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RE: [ccp4bb]: magic angle
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Hi There,
The 'magic' angle name comes from solid state NMR, where a sample is spun
simultaneously about 2 different axes, in order to maximise the randomness
of the orientation of the sample. Solution samples don't have a problem with
orientation, they have none. Powder diffraction often suffers from preferred
orientation, so some people use magic angle spinning to get rid of that
effect. This is how it goes:
An X-ray beam travels towards the sample, a powder in a tube. It is not
stationary, but spinning along the long axis of the tube. The beam
intersects the tube at the point where the sample is. The tube is also
spinning about an axis normal to the beam and the detector, where the angle
between that axis and the tube is the magic angle. These two mutual
rotations bring all orientations in the sample into diffracting position, so
you get a powder pattern even from a single crystal, with reasonably even
distribution of diffracted intensity around the rings (but then you have to
account for the polarisation). On a four-circle diffractometer, you can
achieve this by continuous and simultaneous rotation in the phi and kappa
axes (mind the collimator housing!)
When it comes to our work, single crystal diffraction wants the opposite of
this effect. We want sharp spots, although extremely sharp spots have their
own problems. However, like some people have already said, the reflections
near the rotation axis are often inaccessible or unreliable due to their
remaining in diffracting position for a long time, so they are partials over
many frames. Even 3D profiling will not retrieve all of these reflections.
The only way to collect them, if symmetry is not helpful, is to reset the
crystal into a new orientation. The minimum angle of offset has already been
commented on, but 90 deg. is the maximum needed. The arcs on some goniometer
heads may not be enough for some severe cases. You need to help the
situation by using a kappa axis, for another fixed (kappa) position. When
kappa is zero, phi and omega are identical, but when kappa is non-zero, you
must rotate on omega. Note here that the shadow of the beam stop will not
change in this procedure.
I must say here that a Chi axis is equally capable of achieving the same
thing (for single crystals not powders). The historical reasons for choosing
chi or kappa escape me, but my impression was that some engineers thought
kappa geometry allows more easy access to the crystal for better alignment.
I suppose it is a matter of taste, or perhaps a unique selling point.
Whatever, most of our work these days can be done with a single axis
diffractometer, the plain phi axis camera. We are not restricted anymore to
measuring each reflection separately, with a single counter which could only
move in the 2-theta direction.
Well, I hope this is going to be helpful. Enjoy.
Pierre Rizkallah
-----Original Message-----
From: marius [mailto:marius@hexa.e17.physik.tu-muenchen.de]
Sent: 11 December 2001 16:52
To: ccp4 bulletin board
Subject: [ccp4bb]: magic angle
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Can someone explain to me (in simple words) the pratical
and theoretical usefullness of a diffractometer with kappa
geometry with magic angle (54.? deg) using pre-oriented
and randomly oriented crystals.
Best regards
Marius
____________________________________________________________________________
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Dr. Marius Schmidt
Physikdepartment E17
Technische Universitaet Muenchen
James Franck Strasse
85748 Garching
Germany
Email: marius@hexa.e17.physik.tu-muenchen.de
Tel : +49/89/2891-2550
Fax : +49/89/2891-2548
E17 im WWW:
http://www.physik.tu-muenchen.de/lehrstuehle/E17/bio_server.html
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