Vacuum systems differential pumping

Moving-belt (ribbon or wire) interface. An interface that continuously applies all, or a part of, the effluent from a liquid chromatograph to a belt (ribbon or wire) that passes through two or more orifices, with differential pumping into the mass spectrometer s vacuum system. Heat is applied to remove the solvent and to evaporate the solute into the ion source. 

Two vacuum systems are used to provide both the high vacuum needed for the mass spectrometer and the differential pumping required for the interface region. Rotary pumps are used for the interface region. The high vacuum is obtained using diffusion pumps, cryogenic pumps, or turbo pumps. 

The complications of windows can be avoided by substituting small apertures above and below the sample to restrict the diffusion of gas molecules while allowing penetration of the electron beam. Typically, pairs of apertures are added above and below the sample, with differential pumping lines attached between them. In the early in situ experimentation, an ECELL system (69) could be inserted inside the EM column vacuum between the objective lens pole pieces. 

Example The vacuum system of non-benchtop mass spectrometers consists of one to three rotary vane pumps and two or three turbo pumps. Rotary vane pumps are used for the inlet system(s) and as backing pumps for the turbo pumps. One turbo pump is mounted to the ion source housing, another one or two are operated at the analyzer. Thereby, a differentially pumped system is provided where local changes in pressure, e.g., from reagent gas in Cl or collision gas in CID, do not have a noteworthy effect on the whole vacuum chamber. 

Wet samples can be analyzed without a previous preparation by the so-called environmental scanning electron microscopy (ESEM). In this technique, instead of the vacuum conditions, the sample chamber is kept in a modest gas pressure (Bache and Donald, 1998). The upper part of the column (illumination source) is kept in high vacuum conditions. A system of differential pumps allows to create a pressure gradient through the column (Bache and Donald, 1998 Stokes and Donald, 2000). The choice of the gas depends on the kind of food hydrated food is kept under water vapor. 

In all vacuum systems, the pressure obtained is determined by the gas load and the effective pumping speed. In UHV systems at equilibrium, the predominant gas load arises from the outgassing of the internal surfaces. Although gas sources in vacuum systems have been discussed in Chapter 4, no differentiation was made between gas adsorbed on a surface and that absorbed within its structure. Various applications of UHV technology involve the use of vacuum systems that cannot be baked in situ and consideration of the choice of material for the vacuum envelope and also its surface treatment is critical. An important starting point for this is an understanding of the interaction of gas with materials. 

Chapter 6 examines what, in the authors opinion, are three important applications of vacuum technology in the chemical sciences. First, its use in chemical technology is clearly defined and, in many applications, the requirement for systems operating below 10 6 Pa is obvious. In both cases, typical systems are considered and quantified. The third topic concentrates on a technique (differential pumping) which is widely used in systems where high- and low-pressure areas must be interfaced. Specific systems are discussed to illustrate the usefulness of the technique. 

A miniature cylindrical ion trap mass spectrometer with APCI and ESI capabilities was developed [22], The system includes a three-stage, differentially pumped vacuum system and can be interfaced to many types of atmospheric pressure ionization sources. 

This experiment presents the measurement of uranium with an inductively coupled plasma mass spectrometer (ICP-MS). In this system, a nebulizer converts the aqueous sample to an aerosol carried with argon gas. A torch heats the aerosol to vaporize and atomize the contents in quartz tubes. The atoms are ionized with an efficiency of about 95% by an RF (radiofrequency) coil. The plasma expands at a differentially-pumped air-vacuum interface into a vacuum chamber. The positive ions are focused and injected into the MS while the rest of the gas is removed by the pump. The ions are then accelerated, collected, and measured as a function of their mass. Losses at various stages, notably the vacuum interface, result in a detection efficiency of about 0.1 %, which is still sufficient to provide great sensitivity. The amounts of uranium isotopes in the sample are determined by comparisons to standards. Because different laboratories have different instruments, the instructor will provide instrument operating instmctions. Do not use the instrument until the instructor has checked the instrument and approved its use. 

J. 2.2.2 Moving Belt System. Initially developed by McFadden et al. [13], the moving belt system was based on the physical method of evaporation of the mobile phase through heat and vacuum that leave analytes as a thin coating on a continuously cycling polyimide belt. The analytes were transported from atmospheric pressure region to the vacuum of the ion source through differentially pumped vacuum locks. Ionization methods used... 

The magnetic coil shown in figs. 2.15 and 2.16 was used to orient the electron polarisation vector P parallel to the axis of the analysing target—Mott detector system. The deflection system is part of the differential pumping stage which is necessary for the maintenance of the required ultra-high vacuum in the source chamber. 

The momentum separator can be considered as a two-stage differentially-pumped nozzle-skimmer system, thus featuring a nozzle, a skimmer, and a colhmator. In this respect, the FBI resembles an atmospheric-pressure ionization system (Ch. 5.4). However, in the FBI, the analyte is sampled from a closed desolvation chamber, and the analyte is transferred to the high-vacuum region prior to ionization Generally, little attention is paid to the design characteristics of the momentum separator with respect to optimized molecular-beam performance. 

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