Pump, diffusion helium

Liquid-nitrogen cold traps stop condensable vapors from traveling between mechanical pumps, diffusion pumps, and the rest of the system. They also protect the helium leak detector from possibly contaminating materials (such as silicon-based diffusion pump oil). 

The flow of droplets is directed through a small orifice (Skimmer 1 Figure 12.1) and across a small region that is kept under vacuum by rotary pumps. In this region, approximately 90% of solvent and injected helium is removed from the incipient particle beam. Because the rate of diffusion of a substance is inversely proportional to its molecular mass, the lighter helium and solvent molecules diffuse away from the beam and are pumped away. The heavier solute molecules diffuse more slowly and pass through the first skimmer before they have time to leave the beam the solute is accompanied by residual solvent and helium. 

Again, helium degassing as described above is an appropriate way to remove oxygen, especially when combined with heating (40 to 50°C) of the mobile phase. Special care must be taken that oxygen cannot diffuse back into the mobile phase replace PTFE tubing by steel tubing between mobile phase container and pump, and between column and detector. 

The most common separators include the Ryhage or jet diffusion separator (74), the Watson-Biemann or pore diffusion separator (75), and the membrane solution diffusion separator originally developed by Llewellyn (75). The first two separators involve direct passage of the sample into the mass spectrometer the low molecular weight helium diffuses more readily and is pumped away. The membrane separator involves diffusion of the sample through a silicone membrane while the carrier gas vents to the atmosphere carrier gas is thus not confined to helium. There is no best separator the choice depends on the nature of the compounds, the temperature range over which it will be operated, and most usually what is available in a particular laboratory. A convenient configuration for a double-beam mass spectrometer such as the AEI MS-30 is two different separators, one into each beam, which permits rapid evaluation of separator performance. 

Within a helium leak detector, there are valves that isolate the mass spectrometer and gauge sections, the diffusion pump, and the trap from the rest of the system. These valves are used to prevent contamination or damage to these sections when cleaning, adjusting, or venting is required during use. Find and use them. 

Details of the operation of the vacuum system (Fig. 3) should be reviewed earefully in consultation with an instructor before beginning the experiment. In the following it is assumed that both the helium and nitrogen storage bulbs have already been filled to a pressure slightly over 1 atm, the diffusion pump is operating, and the sample has been degassed. 

The tritium may be purified by diffusion through palladium or separated from helium by preferential adsorption on charcoal at 77 K. The most convenient way of storing tritium gas is by reacting it with finely divided uranium to produce UT3. The tritium may be released when required by heating the UTj, having first removed any decay product (helium) by pumping. 

The jet concentrator consisted of a succession of jets that were aligned in series but separated from each other by carefully adjusted gaps. The helium diffused away in the gap between the jets and was removed by appropriate vacuum pumps. In contrast, the solute vapor, having greater momentum, continued into the next jet and, hnally, into the mass spectrometer. The... 

The two stage momentum separator used in this interface is shown schematically in Figure 3 coupled to the combination Thermospray/EI source. This device is conceptually similar to those used in other MAGIC (2) or particle beam (3,4) interfaces. However, since most of the solvent vapor is removed in the gas diffusion cell, this separator is required primarily to remove sufficient helium to allow the standard MS pumping system to achieve the good vacuum required for El operation. The performance of this device can be optimized much more readily for separating helium from macroscopic particles than when copious quantities of condensible vapors are present as in the more conventional particle beam systems. 

A schematic of a particle beam interface is shown in Figure 21.13. The eluent from the HPLC column is nebulized using helium gas to form an aerosol in a reduced pressure chamber heated at 70°C. A cone with a small orifice is at the end of the chamber, which leads into a lower pressure area. The difference in pressure causes a supersonic expansion of the aerosol. The hehum and the solvent molecules are lighter than the analyte molecules and tend to diffuse out of the stream and are pumped away. The remaining stream passes through a second cone into a yet lower pressure area, and then the analyte vapor passes into the ion source. The particle beam interface produces electron ionization (El) spectra similar to those of GC-MS, so the vast knowledge of El spectra can be used for analyte identification. 

Unless otherwise indicated the catalysts studied are degassed at 350°C. for sixteen hours by means of a mercury diffusion pump system. The helium used for dead space measurements is 99.9% pure and is obtained in special Pyrex flasks from the Ohio Chemical Company. The nitrogen which is over 99 % pure as received from the Ohio Chemical Company is purified further by passage over copper gauze at 500°C. and phosphorous pentoxide with a final distillation at liquid nitrogen... 

This design incorporates several desirable features. (1) The microscope is continuously pumped so that a variety of experiments can be carried out in it. (2) Only minimal amounts of liquid hydrogen (or helium) are required (70 cc for the central Dewar and 150 cc for Dewar //2). (3) Surface diffusion of any gas which can be trapped on a liquid helium cooled surface can be studied, merely by changing the gas bottle. 

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