Liquid nitrogen cryostat

Key words Cryostat, Liquid nitrogen, Fixation, Frozen tissue, Isopentane, Charged slides... 

Because Raman spectroscopy requires one only to guide a laser beam to the sample and extract a scattered beam, the technique is easily adaptable to measurements as a function of temperature and pressure. High temperatures can be achieved by using a small furnace built into the sample compartment. Low temperatures, easily to 78 K (liquid nitrogen) and with some diflSculty to 4.2 K (liquid helium), can be achieved with various commercially available cryostats. Chambers suitable for Raman spectroscopy to pressures of a few hundred MPa can be constructed using sapphire windows for the laser and scattered beams. However, Raman spectroscopy is the characterizadon tool of choice in diamond-anvil high-pressure cells, which produce pressures well in excess of 100 GPa. ... 

The sample is put in a 4He cryostat and enclosed by a shield thermally connected to the liquid helium reservoir. A second thermal shield connected to a liquid nitrogen reservoir encloses all the liquid helium system, allowing for a slow warming-up cycle in order to ensure thermal homogeneity of sample and holder. A window in the dewar enables the laser beam to enter the chamber and to reach the sample through small bores in both thermal shields. The sample is fixed onto a copper support that is in good thermal contact... 

In the case of bath cryostats - in the most simple case a cold trap filled with LNj (liquid nitrogen) - the pumping surface is cooled by direct contact with a liquefied gas. On a surface cooled with LNj (T = 77 K) HjO and COj are able to condense. On a surface cooled to = 10 K all gases except He and Ne may be pumped by way of condensation. A surface cooled w/ith liquid helium (T = 4.2 K) is capable of condensing all gases except helium. 

In a seminal review article, Larsen [1] described the state-of-the-art of the cryogenic techniques most commonly adopted for X-ray diffraction and lucidly showed the applications to all kind of crystallographic studies, stimulated by a variety of research interests. Already at that time, the availability of liquid nitrogen cryostats in many laboratories was highlighted, emphasizing the possibility to carry out many... 

Inner cryostat wall Liquid nitrogen coolant Copper shield wire... 

Cryostats described elsewhere (14) were used between 123° and 273°K. Temperature control was within 0.1°C and temperature was read using appropriate vapor-pressure thermometers. For still lower temperatures liquid nitrogen and liquid oxygen were used. 

Tissue specimens are snap-frozen in liquid nitrogen for 30 sec immediately after removal and then transferred to a cryostat (Kammerer et al., 2001). Serial frozen sections of 5 pan thickness are cut and placed on silane-coated slides. They are air-dried for 30 sec, fixed in acetone for 1 min at room temperature (22°C), and air-dried at 22°C (Fig. 6.11). The sections are incubated with primary antibody in the antibody diluent (Dako) for 3 min by placing the slide horizontally on a hot plate at 37°C. (All incubation steps are carried out by placing the slide horizontally on the hot plate at 37°C.) Following a brief rinse in TBS, the sections are incubated with the goat-anti-mouse EnVision-HRP-enzyme conjugate for 3min at 37°C. 

Kidney tissue is fixed with paraformaldehyde-lysine-periodate by vascular perfusion (Brown et al., 1996). Tissue slices are further fixed overnight at4°C with the same fixative and stored in PBS (pH 7.4) containing 0.02% sodium azide. They are placed in 30% sucrose in PBS for at least 1 hr, and then surrounded by a drop of Tissue-Tek embedding medium on a cryostat chuck before freezing by immersion in liquid nitrogen. Cryostat sections about 5 p,m thick are cut at a chamber temperature of -25°C, collected on Fisher Superfrost Plus charged slides, and stored at —20°C until use. 

For cryostat sectioning, the tissue specimens are cryoprotected in 30% sucrose in 0.1 M phosphate buffer for 12 hr or until they sink to the bottom of the container. They are embedded in O.C.T compound (Miles, Elkhart, IN) and frozen in N-heptane cooled to the temperature of liquid nitrogen. Alternatively, if the antigens are resistant to paraffin embedding, the specimens can be dehydrated in graded ethanol, cleared in xylene, and embedded in paraffin. 

Raman spectra were measured on fresh, chemically etched surfaces in quasi-backscattering configuration using a triple DILOR XY spectrometer, a liquid nitrogen cooled CCD detector, and a 514.5-nm Ar-ion laser. The laser beam of power level 20 mW was focused on an area of 0.1 mm2 on the mirror-like plane (it was the (ab) plane of the single crystals). The measurements were performed in a cryostat with a helium gas atmosphere in the temperature range 5-295 K below temperature of metal-insulator phase transition. 

Liquid nitrogen is stored in large vacuum vessels (cryostats). The liquid nitrogen is evaporated through a heat exchanger and the gas used in the factory. The units are normally supplied and maintained by the gas company. The cryostats can be easily topped up by a road tanker. This system is more economic in medium to large units. 

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