 5 ms is expected. In disordered mate-
rials where the line width is broader and determined by chemical shift distribution
a total acquisition time in F1 of 1 ms may be sufficient. The total acquisition time
aq in F1 equals (
td
/2)*
in_010
.
For rotor synchronized experiments
in_010
= 1/spinning frequency so will typical-
ly be between 100 µs (10 kHz spinning) and 28.5 µs (35 kHz spinning), so only 10
to 40 experiments in amorphous samples but 50 to 200 experiments in crystalline
samples might be required. The rotor synchronization means that the spectral
range in F1 is limited. Depending on the chemical shift range, spinning frequency,
and quadrupole interactions the positions of the peaks may fall outside this range.
In such a case care must be taken when interpreting the spectrum. Acquisition
with half-rotor synchronization to double the spectral window in F1 may help.
However, in this situation one set of spinning sidebands appears and it must be
avoided that the spinning side bands of one peak fall on top of other peaks. Some
sort of rotor synchronization is always recommended because spinning side
bands in the indirect dimension extend over a very wide range, which cannot be
truncated by e.g. filtering. Therefore, rotor synchronization together with States or
States-TPPI phase sensitive acquisition helps to fold spinning sidebands from
outside back onto centre bands or other side bands.