Minimizing Sample Motion in Cold Gas Streams

Sample motion in the cold gas stream can be problematic, especially when working with small samples and small X-ray beams. Sample motion can broaden the apparent mosaicity and corrupt diffraction spot intensities so as to make indexing and refinement difficult. Sample motions that are large enough to have significant effects on diffraction are generally easily visible in home source sample alignment telescopes and synchrotron source video displays.

What causes sample motion? A partial list:

• Improper adjustment of the cold gas stream and the surrounding shield flow. With modern gas stream equipment, following the manufacturer’s instructions generally gives the best results.
• Partial loss of vacuum (and thus thermal insulation) in the gas stream’s flexible hose, or ice accumulation inside the flow tubes. These can cause fluctuating gas flows and gas temperatures, and may require larger flows to achieve adequate cooling.
• Improper centering of the gas stream on the sample, and improper distance between the gas stream outlet and the sample.
• For off-axis gas streams, orienting the sample so that the gas flows perpendicular to (rather than parallel to) the plane of the sample mount.
• Excessive drag around large samples encased in large amounts of liquid.
• Inadequate mount rigidity. Nylon loop mounts are prone to motion , especially if the twisted nylon necks are not encased in frozen water.

Perhaps 90% of sample motions are due to turbulence associated with the first three items in this list. Aside from making sure that your gas stream hardware is functioning properly, is properly aligned, and has the correct flow speeds, what can you do if sample motion is a problem? Some suggestions:

• Reduce the amount of excess liquid around your sample, to reduce mass and drag.
• Mount a smaller sample.
• Mount your sample with an orientation that minimizes data collection when the gas stream is nearly perpendicular to the plane of the mount.
• Use a thicker sample mount. Regular MicroMounts and MicroLoops are designed to minimize X-ray background scatter for the most demanding applications. If you can tolerate a bit more background scatter from the mount, try using MicroMounts HT or MicroLoops HT, which are significantly thicker and more rigid.
• Leave a thin layer of liquid (mother liquor or oil) on the mount, including the neck region. When it freezes, it will increase the mount's rigidity.

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