Dewar flasks, filling

When 5.00 mL of ether has been delivered by the syringe pump, the pump is shut off The reactor is allowed to run an additional 15 min before the fluorine and the mercury arcs ate shut off. The preaerosol furnace, the evaporator heater unit, and the coolant pump are shut off. Once the system approaches ambient conditions, all the helium carriers are shut off and the product trap valves are closed The product trap and its Dewar flask filled with liquid nitrogen are removed to the vacuum line where the trap is evacuated... 

Figure 2.89 illustrates a commercially available Quickfit lyophiliser (Bibby Science Products) accommodating a single flask of such a size that the volume of aqueous solution to be treated is one-quarter its total capacity. The charged flask is rotated in a dry ice-acetone bath so that an even layer of frozen solution is obtained over the inside. The flask is immediately attached to the refrigerant chambers which are filled with a Cardice-acetone mixture. An oil vacuum pump is connected to the refrigerant chamber via the supplementary trap, which if possible should be immersed in a Dewar flask filled with liquid nitrogen such a cooled trap provides maximum protection for the vacuum pump. Vacuum is... 

Procedure. The furnace should be turned on the day before measurements are to be made so that it will achieve a steady temperature. Set the power supply to a predetermined voltage appropriate for the desired reaction temperature. The heating current should be measured with a series ammeter and recorded periodically. At the start of the experiment, place the reference thermocouple junction in a Dewar flask filled with ice and distilled water (see Chapter XVII) and connect the thermoconple to a potentiometer or digital voltmeter (see Chapter XVI). Measure the thermocouple emf and check to see if the furnace is at the proper... 

Slowly raise a narrow-mouth Dewar flask filled with liquid nitrogen into plaee, and clamp it so that the sample bulb is completely immersed. Then plug the mouth of the Dewar loosely with glass wool or a piece of clean towel to retard condensation of water and oxygen from the air. (Liquid O2 dissolved in the liquid N2 would raise the temperature of the bath.)... 

For ESR measurements, the ball milled or cut fibers were transferred to a Dewar flask filled with liquid nitrogen immediately after mechanical treatments in order to avoid significant decay of unstable free radicals. The fibers were then transferred slowly to ESR sample tubes together with liquid nitrogen, which was removed by a vacuum pump afterward. Subsequently, the ESR tube was sealed in vacuum for ESR measurements. 

Phosphorimeters. The design of a phosphorimeter is the same as that of the fluorometer, except for the use of a rotating can and Dewar flask. The sample solution is transferred into a small round quartz tube and the tube placed in a special Dewar flask filled with liquid nitrogen to freeze the solution into a glass. The lower part of the flask is smaller than the upper (see Fig. 9.6) and is also made of quartz to permit transmission of the exciting radiation and the phosphorescence emission. The Dewar is placed inside the rotating can, which has two apertures, or slits. As a slit moves into line with the monochromator beam, the sample is excited but the speed of rotation is such that any fluorescence emission ceases before the slit moves into line with the emission detector, so that only phosphorescence is observed. 

Fig. 6. Simple liquid N2 temperature cryostat for RR studies of frozen protein solutions. The protein solution (typically 5-15 /xl of 1-10 mM concentration) is placed in a copper cup on the end of a cold finger to give a flat surface. The glass or quartz shroud is clamped over the cold finger, and the sample is frozen by pouring liquid N2 in the horizontal Dewar flask. Once the sample is frozen, the Dewar flask is turned to a vertical position and evacuated to lO -lO" torr. The Dewar flask, filled with liquid N2, is transferred to the Raman sample compartment, and the scattered light is collected via 135° backscattering geometry directly from the surface of a frozen protein solution. ...

To a quartz cuvette fitted with a Teflon stopcock was added 53 mg 2,6-dichloro-benzoquinone and 63 mg phenyl m-xylyl acetylene in CH2CI2 in a 0.1 M solution of each substance under argon. The cuvette was then placed in a clear Dewar flask filled with water at 25°C and irradiated with focused light from a medium-pressure mercury lamp (500 W) passed through an aqueous IR filter and an ESCO 410 nm filter. This ensured that the quinone itself was excited and not the diarylacetylene. After 22 h, the solvent was evaporated, and the residue was subjected to flash chromatography. 1-(3,5-Dimethylphenyl)-l-benzoyl-1-methylene-3,5-dichlorocyclohexa-2,5-dien-4-one was obtained as the major product, which was further recrystallized from acetonitrile, m.p., 191-192°C. The minor product was identified only by GC/MS. The total yield of quinone methide was 88% based on 62% of conversion. 

Small Dewar flask filled with liquid nitrogen. 

A 1.4 Apparatus—A suitable apparatus is shown in Fig. Al.l. For the freezing point of water, a Dewar flask filled with at least 50 % crushed ice in water may be substituted. 

Plunge tubes into a dewar flask filled with liquid nitrogen. Store at -SO C. 

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