Remember, the crystal you choose
largely determines how much effort you will have to expend later on.
Good crystals can give good data or crummy
data but lousy crystals will only ever give crummy data.
It usually takes a bit of practice to
routinely pick the most appropriate sample.
people get the hang of it almost immediately while
others apparently never get it.
You are aiming to pick a crystal with
sufficient volume, so blocky
crystals tend to be better than needles.
Crystals come in all shapes and sizes.
A good size for organics with (say) all
atoms lighter than sulphur would be a block about 0.25mm on a side, but
this is just a guide. Some visually stunning crystals end up being much
worse than crystals that are not so pretty - the golden rule is try
them. Even if they are < 0.1 mm on a side, try
Large crystals with heavy atoms will cause
absorption problems, e.g.
when the x-ray beam is absorbed by
different amounts for different reflections. The problem is that
each reflection is attenuated differently, by some amount that is not
known a priori. Although it
possible to correct for this to varying degrees, it is much better to
pick a crystal that will minimize the problem to begin with.
Absorption corrections will be covered elsewhere.
Sometimes none of the crystals seem to be
free of problems. Do not despair - all the samples below
acceptable data - even the last one, but again, they all required
surgery. Cutting, breaking and cleaning are all important skills
that you will have to learn.
The oil makes it easy to manipulate
the crystals using a needle and to perform surgery because it holds the
crystals steady while you work on them.
Transparent crystals can be checked
between crossed polarizers.
the polarizer on the very bottom of the microscope objective - see
how the view
goes light and dark.With the
polarizers crossed (dark) slowly turn the microscope stage.See how the crystals go light and dark as you
This allows you to tell
if your crystals are twinned. If your crystal appears to have two or
regions that disappear at different times or if it doesn't
disappear at all,
then you have a problem. Twinning is a complicated business that needs
separate document, just to cover the basics.
Opaque crystals usually require
reflected light and can be viewed with either a white, black or some other background.
At some point you will need to sketch
and measure the crystal.If you
know that your crystals are stable you can do this now.Otherwise you should measure it after data collection.
When the SZX12 microscope ZOOM is set to "25",
then one small division in the eyepiece is a tenth
of a millimetre. The SZX9 is a little different - on this microscope, with a ZOOM of "25", one tenth of a millimetre is 2.5 small divisions.
Feel free to play with the microscope and figure out what the scale
is on other ZOOM settings.