Split and Nested Sample Loops

The external sample loop of a rotary valve may, beside serving as a simple volumetric cavity accommodating a selected sample volume, be split or nested in order to perform more sophisticated functions (Fig. 5.10). So far only individual components, such as columns, dialyzers, and gas-diffusion units, have been nested in the loop, but it is likely that in the future a series of components, perhaps even an entire FI A microsystem, will become nested in a loop to perform a complex assay. Although a four-port rotary valve scheme was used in this section to review the techniques, it should be kept in mind that a sample loop can be split or nested equally well when mounted in a six-port valve—as implied in the previous section (Fig. 5.8)—or in the Brazilian commutator [1060]. Among many such schemes, new exciting techniques will undoubtedly be discovered. 

In the volumetric sample injection (Fig. 5.10a) the sample loop has its simplest function, that is, merely to meter the volume of the analyte to be injected. The next step is to inject the reagent and analyte simultaneously with the purposes discussed previously in Chapters 2 and 4. This can be done in two ways (1) by nesting another loop by means of a second valve in the way shown in Fig. 5.10/ , or (2) by splitting the sample loop as shown in Fig. 5.10c. 

The nested packed reactor (Fig. 5A0d) allows sample pretreatment prior to injection by means of solid oxidants, reductants, ion exchangers, immobilized enzymes, or suitable surface-active sorbents. The potential of this approach is largely unexploited, since so far such sample pretreatment has been used only to remove unwanted matrix, which is not retained on the column, for sample preconcentration, and for analyte conversion in connection with AAS and ICP (cf. Chapter 4.7). 

The nested detector (Fig. 5.10/) will find its use whenever an intrinsic property of the sample should be measured prior to its further treatment in a FIA system. This may be the determination of the original pH value of the sample, its conductivity, color, or content of solid particle or blood cell. The nested detector may also be tuned to a given value of a selected parameter, which, when reached, will trigger the valve to inject the entrapped portion of the analyte into the FIA system for further investigation. 

Inherently, all injection valves have one thing in common, that is, they have moving parts that might eventually become worn. Yet, the recent generation of FIA valves, made of a suitable combination of polymers, have proven very reliable in continuous use—in fact, van der Linden [1059] has reported that rotary valves subjected to exhaustive tests even after 50,000 switches still functioned properly, and no wear or tear was noticeable provided that the sample and carrier stream were virtually free from particles. Even better performance is feasible with valves if powered by stepper motors, which results in superior reproducibility of injection and excellent duration. Nevertheless, it would, particularly for special applications, be beneficial to be able to meter the sample volume and subsequently transfer it reproducibly into the carrier stream by means requiring no mechanically moving parts. 

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