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KappaCCD User Manual
Revision 1.10, Last Update 1-July-1998
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Sample preparation
There are several stages in which you can influence the final
quality of your measurements.
One of them is selecting the right crystal and mounting
it properly.
Selecting the right crystal
The following lists the requirements a nice crystal would
fulfill. In practice there are not so many choices available. A batch of
crystals comes as is. Nevertheless you may bear these remarks in mind when
you select the most appropriate crystal.
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Select a crystal with a size around 0.2 - 0.3 mm for organic
compounds. Use smaller sizes for compounds containing heavier atoms to
reduce the absorption effects. Crystal sizes in general can be selected
smaller than on a conventional serial diffractometer. Since crystal size
is related to spot size, you will have less overlap from smaller crystals.
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Select a crystal with a simple, regular shape.
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Select a crystal with equal dimension, i.e. avoid exaggerated
crystal dimension ratios.
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Check your crystals under a polarizing microscope. A good
single crystal will extinguish completely on a distinct position when between
crossed polarization filters.
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Scan your crystals with the KappaCCD, i.e. make a few quick
exposures and check the images visually on spot shape, spacing and distribution
Crystal mounting - Influence of the
support
The amount of glue and crystal support material present in
the X-ray beam influences the background considerably. This effect diminishes
rapidly with distance. Since the KappaCCD is most effective at short distance
you should pay some attention to this point.
Glass rods should be avoided as they offer the worst
ratio between stability and scattering mass in the beam.
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Capillaries are a better choice, especially when you
can reduce the scattering mass even further by stretching them in a flame
and then breaking them at the thinnest part. You can form a kind of scoop
by pricking in the capillary opening with a needle, breaking out one side
of the glass
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Stretched capillary
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Massive glass rod
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Very fine layers of glue can keep small crystals very well
in place. You do not have to see the glue in order to make it stick!
The length of the capillary plus the mounting pin should
be around 30 mm in order to bring the crystal conveniently in the center
of the goniometer without an extender ring. A standard pin is typically
20 mm long and has a diameter of 3 mm.
If you use a sealed capillary, do not let it stick out
further then 5 mm above the crystal. Otherwise take care that the top end
of the capillary does not collide with the beam stop holder.
Crystal mounting - Influence of the orientation
Most people tend to align the longest dimension of their
crystals with the mounting support. While this is nice to reduce absorption
effects, it is not the most favorable orientation for use on the KappaCCD
as it generally complicates measurement strategy.
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Try to tilt the crystal axis some 30 degrees away from the
mounting support. In this way also simple phi axis rotations can result
in more complete data sets, depending on the crystals’ symmetry. Absorption
is not too much influenced with this change.
Crystal mounting - Influence of the
goniometer head.
The minimum distance of 25 mm can be reached with the
Nonius ECON heads, provided no extender ring is used.
Note that goniometer heads of other
make may lead to collisions when the minimum distance of 25 mm is used.
Make sure to check that the setup
for hardware description in the server program reflects the current goniometer
head setup you are using.
Scanning crystal samples with the KappaCCD
Crystal pre-scanning on the KappaCCD is a simple but effective
way to select a good crystal. Visual inspection of some test exposures
can be made to find out if obvious problems can be avoided in an early
stage.
Two exposures of 1 degree and 90 degrees apart from each
other may give you already an impression of the scattering range and axes
size of the crystal.
If you have doubts about the quality of the crystals,
it can be handy to mount a few samples of the same compound and put them
all up for visual inspection.