Pumps flow rate stability

The constant-flow pumps is the most widely used in all common IC applications. Flow rate stability is an important pump feature that distinguishes pumps. For size exclusion chromatography, the flow rate has to be extremely stable. External electronic control is a very desirable feature when automation or electronically controlled gradients are to be run. 

The accuracy and stability of the pump flow rate can be checked at 1 mL/min, with the column in place, by measuring the time required to fill a 10-mL volumetric flask... 

However, an important demerit of piston pumps, which limits their versatility, is the small number of available channels (usually two). A piston pump with an equal number of channels as an eight-channel peristaltic pump would be much more expensive and bulkier than the latter. Piston pumps are therefore often supplemented by peristaltic pumps, using the laner for purposes where flow-rate stabilities are less critical, such as filling samples into a sample loop. 

In the pneumatic pumping system, the pressure (and not the flow rate) is maintained constant as variations in chromatographic conditions occur. Thus, a change in mobile phase viscosity (e.g. gradient elution) or column back pressure will result in a change in flow rate for these types of pumps. The gas displacement pump in which a solvent is delivered to the column by gas pressure is an example of such a pneumatic pump. The gas displacement system is among the least expensive pumps available and is found in several low cost instruments. While the pump is nonpulsating and hence, produces low noise levels with the detectors in current use, its flow stability and reproducibility are only adequate. In addition, its upper pressure limit is only 2000 psi which may be too low in certain applications. 

The different adsorption and desorption events are controlled via the flow rates adjusted by the means of 3 or 5 external pumps and the column switch times, Fig. 3. The key element for success is the proper selection of the respective flow rates, which must be chosen in such a way that the extract front between zones I and II and the raffinate front between zones III and IV are stabilized, while the separation between zones II and III is assured. A simple trial-and-error approach to such an optimization of the system parameters is unlikely to be successful. Instead, the chromatographic behavior of all compounds has to be modeled and simulated. 

Stopping this pump allows the solution to stabilize before measurement. After measurement, activation of pump 2 (which runs at a faster flow-rate), cleans the cell of liquid. Two flushes of a sample were found to give a representative measurement even when changing from high to low conductivity in the sample line. The pumps, in this instance, act as a series of valves to wash the cell, set up a representative sample, take the measurement and then wash the cell prior to the next sample. 

A schematic representation of a laboratory apparatus for CDJP is given in Figure l.l.l. In principle, the reacting solutions are introduced into a constant temperature chamber at desired flow rates by means of peristaltic pumps. The predetermined volume of solutions in the reactor may contain stabilizing, reducing, or other agents, or it may be used to control the reaction pH. 

In a microstructured reactor plant, in contrast, the flow rate will be dominated by the pressure loss. Typical pressure losses in micro devices are of the order of 1 bar at a flow of 11 h 1 (water) [50,93], If sufficient pump capacity is available, the pressure loss in a micro structured device is limited by the mechanical stability of the reactor housing, which is often made of steel and hence a loss of several bar is certainly acceptable. Even the combination of up to 10 different micro devices only amounts to about 10 bar in this example. The main advantage of a micro structured reactor plant is that the flow rate can be adjusted more freely because the flow is pressure driven and not influenced by a single gravity-driven device as in a miniplant. 

The amount of electrons extracted from the microplasma, the ionization rate of the sample gas, and the mean free path length depend on the pressure at the different locations of PIMMS-chip. Therefore, exact knowledge and control of pressure and gas flow rates will be necessary for quantitative analysis. Pressure sensors, valves for gas inlets, and vacuum pumps are the components, which have to be read out and controlled, respectively, by electronic and software to install and stabilize appropriate pressure regimes. 

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