Differential pumping system

Fig. 14. Schematic of the basic geometry of the aperture system and objective lens pole pieces incorporating radial holes for differential pumping system in the novel atomic resolution-ETEM design of Gai and Boyes (85-90) to probe catalysis at the atomic level.

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. 

The basic ECELL geometry consists of small apertures above and below the sample and the apertures are mounted inside the bores of the objective lens polepieces (figure 2.10(d)). The controlled environment ECELL volume is the normal sample chamber of the microscope. It is separated from the rest of the column by the apertures in each polepiece and by the addition of a gate valve, which is normally kept closed, in the line to the usual ion-getter pump (IGP) at the rear of the column. Differential pumping systems are connected between the... 

Fig. 1. Photo and illustration of the HRTEM allowing acquisition of images of catalysts under working conditions (4). The microscope is equipped with an FEG, a quadrupole mass spectrometer (QMS), a Gatan image filter (GIF), and a Tietz F144 CCD for data acquisition. The differential pumping system consists of IGPs, turbo molecular pump units (TMP, MDP), and an oil diffusion pump (ODP). The differential pumping stages are set up by apertures inside the TEM column (denoted by black bars) at the objective lens (OL), the first (Cl) condenser aperture, the second (C2) condenser aperture, and the selected area aperture (SA).

A miniaturized mass spectrometer with APCI was built [21], The analyzer was a monopole with 54 mm rod length and 2 mm radius. A two-stage differentially pumping system and sampling nozzle of 80p,m enabled an inlet gas flow rate of 1 ml/s. The ion current generated by the corona discharge was 0.01-10 p,A. 

In March, 2004, the company Gammadata Scienta announced a commercial XPS system based on a spectrometer designated SES-100. Pressure drop in the four-stage differential pumping system was specified to be better than six orders of magnitude, which should correspond to a pressure limit in the sample cell of about 0.1 mbar (assuming a pressure in the analyzer not higher than 10 7 mbar). 

Scheme of an atmospheric interface with differential pumping system using three stages. 

The operation of an ESEM is made possible through use of a differential pumping system (Figure 7.36) that maintains a UHV environment (10 Torr) required for the electron gun, while allowing the presence of gases in the sample chamber (10-20 Torr). The pressure and temperature of the sample chamber may be strictly controlled, inducing evaporation or condensation events. 

Figure 3 Schematic of a generic atmospheric pressure ionization source showing a differential pumping system.

Faraday cup (VG 9000, courtesy of VG Instru-ments) and B a quadrupole based low resolution glow discharge mass spectrometer (a) source (b) ion optics (c) quadrupole mass filter (d) differential pumping system (e) detector. Similar to described in Ref. [611]-... 

Figure 5. Schematic of ion sampling unit, composed of a sampling orifice, a set of electrostatic lenses, and a differential pumping system. The electric lines of force under the applied potential are drawn schematically together with an imaginary convex lens.

Recently, controlled atmosphere electron microscopy (CAEM) or in situ TEM has allowed in situ observation of gas-solid reactions at a resolution of about 25 A. The principal feature of the technique is that a relatively high gas pressure is obtained at the specimen while preserving the low pressure needed for operation of the microscope. Various types of cell and differentially pumped systems have been described (164). The applications of CAEM to catalysis have been reviewed with special emphasis on carbon deposition, carbon gasification, and sintering studies (165). 

A vacuum system maintains a very low pressure in the mass spectrometer. The ion source region is usually maintained at a pressure of 10 " to 10 torr somewhat lower pressure is required in the mass analyzer region (around 10 torr). Most instruments use a differential pumping system to maintain an optimal vacuum. 

MS, which requires vacuum. This membrane needs to be impermeable to the liquid electrolyte, yet permit the transport of the generated gases [79]. In the case of polar solvents such as those in Li-based batteries, polytetrafluoroethylene (PTFE) is the most popular choice [80]. Typically, differential pumping systems are used to progressively approach the vacuum required by the MS. An extensive discussion of the specific modifications that enable different types of electrochemical experiments and the corresponding analytical sensitivities can be found in the literature [79]. 

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