Bruker BioSpin Solid State NMR, User Manual

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56 (327) BRUKER BIOSPIN User Manual Version 002
Basic Setup Procedures
This chapter will include:
"Setting the Magic Angle on KBr"
"Calibrating 1H Pulses on Adamantane"
"Calibrating 13C Pulses on Adamantane and Shimming the Probe"
"Calibrating Chemical Shifts on Adamantane"
"Setting Up for Cross Polarization on Adamantane"
"Cross Polarization Setup and Optimization for a Real Solid: Glycine"
"Some Practical Hints for CPMAS Spectroscopy"
"Literature"
General Remarks 4.1
Despite the fact that most spectra taken on a CP/MAS probe look like liquids
spectra, the conditions under which they are taken must account for the presence
of strong interactions. This basically means that:
•
Fast spinning, and
•
high power pulses are applied.
Fast spinning requires a high precision mechanical system to allow spinning near
the speed of sound. This requires careful operation of the spinning devices.
Please read the probe manual carefully!
High power decoupling in solids requires 20-fold RF fields compared to liquids
spectroscopy, since we are dealing with >20 kHz dipolar couplings rather than
maximum 200 Hz J-couplings! This means that RF voltages near the break-
through limit must be applied, and that currents of far more than 20 A occur.
It is therefore essential that:
•
Power levels for pulses must be carefully considered before they are applied.
Always start at very moderate power levels with an unknown probe, find the
associated RF field or pulse length and then work your way towards specified
values. The same applies for pulse lengths, especially decoupling periods,
since the power dissipation inside the probe is proportional to pulse power and
duration. Always observe the limits for duty cycle and maximum pulse power.
Please refer to the probe specifications for more information. Never set acqui-
sition times longer than required!
•
Spinners and turbine must be kept extremely clean. Any dirt, especially oil,
sweat from fingers, water will decrease the breakthrough voltage dramatically.
Make sure the spinner is always clean (wipe before inserting, touch the drive
cap only) and the spinning gas supply is carefully checked to provide oil-free
and dry (dew point below 0 °C) spinning gas. Compressors and dryers must be
checked and maintained on a regular basis. Any dirt inside the turbine will
eventually cause expensive repairs.
The following setup steps need only be executed upon installation or after a probe
repair. The test spectrum on glycine should be repeated in regular intervals to as-
sure probe performance.