The diffractometer is enclosed with a leaded Plexiglass shield to minimize exposure to scattered X-rays. Shown below is a closer view inside the shielding.
X-rays are produced when a 35 keV electron beam strikes a copper target. The x-rays are emitted horizontally through a shutter (A) on the left, travel through a focusing slit (B) and then impinge on the sample (C, which can not be seen in this photo because a safety cover is in place). The emerging beam travels through two more slits (D and E) as well as a thin nickel foil (to remove most of the k-beta and white radiation). Finally, the x-rays impinge on the detector (F).
The arm holding the detector assembly (D-F) pivots during the run. If data were collected from 0 to 90 degrees 2-theta, then the detector assembly would be positioned directly above the sample (C) at the end of the run.
Older instruments simply record the output file to a strip chart recorder. Modern instruments can store and manipulate the pattern and have many powerful features. One of the most useful is the ability to match an unknown material against a database of 70,000+ known phases.
For materials that diffract less strongly such as organic molecular crystals, more sample (100-200 mg) must be used. One way of doing this is to tape or glue a microscope slide to the back side of the sample holder. The depression in the holder is then filled with sample and smoothed flat. Air sensitive samples are handled in special holders that have an airtight seal and Mylar windows. Bruker has a product sheet titled Specimen Holders for X-ray Diffraction that has pictures of these and other types of sample holders. Mylar windows are available from various vendors including Chemplex
XRD samples should be well-ground in a mortar and pestle or a ball mill. This creates a uniform particle size and ensures that all possible crystallite orientations are present in the sample. A special problem that can arise in sample preparation is called preferred orientation which usually occurs with rod or plate-like crystals. For example, plate-like crystals tend to lie flat on the sample holder; very few will have a perpendicular orientation. As there is no longer a random orientation of crystallites some of the x-ray reflections that would be expected are unusually weak or missing altogether.
This page was last updated Sunday, June 12, 2016.
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