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Mounting single crystals
Mounting single crystals
Picking a suitable single crystal
Picking a suitable single crystal
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The section on identifying suitable crystals covers the requirements making crystals suitable for diffraction. The crystallographer will remove crystals from the sample vial and place some onto a drop of oil on a microscope slide. Using a needle tool, the crystals are separated on the plate and a suitable crystal for mounting is identified.
Crystals are taken from a tube and placed onto a drop of oil on a microscope slide.
The crystals are spread out in the oil on the slide to separate the individual crystals.
Using the microscope and polariser, a suitable crystal is identified. The circled crystal was selected and this crystal will be followed through the rest of this guide.
Mounting the crystal
Mounting the crystal
The crystal is mounted into a crystal mounting loop. These have varying sizes of loops, with 50, 100 and 200 µm loops shown in the images. The loops have a magnetic base, which is fitted to a magnetic hand tool during mounting, and the goniometer also uses a magnetic mount. Visualising the crystal and loop down the microscope, the crystallographer can identify the right loop size for the crystal. The crystal is then carefully picked up into the loop, and is held in place from the surface tension of the oil.
Crystal mounting loops are available in a range of sizes.
The loop heads have a magnetic base. These are placed onto a hand tool to use whilst mounting.
Microscope closeup image of the different loop sizes.
A crystal mounting loop above the microscope slide.
The loop and desired crystal viewed down the microscope. The crystal of interest is directly above the loop.
A microscope image of the crystal which has now been mounted into the loop.
Loading onto the Goniometer
Loading onto the Goniometer
The loop containing the crystal is then removed from the hand tool and placed onto the goniometer. The pictures below show the crystal mounted on the loop which has been placed onto the goniometer of the diffractometer. Note the cryojet stream of cold nitrogen gas being blown over the crystal which both cools the crystal and provides a protective atmosphere. The cold temperature improves the quality of the crystal structure by reducing molecular vibrations (typically cooling to 110 K, but other temperatures can be used). The cold temperatures can result in the formation of ice (despite the dry nitrogen being used), which can be problematic in data collection.
The crystal is mounted onto the goniometer head which is magnetic, allowing the loop to be easily transferred and be securely held in place.
A closer image of the mounted crystal in the cryostream jet of the diffractometer
A very close image of the crysal in the loop mounted on the goniometer.
Centering the crystal
Centering the crystal
Once the crystal has been loaded onto the goniometer, the crystal needs to be aligned so that it is centred in the X-ray beam. The diffractometer includes a mounting camera with crosshairs for this process. A small tool is used to alter the position of the crystal on the goniometer until it is centred in all dimensions. This is achieved by repositioning the goniometer with the crystal to view each crystal face in turn. Only a small part of each face will be irradiated with the X-ray beam, which is indicated by the crosshairs.
The screen for the crystal mounting camera showing the crystal is not centred within the X-ray beam.
The crystal is carefully repositioned to centre the crystal within the beam in all dimensions.
The tool used to adjust the goniometer head in three dimensions.
Following the crystal mounting process, the setup is ready for data acquisition.
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