Cryogenic baths

Fig. 3.25.1. Porous structure of collagen sponge produced by freezing in a cryogenic bath at -25 °C and subsequently freeze dried (scanning electron microscope, white bar = 1 mm) (from 13.67]).

Plunge freezing Plunge specimen into a container of cryogen with forceps cell suspensions can be adhered to coverslips coated with poly-L-lysine design of cryogen baths is important (27) baths are cheaper than other freezing methods... 

One of the drawbacks of this CAVERN device is the occurrence of a nonuniform distribution of reactant on catalysts because adsorption occurs on a deep bed of catalyst packed in a MAS rotor. To overcome this problem, we developed several shallow-bed CAVERN devices (95), and Fig. 10 shows a version of one such design. A thin layer of catalyst is supported on a glass trapdoor, and the device is evacuated. A furnace is clamped in place so that the catalyst can be activated if necessary. The catalyst is cooled with a cryogen bath, and a controlled amount of adsorbate is introduced from the vacuum line. The trapdoor is raised, the loaded catalyst falls into the MAS rotor, and the seal is driven into place. Finally the cold, sealed rotor is manually transferred into the cold MAS probe. The added advantages of the shallow-bed CAVERN is that all manipulations can be carried out without using a glovebox in any step. 

Disgorging involves separating the wine from the lees accumulated in the previous step at the neck of the bottle. This deposit of lees is frozen by immersing the bottle in a cryogenic bath. The bottle is then placed upside down, the cap is removed and the pressure inside causes the frozen deposit to be expelled. 

The relative pressure domain close unity is subject to experimental limitations. This is because the temperature of the sample may be slightly greater than the temperature of the cryogenic bath owing to the cxothermicity of the adsorption and heat transfer phenomena. There is thus a great deal of uncertainty in the application of Kelvin s law to high relative pressure levels. In practice, application is limited to pores less than 50 nm in size. 

Arndt et al. (124) found that controlled cooling based on natural convection currents could not be used in the temperature range 30-120 C at cooling rates of 10°CVmin. They proposed the use of the simple controlled cooling apparatus shown in Figure 6.13. The jacket consisted of an aluminum cylinder of about 8 cm in diameter in which ice water from a cryogenic bath was circulated by means of a pump. The entire jacket was covered by a PVC... 

Table 8.2 Working temperatures of cryogenic slush baths ...

Elemental composition C 12.14%, 0 16.17%, Cl 71.69%. Phosgene can be analyzed by GC using FID or a balogen-specific detector or by GC/MS. Ambient air may be collected in a metal container placed in an argon bath or condensed into any other type cryogenically cooled trap. Alternatively, the air may be collected in a Tedlar bag. The sampled air may be sucked by a condensation mechanism into tbe GC column. 

For cryogenic freezing, nitrogen is used in several forms—as a shower of liquid droplets, as a liquid bath for direct immersion, or as a cold gas. Carbon dioxide is used as a liquid or in solid snow" form. When used in a tunnel for 1QR applications, liquid carbon dioxide can freeze products at a temperature from -62 lo -78°C (-80 to 109°F). Fluorocarbons and halocarbons also have been used in conjunction with tunnel and spiral-type freezers thill are used in IQF methods. 

Cryogenic materials and the surfaces they cool can cause severe cold bums if they are allowed to contact the skin. Thick gloves and a face shield may be needed when preparing or using some cold baths. 

Foam-insulated containers are adequate for some slush baths, but will require more effort to maintain the coolant. Foam-insulated containers should never be used for long- or short-term storage of cryogenic liquids. They may be used for cryogenic liquid transport, but there will be significantly more loss of the liquid coolant (even in limited transport) than from a regular Dewar. 

Such obvious benefits have stimulated solid-state MAS experiments at cryogenic temperatures in several research laboratories. In the most successful and reproducible case, a sealed sample with a liquid-nitrogen temperature spinning module was cooled in a helium bath, but rotation speeds did not exceed 2 kHz at 40 K and 1 kHz at 5 K [ 15]. This is a serious limitation for practical applications. 

---