Flow-rate independence

Figure 4 shows breakthrough profiles during the adsorption operation of Pb(II) by both FPS-f and FP-f packed columns. Surprisingly, both columns gave flow rate independent breakthrough profiles of Pb(II), although the... 

Figure 4. Flow rate Independent Viscometer Response. Sanple polystyrene 17500 MW Solvent THF, concentration 1%.

Figure 9. Flow Rate-Independent SEC-Viscoslty Detection Sanple polystyrene mixture (1.8M + lOOK + 9K) Solvent THF, Concentration 0.08%...

Knox (6) described a constant flow device that ensured a flow-rate independent of the pressure at the column inlet while Guild,... 

It is also possible to employ detectors with solutions flowing over a static mercury drop electrode or a carbon fiber microelectrode, or to use flow-through electrodes, with the electrode simply an open tube or porous matrix. The latter can offer complete electrolysis, namely, coulometric detection. The extremely small dimensions of ultramicroelectrodes (discussed in Section 4.5.4) offer the advantages of flow-rate independence (and hence a low noise level) and operation in nonconductive mobile phases (such as those of normal-phase chromatography or supercritical fluid chromatography). 

Successful and reproducible separations require the use of a pump that delivers the buffer at a constant flow rate independent of the resistance to the flow. The choice of flow rate is dependent upon the type of resin, the diamensions of the column and the overall design. 

The most commonly used pump for HPLC is the reciprocating pump. This has a small cylindrical piston chamber that is alternately filled with mobile phase and emptied via back-and-forth movement of the piston. This produces a pulsed flow that must be damped. Reciprocating pumps have a number of advantages. They have a small internal volume, are capable of high output pressures, and they can readily be used for gradient elution. They provide constant flow rates, independent of solvent viscosity or column backpressure. Other pumps used are motor-driven syringe pumps and pneumatic (constant-pressure) pumps. 

Detectors to be used in FIA should idealiy be endowed with a number of attributes such as low flow-cell volume and noise, flow-rate-independent signal, fast and linear response over a wide concentration range and high sensitivity. FIA methodology utilizes a variety of analytical detection techniques such as optical (spectroscopic and non-spectroscopic), electric (amperometric, po-tentiometrlc, conductimetric, coulometric) and thermochemical. 

The sample holder environment was purged with helium during these experiments, at the relative high flow-rate of 200 ml/minute. Such a flow-rate proved to be necessary to obtain flow-rate independent results. Apparently, the formation of a the vapour cloud above the sample container is prevented in this way. 

W. L. Caudill, J. O. Howell, and R. M. Wightman, Flow Rate Independent Amperometric Cell. AnaL Chem., 54 (1982) 2532. 

It seems generally accepted that these pumps offer a very constant flow-rate, independent of the value of the downstream back-pressure and of its possible variations. In fact, it has been shown that, because of the compressibility of liquids, the time necessary to achieve a steady-state flow-rate can be long, typically 15-60 min. During this time, the flow-rate increases continuously. 

Pumps These are piston-type precision pumps. They pump the polymer in solution through the system. The pump must deliver the same flow rates throughout the time in order to maintain the same pressure inside the system. Any variation in flow rates affects directly the results. The pump also has to deliver the same flow rates independently of the viscosity differences. In addition, some detectors are highly sensitive to the solvent flow rate precision. Such constant flow is a critical feature of the instrument. 

Flow controllers Many detectors are flow sensitive and, thus, a flow controller may be essential. The controllers are simple pneumatic devices that will maintain a constant mass flow rate independent of the column temperature or column impedance. 

Carbon fibers have been used for array construction because they are more rigid than gold. The construction of carbon fiber arrays consisting of five layers of 20 electrodes was introduced by Caudill et al. (29) and used as a flow rate independent amperometric detector. Sleszynski et al. (25) reported the design of ensembles based on reticulate vitreous carbon embedded in epoxy resin. 

An example of a pulsatile-flow bioreactor for vascular grafts is shown in Figure 44.3. This design, used in the Laboratory of Cardiovascular Tissue Engineering at the University of Oklahoma, resembles the shell and tube concept of some heat exchangers. Flow circuits are separated into shell-side and tube-side to monitor (and control) system pressure and flow rates independently. Control valves downstream... 

The volumetric pump provided a fixed flow-rate independent of other test conditions, while the inlet pressure could be varied and controlled by means of the pressurizing system and the control valve downstream of the test section. 

Ion transport induced by convective transport effects increases the fuel utilization up to 58% and enables control of fuel and oxidant flow rate independently. Experimental results indicated that by increasing the fuel flow rates, fuel utilization is decreased from 58% at lOOplmin to 4% at 5mlmin . Increasing the concentration of sulfuric acid as supporting electrolyte increases the maximum power density from —1.5 to —3mWcm due to decreasing charge transport losses. 

MCR 301 and Vilastic V-E rheometers at shear rates between 0.01 and 1,000 s". For the xanthan solution, the viscosity at low shear rates exhibited a plateau value of 8 (i.e., 8 cp or a flow resistance 8 times greater than water solid curve in Fig. 2). Consequently, on the basis of viscosity (vs. shear rate) data, we expected to see Newtonian (flow-rate-independent) behavior at low flux values. Instead, in 55-md Berea sandstone, resistance factors at low flux reached as high as 30 (solid circles in Fig. 2). This behavior was also reported for xanthan solutions by Cannella et al. (1988) and Hejri et al. (1991). In 269-md Berea and 5120-md porons polyethylene, we did not achieve sufficiently low velocities to observe this phenomenon (because of limitations in the accuracy of our pressure transducers at low pressure drops). 

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