Preparation-vacuum system

The chemistry required to convert the oxide to other binary compounds is independent of the scale of operation. However, with microscale synthetic methods applied to radioactive materials, successful preparations are achieved more readily by carrying out the chemistry in situ, that is, in such a manner that eliminates, or at least minimizes, the necessity of having to "handle" the sample during or following its synthesis. Thus, actinide compounds are usually prepared in silica capillary tubes which can be flame sealed at the conclusion of a synthesis to provide the desired sample for study in a small volume, quartz container. A special feature of the preparation/vacuum system in the TRL is the capability to interrupt a synthesis, isolate (by means of a stopcock) and remove the sample, examine it in... 

Figure 2. Schematic of preparation-vacuum system used for synthesizing transcurium element...

Barium is prepared commercially by the thermal reduction of barium oxide with aluminum. Barium metal is highly reactive, a property which accounts for its principal uses as a getter for removing residual gases from vacuum systems and as a deoxidiser for steel and other metals. 

Electron spectroscopic techniques require vacuums of the order of 10 Pa for their operation. This requirement arises from the extreme surface-specificity of these techniques, mentioned above. With sampling depths of only a few atomic layers, and elemental sensitivities down to 10 atom layers (i. e., one atom of a particular element in 10 other atoms in an atomic layer), the techniques are clearly very sensitive to surface contamination, most of which comes from the residual gases in the vacuum system. According to gas kinetic theory, to have enough time to make a surface-analytical measurement on a surface that has just been prepared or exposed, before contamination from the gas phase interferes, the base pressure should be 10 Pa or lower, that is, in the region of ultrahigh vacuum (UHV). 

Anionic polymerization of ethylene oxide by living carbanions of polystyrene was first carried out by Szwarc295. A limited number of methods have been reported in the preparation of A-B and A-B-A copolymers in which B was polystyrene and A was poly(oxyethylene)296-298. The actual procedure was to allow ethylene oxide to polymerize in a vacuum system at 70 °C with the polystyrene anion initiated with cumyl potassium in THF299. The yields of pure block copolymers are usually limited to about 80% because homopolymers are formed300. ... 

Figure 1. Apparatus for the preparation of radical anions (11). On connection of the entire vessel to the vacuum system, traces of water and oxygen on the wall are removed by heating and discharging with a tesla coil. When the apparatus is filled with purified nitrogen through A, the weighed sample of the hydrocarbon is put into B through C, a piece of sodium is put into D, and dimethoxyethane is distilled into E, where a small amount of an Na-K alloy is added. After the system is again evacuated the solvent is distilled from E into B, the bulb E is,sealed off at F, and the sodium is sublimed to form a mirror on the wall of the bulb G. After tubes at C and H are sealed off, the apparatus is pumped to high vacuum for 1 hr and then sealed off at J. Then the solution of the hydrocarbon is poured from B into G. After a time varying from several minutes to several hours, a color is observed, and the sample is ready for optical and esr measurements.

The prepared flask is supported so that the tube B can be sealed to a vacuum system. If a suitable furnace is available the flask is surrounded by this, with the constriction A still within reach of the hand torch flame otherwise the heating must be done by flame. A liquid air trap is included between the flask and diffusion pump. With a vacuuih of below 10 mm of mercury the temperature of the flask is raised to near the softening point—the heating may be rapid... 

The decomposition of formic acid over evaporated Pd-Au alloy films has been studied by Clarke and Rafter (69) the same reaction on Pd-Au alloy wires was studied by Eley and Luetic (128). The alloy films were prepared in a conventional high vacuum system by simultaneous evaporation of the component metals from tungsten hairpins. The alloy films were characterized by X-ray diffraction and electron microscopy. The X-ray diffractometer peaks were analyzed by a method first used by Moss and Thomas (SO). It was found that alloys deposited at a substrate temperature of 450°C followed by annealing for one hour at the same temperature were substantially homogeneous. Electron microscopy revealed that all compositions were subject to preferred orientation (Section III). 

Recently, Hologic, Inc. developed a fully automated cell block system, Cellient system (Bedford, MA), expecting to improve capture, presentation, consistency, and efficiency of cell block preparation. This system is built on ThinPrep technology with vacuum-assisted filtration to maximize cell collection. The cell block can be produced in less than an hour.39 However, the biggest concern of this methodology is the fixation issue. Cellient system adopts alcohol instead of formalin for fixation, which unfortunately creates problems for ICC analysis. 

Phials of anhydrous perchloric acid in methylene dichloride were made up as described [11]. Sodium ethoxide was prepared i.vac. from purified ethanol and sodium purified by cascade fusion . Solutions of it in diethyl ether were prepared and filled into phials [11,12] by standard techniques on a tapless, jointless high-vacuum system. These solutions were colourless, whereas those prepared i.vac. from vacuum-distilled sodium were always yellow or brown. 

The probable impurities in radioactive phosphorus trichloride are hydrogen chloride and phosphorous acid the trichloride is, therefore, purified by fractional evaporation and fractional condensation in the high-vacuum system. Pure ethylphosphonous dichloride is prepared according to the equation ... 

The continuous polycondensation process consists of four main process units, i.e. (1) slurry preparation vessel, (2) reaction unit, (3) vacuum system, and (4) distillation unit. The molar EG/TPA ratio is adjusted to an appropriate value between 1.05 and 1.15 in the slurry preparation vessel. In most industrial processes, the melt-phase reaction is performed in three up to six (or sometimes even more) continuous reactors in series. Commonly, one or two esterification... 

Figure 8. Gas treatment cell for transmission XAS. The sample is prepared as a pressed self-supporting pellet in the sample holder, diluted with BN. The liquid nitrogen dewar enables data collection at 77 K, and the connection to gas-flow or a vacuum system enables control of the sample environment. (Reproduced with permission from ref 154. Copyright 1997 B. L. Mojet).

Contamination may result from incomplete evacuation of the vacuum system and/or from degassing of the sample. The system blank should be normally less than 1% of the amount of gas prepared from a sample for analysis. For very small sample sizes, the blank may ultimately limit the analysis. Memory effects result from samples that have previously been analyzed. They will become noticeable, when samples having widely different isotopic compositions are analyzed consecutively. 

Figure 1 is a sketch of a complete apparatus for the preparation of tetrachloro-diborane(4). Boron chlorides are very corrosive and attack many elastomers and vacuum greases, so only Teflon, Viton, and halocarbon greases should be used for seals and lubrication in the vacuum system. 

Caution. The disilathianes should be regarded as toxic and are vile smelling. Their exposure to air and/or moisture is likely to promote rapid oxidation. Manipulations should be carried out in a sound vacuum system in a well-ventilated area. All preparations may be scaled-up to use 10 mmole of starting material. 

I. Sodium. Probably the best known active hydrogen remover is sodium. When used outside a vacuum system, for instance as sodium wire to dry solvents, the sodium is little more than a support for a skin of sodium hydroxide. Inside a vacuum system, however, one can prepare films of sodium metal and one can prepare really clean sodium which will give a colourless solution of sodium ethoxide (see Section 5.2.1.). The method of making sodium films for the removal of acidic compounds from liquid reagents will be described and also a very much less well-known method involving sodium vapour and colloidal sodium. 

Crystallisable salts and related compounds. Almost all crystallisable catalysts, such as sodium and lithium aromatic compounds (e.g. sodium naphthalide), -oyl salts such as aroyl hexafluorophosphates, alkoxides, and many others can be prepared in a vacuum system and then purified by repeated crystallisations and washings in a closed system (see Chapter 5) thereafter they can be distributed into breakable phials or other devices as described in Chapter 3. 

Evans and Lewis (1957) have described an ingenious method of preparing SnCl electrochemically in a high vacuum system from SnCl which, being crystalline at ambient temperature, is easier to purify than the tetrachloride. The apparatus is shown in Fig. 5.5. 

The sample preparation vacuum line (often called spray-on line ) should allow for (a) controlled mixing of the host gas with the substrate (or with other components that are added to the matrix) by manometric techniques and (b) the controlled release of the gas (mixture) toward the inlet system of the cryostat. These conditions are met by a vacuum line that incoporates a storage bulb for the gas (mixture), inlets for attachment of evacuable containers that allow degassing of the substrate prior to its mixing with the host gas, pressirre gauges that cover suitable ranges, a needle valve that allows the controlled release of the gas, possibly via a flowmeter, and interfaces to the bottles that contain the host gas(es), and to the inlet system that is attached to the vacuum shroud of the cryostat. 

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