Role of valves in non-segmented flow systems

Valves are essential components in non-segmented flow systems, particularly in Flow Injection Analysis (FIA). Although their usual role is associated with sample 'injection' into a carrier stream, this is not by any means their sole use. In hydrodynamic systems, valves must also be regarded as components allowing the manipulation of the flow and affording a high degree of automation of the operation or function they are to carry out. Valves permit simple alterations in the flow-rate to be performed, as well as fully controlling sample treatment (chemical reaction etc.). Sometimes, by coupling the two functions (injection and manipulation of the different flow-rates) extremely difficult problems can be solved. The different types of valves available can be classified according to their performance into the three groups listed in table 1; this includes their potential functions. While rotary valves switch between various streams by a simple turn, commutation valves work by sliding one of the parts with respect to the others. Solenoid valves are more sophisticated and their operation (circulation of the sample through the system for a preselected time) is electronically controlled. As it is sometimes of interest to automate these elements with a minicomputer, the classification shown in table is important because the different categories of interfaces needed in each case is related to their type of movement (or time control). Injection valves Injection valves introduce the sample into the flow system. The term 'injection' is an old-fashioned one, referring to the early systems described by Ruzicka and Hansen [1 and 2], which are based on the use of a needle-less syringe, whose plunger presses and displaces a septum, thus creating a transient chamber which holds the sample. As the septum returns to its original position, the sample is introduced into the carrier solution. These systems were described by Betteridge in a review published in 1978 [3]. A syringe with a hypodermic needle injecting the sample into the carrier through a septum has also been used. Injection proper results in a transient change in the flow which yields a sharp and irreproducible peak. These systems have been superseded by valves and commutator, which allow accurate and reproducible volumes of the sample solution to be inserted (rather than injected) with no disturbance to the flow, the injected volume varying over a wide range. Their easy and fast handling allows their operation to be automated. Rotary valves Rotary valves are commonly used …