Crystal Handling at Low Temperature*
The sample in its mother liquor is stored in an ordinary Schlenk flask as generally used in chemical laboratories. The temperature of the flask is kept at the crystallization temperature employing a suitable cooling bath. If necessary, the flushing gas may be cooled as well.
Through the neck of the flask, a small spoon is allowed to cool down in the mother liquor. A portion of crystals is scooped up and covered with cold inert oil from a syringe (if the crystals dry up very quickly this can be done even within the mother liquor). The spoon is now rapidly transferred into the cold gas-stream of the X-Temp 2 device and the crystals are applied to the cold mixture of inert oils on the microscope slide (Fig. 1).
|Fig. 1: Crystal selection under a microscope at low temperature.|
The mother liquor is removed by using a soft tissue and the crystals are completely immersed in the inert oil. The cold inert gas atmosphere and the surrounding inert oil prevent the crystals from being attacked by moisture and/or oxygen. The transparent oil allows washing, splitting and selecting of crystals with out restrictions as well as investigating the crystal quality by utilizing the polarizing effect of the microscope.
The selected crystal is mounted on the tip of a glass fibre where it is held by the adhesive power of the inert oil. A portable cooler is employed to transfer the crystal mount safely to the diffractometer. The crystals can be transferred in a dry-ice block which is provided with a drill-hole. For lower temperatures, a small Styrofoam container is filled with liquid nitrogen. By introducing metal blocks a cold nitrogen boil-off is generated for a few minutes which effectively cools the crystal mount above the surface of the liquid phase.
When subjected to the low temperature of the diffractometer cooling device, the oil-drop embedding the crystal on the glass fibre freezes and the crystal orientation is fixed.
*T. Kottke, D. Stalke, J. Appl. Crystallogr.1993, 26, 615-619.