Alio News Letter….March 2009 issue: crystallography

Alio News Letter….March 2009 issue: crystallography
The application shown here is a four axis system ideally suited for protein crystallography.

The application was at Argonne National Labs where the ALIO solution was chosen over Aerotech for two main reasons, first by having the ability to produce equipment that could deliver tighter rotational run-out specifications than could be manufactured by the competition. Second, by being able to offer a complete solution, here again the competitor could not include the XY on the face of the stage therefore their solution was only partial, thus meaning a second vendor would have to be included and therefore a second control system.

The application required a sample to be loaded into the tooling that is mounted to the face of the XY then the lower Z axis moves the sample into the beam line. The air bearing R rotates the sample on the centreline of rotation for perfect positioning.

Micromech is the UK distributor for and if you would like more details about these exciting precision stages then contact Stirling Morley on 01376 333333 or email on stirling@micromech.co.uk

‘Nano precision a Standard’

Ps. ALIO was also involved with a recent project for the Diamond Light Source in the last year supplied and commissioned by Micromech.


What is Crystallography?

Crystallography is the experimental science of determining the arrangement of atoms in solids. In older usage, it is the scientific study of crystals. The word ‘crystallography’ is derived from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and graphein = write.
Before the development of X-ray diffraction crystallography (see below), the study of crystals was based on the geometry of the crystals. This involves measuring the angles of crystal faces relative to theoretical reference axes (crystallographic axes), and establishing the symmetry of the crystal in question. The former is carried out using a goniometer.
The position in 3D space of each crystal face is plotted on a stereographic net, e.g. Wulff net or Lambert net. In fact, the pole to each face is plotted on the net. Each point is labelled with its Miller index. The final plot allows the symmetry of the crystal to be established.
Crystallographic methods now depend on the analysis of the diffraction patterns that emerge from a sample that is targeted by a beam of some type. The beam is not always electromagnetic radiation, even though X-rays are the most common choice. For some purposes electrons or neutrons are used, which is possible due to the wave properties of the particles. Crystallographers often explicitly state the type of illumination used when referring to a method, as with the terms X-ray diffraction, neutron diffraction and electron diffraction.
These three types of radiation interact with the specimen in different ways. X-rays interact with the spatial distribution of the valence electrons, while electrons are charged particles and therefore feel the total charge distribution of both the atomic nuclei and the surrounding electrons. Neutrons are scattered by the atomic nuclei through the strong nuclear forces, but in addition, the magnetic moment of neutrons is non-zero. They are therefore also scattered by magnetic fields. When neutrons are scattered from hydrogen-containing materials, they produce diffraction patterns with high noise levels. However, the material can sometimes be treated to substitute hydrogen for deuterium. Because of these different forms of interaction, the three types of radiation are suitable for different crystallographic studies.

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