Vacuum Requirement

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). 

The requirement for the achievement of UHV conditions imposes restrictions on the types of material that can be used for the construction of surface-analytical systems, or inside the systems, because UHV can be achieved only by accelerating the rate of removal of gas molecules from internal surfaces by raising the temperature of the entire system (i.e. by baking). Typical baking conditions are 150-200 °C for 

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The vacuum requirements in the target chamber are relatively modest (10 Pa) and are comparable to those in the accelerator beam lines. All that is required is that the ion beam does not lose energy on its path to the sample and that there is minimal deposition of contaminants and hydrocarbons on the surface during analysis. 

Vapor Distribution Relatively large shell inlet nozzles, which may be used in condensers under low pressure or vacuum, require provision for uniform vapor distribution. 

NFPA 30 and API Standard 2000 provide gmdance for design of overpressure protec tion involving storage tanks that operate at or near atmospheric pressure. In particular, NFPA 30 focuses on flammability issues, while API 2000 addresses both pressure and vacuum requirements. The ASME code (Sections I and TII) and API RP 520 are the primaiy references for pressure rehef device sizing requirements. 

From a practical sense, MOKE is a versatile technique it is an optical method the polarization measurement is fairly easy to do the necessary optical components are common and relatively inexpensive and it has no intrinsic vacuum requirements. 

Determine vacuum required at the critical process point in system. 

Nonfills Air entrapment Gel and/or resin timet too short Additional air vents and/or vacuum required Adjust resin mix to lengthen time cycle... 

Operation of the interface should not compromise the vacuum requirements of the mass spectrometer and should allow all capabilities of the mass spectrometer to be utilized, i.e. ionization modes, high resolution, etc. 

Typically, flow rates in HPLC are around 1 mlmin , while the vacuum requirements of the mass spectrometer preclude liquid delivery of more than around 15 p.lmin at the probe end. To achieve compatibility therefore requires either the splitting of the flow from a conventional column or the use of some form of HPLC, such as a packed microcolumn, which provides directly compatible flow rates. Whichever of these solutions is employed, the amount of analyte reaching the mass spectrometer, and thus the overall sensitivity of the analysis... 

No satisfactory, single, intermediate glass for seahng platinum metal to Pyrex glass is readily available. Very fine platinum wire and very thin-walled platinum tubing can, however, be satisfactorily sealed directly to Pyrex, but such seals are not usually serviceable for vacuum requirements. 

The production of vacuum (sub-atmospheric pressure) is required for many chemical engineering processes for example, vacuum distillation, drying and filtration. The type of vacuum pump needed will depend on the degree of vacuum required, the capacity of the system and the rate of air inleakage. 

Equation 14.11 introduces the notion of radius of influence, which is one of the important design parameters of S VE systems. Theoretically, the maximum radius of influence of a well is the distance at which the pressure becomes equal to the ambient atmospheric pressure, i.e., P = Patm. In practice, Pj is determined as the distance at which a sufficient level of vacuum still exists to induce airflow, e.g., 1% of the vacuum in the extraction well.912 The extraction wells are usually constructed using pipes with a standard radius, e.g., Pw = 5.1 cm (2 in.) or 10.2 cm (4 in.), and the vacuum applied in the wells typically ranges from 0.05 to 0.15 atm, i.e., Pw = 0.95-0.85 atm.9 12 If the vacuum required in the radius of influence is 1% of the vacuum in the extraction well, the... 

This requires a heavy-walled flask. Ordinary lab vacuums are about 15 mm Hg. A simple water-forced suction vacuum requires only a water source to produce a vacuum of about 25 mm Hg, which is satisfactory for most purposes. Evaporation causes the temperature to drop which slows evaporation. Running a stream of warm water over the flask or putting it in a warm water bath avoids this. To avoid difficulties in getting residues out of the bottom of the flask, it is useful to do the evaporation in a vacuum exsiccator shaped as shown or in a flat dish in the exsiccator. Whenever a forced water vacuum is used, it is wise to place a water trap between the vacuum and the solvent being evaporated to prevent water from entering when the pressure fluctuates. 

Pressure and vacuum requirements for effective treatment and capture of volatilized materials. 

With respect to the practical considerations of gas flow and vacuum requirements, the PHPMS experiment might, upon cursory consideration, appear to be easily extended into the VHP region. That is, several MS-based analysis techniques routinely use ion source pressures of 1 atm. However, when an attempt to increase the pressure within a PHPMS ion source is made, the factors that do become problematic are those related to the subtle principles on which the method is based. Most importantly, the PHPMS method requires that the fundamental mode of diffusion be quickly established within the ion source after each e-beam pulse, so that all ions are transported to the walls in accordance with a simple first-order rate law while the IM reactions of interest are occurring. This ensures that a constant relationship exists between the ion density in the cell and the detected ion signal. The rates of the IM reactions can then be quantitatively determined from the observed time dependencies of the reactant ion signal because the contribution of diffusion to the time dependencies are well known and easily accounted for. 

The values of H and V are known, r is determined experimentally and e is assumed to be unity thus permitting us to determine the mass m. In practice the magnetic field is scanned so that streams of ions of different mass pass sequentially to the detecting system (ion collector). The whole system (Figure 4.1) is under high vacuum (less than 10 Torr) to permit the volatilisation of the sample and so that the passage of ions is not impeded. The introduction of the sample into the ion chamber at high vacuum requires a complex sample inlet system. 

Because of the chemical activity of Metraxt, the equipment used for application and vacuuming requires routine inspection and maintenance. Hoses and gaskets will have to be periodically inspected. Washing the foamer, its hoses and gaskets with soap and water and rinsing with water is recommended after each use to extend hfetime. 

Evacuate the system and determine the maximum obtainable vacuum. Requirements... 

Test the system under production conditions to determine that it can reproducibly reach the vacuum required within the normal time constraints. The results of the testing should then be documented for future reference. 

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