Pumping differential stage

Liquid-Piston Type This type is illustrated in Fig. 10-97. These compressors are offered as single-stage units for pressure differentials up to about 0.52 MPa (75 Ibf/in ) in the smaller sizes and capacities up to 6.8 X 10 mVh (4000 fF/min) when used with a lower pressure differential. Staging is employed for higher pressure differentials. These units have found wide application as vacuum pumps on wet-vacuum service. Inlet and discharge ports are located in the impeller hub. As the vaned impeller rotates, centrifugal force drives the sealing liquid... 

The interface used today between the atmospheric-pressure plasma and the low-pressure mass spectrometer is based on a differentially pumped two-stage interface similar to those used for molecular beam techniques [89-91]. The key to successful development of ICP-MS instruments was the use of a relatively large ( l-mm-diameter) sampling orifice so that continuum flow was attained with an unrestricted expansion of the plasma to form a free jet. When small orifices were used, a cold boundary layer formed in front of the orifice, resulting in substantial cooling of the plasma, including extensive ion-electron recombination and molecular oxide formation. The smaller orifices were also susceptible to clogging. 

FT-MS is the slowest among all MS techniques. The long SAT of FT-MS is due to an extended time for acquiring the transient, which is essential for the accurate determination of frequencies of periodic motions of ions. Moreover, FT mass analyzers need to be installed in ultrahigh vacuum environments, and such environments require long ion transmission paths and several differentially pumped vacuum stages. Because commercial FT mass analyzers are installed downstream of IT mass analyzers, the typical SATs of FT-MS range from hundreds of milliseconds to tens of seconds. 

From the operational point of view, reliable vacuum systems are a prerequisite for mass spectral measurements. In most cases, manufacturers apply differential stage pumping to achieve the required pressure range(s). Rotary pumps are used to provide an initial vacuum of approximately 10 to 10 Torr. High-vacuum pumps such as diffusion pumps (10 to 10 Torr), turbomolecular pumps (10 to 10 Torr), and cryopumps (10 to 10 Torr) are used to reduce pressure ftorther. Adequate knowledge in vacuum technology is essential in instrument design however, this is also beyond the scope of this chapter. 

Fig. I. Experimental setup the clusters are emitted from the cluster condensation cell, passing as a particle beam through a differential pumping stage into the focus of a time-of-flight mass spectrometer, where they are ionized by a laser pulse.

After condensation, the clusters are transported by the He-flow through a nozzle and a differential pumping stage into a high vacuum chamber. For ionization of the clusters, we used excimer and dye laser pulses at various wavelengths. The ions were then mass analyzed by a time-of-flight mass spectrometer, having... 

The chemical compositions of the isolated Au SR clusters were investigated by mass spectrometry [15,16,18, 22,32-35]. TEM was used to confirm that the species detected by the mass spectrometer represents the clusters in the sample. Figure 3a is a schematic representation of the top view of the mass spectrometer, which consists of five stages of differentially pumped vacuum chambers. The apparatus accommodates two t5 pes of ion sources, electrospray ionization (ESI) and laser-desorption ionization (EDI), and a time-of-flight (TOE) mass spectrometer with a reflectron. Details of the apparatus and the measurement protocols are described below. 

Fig. 4.54. Ion transfer optics and differential pumping stages to adapt an ESI source to an FT-ICR instrument. Only the ICR cell is inside the superconducting magnet. By courtesy of Bruker Daltonik, Bremen.

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).

Non-contact seals with defined narrow clearances, such as labyrinth seals in turbo-machines (Fig. 4.1-47 A), are less problematic owing to the relatively low differential pressures however, the fluid should not have an erosive effect. Differential pressures up to several hundred bar occur at the axial-thrust compensation pistons of centrifugal pumps and compressors, but the fluid expansion is distributed over the many labyrinth stages. 

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