Cryocrystallography: Flash Cooling

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In addition to shipping pre-frozen crystals to BioCARS, users can flash cool crystals on site using liquid propane, liquid nitrogen or cold gas stream. For this purpose, users may withdraw small amounts of liquid nitrogen (up to 4 l from a large 50 l tank -50LD-) or request liquid propane (up to 10 ml) from the BioCARS staff. Proper safety procedures for using liquid propane must be followed.

BioCARS Standard Procedure for Flash cooling of Macromolecular Crystals by Liquid Propane

This is a Standard Operating Procedure (SOP) for flash cooling macromolecular crystals by using liquid propane on site of BioCARS lab at Sector 14. Flash cooling crystals is now a common practice in macromolecular crystallographic community. The cooling agents are usually involved liquid nitrogen and liquid propane. The latter, due to its fire hazard, causes safety concerns. The procedure proposed here is adopted from a widely used one by other synchrotron facilities (e.g. CHESS) and institutes (e.g. Yale University) to prepare, collect and store small amount liquid propane. With other restrictions on the quantity of material, location of operation and environment control etc., we are confident that the procedure will meet APS' safety standard.

Hazards

Propane (C3H8, m.w. 44.097) is a colorless unsaturated hydrocarbon gas. Although it may causes potential health effects, the major safety concern is its fire hazard (flash point of 105oC and vapor pressure 6536 mmHg at 20oC, see attached Material Safety Sheet),

Controls

Commercial product of propane is usually supplied in the form of compressed (~100 psi) liquid. The boiling point of propane is high (-42°C), which makes it easy to be liquefied. Compressed liquid propane is handled as flammable material. At room temperature, material released from the liquid container is of propane gas, which is of fire hazard. But if it is immediately cooled to very low temperature, it will then be re-liquefied, which can be handled safely. We propose that the use of liquid propane in limited quantity, within restrict area, at cryogenic temperature and, operate with extremely cautions:

Operating procedure

Set-up

The 5 lb liquid propane container is connected, via the CGA fitting and stainless steel tubing, to a needle valve. The valve is cooled by liquid nitrogen held in a small reservoir soldered directly to the valve. Small quantity of propane gas released from the container will be re-liquefied, when the valve is sufficiently cooled, and collected by a 1.8 ml cryo-vial. The vials are pre-cooled and kept at liquid nitrogen so that the rate of evaporation of liquid propane is extremely low (at 77 K, the propane becomes solid in the vial).. The cryogenic cooled liquid propane in the vials is then facility for flash cooling of the crystals by dipping the crystal in liquid propane and seal the vial with a cap. The sealed vials are kept in a liquid nitrogen dewar of standard cryo-storage system till X-ray diffraction experiment.

Operation