Mechanical pumps direct-drive

Pump, direct-drive (vacuum technology) A mechanical pump, the moving parts of which... 

For general experimentation, large manifold systems, and industrial operations, direct-drive pumps are also strongly recommended. If, however, you are pumping a dirty environment, belt-driven mechanical pumps should be considered because their greater oil chambers can better tolerate contamination (see Sec. 7.3.7). They can easily be placed in pans (to catch any accidental oil leaks) on the floor, and they can be left on for days and/or weeks of continuous operation. 

In addition, mechanical pump oils can be specialized for use in specific environments such as those with high-oxygen contents. Some are blended for use in specific types of pumps such as direct-drive pumps, belt-driven pumps, and rotary-piston pumps. As with most things, no single product fits the bill for all circumstances. Thus, there are many varieties, grades, and types of mechanical pump oils. 

One important aspect which should he pointed out is that there is a basic incompatibihty between GC and MSD, the mass spectrometer operate at pressures of 10- torr or less, whereas the gas chromatograph effluent operates at about 760 torr. An interface device is necessary to handle the pressure differences. The simplest and most efficient interface in GC/MSD is a direct capillary column interface. The low flow rate of the narrow bore column and the high pumping rate of an oil diffusion pump backed by a suitable direct drive mechanical pump assure pressures less than lO " torr and allow for the direct insertion of the colmnn end into the mass spectrometer. Interfacing megabore... 

The natural direction of chemical reactions is much like the natural direction of more familiar changes in everyday life. We aU know that water going over a waterfall is favorable. Likewise, water going back up the waterfall is not favorable. To move water from the bottom of the waterfall back to the top would require energy and work—electricity could drive a mechanical pump that could move the water. 

Three processes are involved in transcellular transport across the intestinal epithelial cells simple passive trans-port, passive diffusion together with an efflux pump, and active transport and endocytosis. Simple passive transport is the diffusion of molecules across the membrane by thermodynamic driving forces and does not require direct expenditure of metabolic energy. In contrast, active transport is the movement of molecules across the mem-brane resulting directly from the expenditure of metabolic energy and transport against a concentration gradient. Endocytosis processes include three mechanisms fluid-phase endocytosis (pinocytosis), receptor-mediated endocytosis, and transcytosis (Fig. 6). Endocytosis processes are covered in detail in section Absorption of Polypeptides and Proteins, later. 

The fused-silica surface also provides another mechanism, electro-osmosis, which drives solutes through the tube under the influence of an electric field. The principle of electro-osmotic flow (EOF) is illustrated in Fig. 1. The inner wall of the capillary contains silanol groups on the surface that become ionized as the pH is raised above about 3.0. This creates an electrical double layer in the presence of an applied electric field so that the positively charged species of the buffer which are surrounded by a hydrated layer carry solvent toward the cathode (negatively charged electrode). This results in a net movement of solvent toward the cathode that will carry solutes in the same direction as if the solvent were pumped through the capillary. This electrically driven solvent pumping mechanism results in a flat flow profile in contrast to... 

FIGURE 22.13 The mechanism of photophosphorylation. Photosynthetic electron transport establishes a proton gradient that is tapped by the CF,CFq— ATP synthase to drive ATP synthesis. Critical to this mechanism is the fact that the membrane-bound components of light-induced electron transport and ATP synthesis are asymmetric with respect to the thylakoid membrane so that directional discharge and uptake of H ensue, generating the proton-motive force. The number of protons pumped through the ATP synthase varies by species and is the subject of active research. 

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