Flow impedance

The second valve controls a sample loop, 5 cm long and 1 mm in diameter, packed with dimethyloctadecyl reverse phase comprising of fairly coarse particles 100-120 im in diameter to reduce flow impedance. The sample pump is supplied via a two-way tap from either of two reservoirs, one containing pure water and the other, normal saline. The output of the pump can be used to either force the contents of the open loop sample tube through the packed loop, or to permit washing with an appropriate solvent. The separate pump is necessary to overcome the impedance of the packed loop. 

In the course of this work two phenomena were noted for which no explanation was sought. When ammonium bicarbonate was added to sea water to prepare a regenerant solution, a white precipitate was formed which was settled out and discarded with no attempt made at chemical analysis. Toward the end of some of the regeneration cycles and at the start of some of the feed cycles, gas was evolved in the ion exchange columns, which eventually disappeared. This gas, which was not identified, caused serious flow impedance. 

The flow rate for pressurized delivery is determined by the difference in the source and destination pressures and any impedance to flow in the delivery path. The flow rates for a positive displacement device are generally quite accurate and reliable and should not change significantly with small variations in the blanketing pressure and the flow impedance of the system. 

The differences in objective and principle between HPLC and FI separations bring major dissimilarities in the related equipments. The FI flow systems produce much less flow impedance during operation, even when packed columns are implemented. Columns are much shorter, and packing materials are much coarser when they are used to serve FI separation purposes. This in turn substantially lowers the requirement to withstand high pressures for the valves and pumps of a FI system, so that high pressure pumps such as those used in HPLC are not necessaiy. On the other hand, the high versatility of FI often demands multi-functional valves, and multi-channel pumps. These differences may be better understood after comparing the related instrumentation described in Chapter 2 to standard HPLC equipment. 

Despile the basic nature of its function in most FI systems, users of peristaltic pumps often do not operate them properly, including people who have used them for years. Since its proper usage is of primaiy importance, particularly in many on-line separation sy.stems where flow impedances are often significantly higher than normal FIA systems, it ma be necessary to stress the important points here, and provide some useful hints for their correct manipulation. 

The pressure on the pump tubes should be readjusted when the type or number of pump lubes or the flow manifolds are changed. Periodical readjustments should also be made when pump tubes gradually age under usage. Note that pressure adjustments optimized for open ended pump tubes (i.e. without load) may have to be readjusted when the pump tubes are later connected to a manifold, particularly when the flow impedance of the manifold is high, as when packed columns are used in the system. Some operators tend to overtighten the screws to make sure that the flows will not be influenced by a change in impedance. However, this may significantly shorten the lifetime of the pump tubes as well as deteriorate the reproducibility of the flow due to panial or complete adhesion of the inner wails of the pump tubes under excessive pressures. 

In order to balance the pressures on the two sides of the channels, the channel with the lower pressure (usually the donor stream) can be connected to a flow restrictor located downstream of the dialyzer. The restrictor is simply a length of thin tubing. Such measures may become necessary when one channel, usually the acceptor stream, is connected with packed reactors which significantly increase the flow impedance. On the other hand, in order to minimize clogging of the membranes, an increase in pressure on the acceptor side may be necessary when samples containing suspended sediments (e.g. urine) are analyzed, possibly with some sacrifice in the dialysis efficiency. 

Capable of collecting a few milligrams of precipitate without developing prohibitive flow impedance which could reduce a sample and reagent flow of 4-S ml min deliverecPthrough a peristaltic pump. 

The split-flow injector is very similar to the packed column injector except that only part of the carrier flow passes to the column, the rest exits to waste. By varying the exit flow-impedance, the split-ratio can be adjusted over a wide range. Without this type of split injection system, the small bore capillary columns would be virtually impossible to use. However, because of the waste of sample and the relatively small mass range obtainable from small bore columns, the large bore capillary column was introduced. 

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