272 (327) BRUKER BIOSPIN User Manual Version 002
CRAMPS: General
Goldburg). To observe the signal, a gap within the pulse sequence must be sup-
plied, which is long enough to observe one or several data points while the mag-
netization vector points along the magic angle. This condition obviously persists
only for a time period short compared to the transverse relaxation of the signal. To
observe the time dependence of the signal, the sequence must be repeated and
more data points accumulated until the signal has decayed under the influence of
residual broadening. Obvious problems of this experiment are the requirement to
observe a relatively weak signal shortly after a strong pulse (dead time problem)
and the requirement to time the sequence in such a way that the magnetization
vector is accurately aligned with the magic angle (requires precise pulse lengths
and phases, and it requires RF fields strong compared to the interaction and shift
distribution). Many sequences have been devised after the original WHH-4 (or
WaHuHa) sequence which yield better results due to better error compensation
(MREV-8, BR-24, C-24, TREV-8, MSHOT). Modern hardware has made the setup
and application of these sequences a lot easier since pulse phase and amplitude
errors are negligible, higher magnetic fields have led to better chemical shift dis
-
persion and also to shorter dead times. The resolution achieved with long, highly
compensated sequences like BR-24 is very good, but their applicability at limited
spin rates (because of the need for the cycle time to be short with respect to the
rotor period) often presents a problem.
References:
1. S. Hafner and H.W. Spiess, Multiple-Pulse Line Narrowing under Fast Magic-Angle Spinning, J.
Magn. Reson. A 121, 160-166 (1996) and references therein.
2. M. Hohwy, J. T. Rasmussen, P. V. Bower, H. J. Jakobsen, and N. C. Nielsen.
1
H Chemical Shielding
Anisotropies from Polycrystalline Powders Using MSHOT-3 Based CRAMPS, J. Magn. Res.133 (2),
374 (1998), and references cited therein.
W-PMLG and DUMBO 21.3
W-PMLG and DUMBO are shorter sequences which avoid turning high power
pulses rapidly on and off, which is what most multiple pulse sequences do. This
avoids undesired phase glitches. Also, they use higher duty cycles during the de
-
coupling period. As a result, the sequences are simpler and shorter, requiring few-
er adjustments and allowing higher spin rates.
Both sequences use repetitive shaped pulses with detection in between. PMLG
uses the principle of a
Frequency Switched Lee Goldburg (FSLG) sequence (con-
tinuous irradiation with a net RF field along the magic angle), where the frequency
shifts are replaced by a phase modulation. DUMBO basically works like a window
-
less MREV-type pulse sequence where the individual pulses are replaced by a
single pulse with phase modulation.
References:
1. E. Vinogradow, P.K. Madhu, and S. Vega, High-resolution proton solid-state NMR spectroscopy by
phase modulated Lee-Goldburg experiment, Chem. Phys. Lett. 314, 443-450 (1999).
2. D. Sakellariou, A. Lesage, P. Hodgkinson and L. Emsley, Homonuclear dipolar decoupling in solid-
state NMR using continuous phase modulation,
Chem. Phys. Lett. 319, 253 (2000).
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