Sample nested loop

Precise introduction of a well-defined portion of the aqueous sample into the carrier stream is fundamental to flow analysis. This is generally accomplished by continuously pumping the sample into the manifold for a fixed period of time (time-based introduction) or by inserting the sample aliquot via a fixed-volume loop attached to a rotary or sliding bar valve (loop-based introduction). Hydrodynamic [48] and nested [49] sample introduction have also been used. 

Nested injection involves the insertion of a sampling loop into a second loop which is simultaneously inserted into the main channel of the flow... 

In order to accomplish nested introduction, two loop-based rotary valves with coincident movements [64] or a two-section injector—commutator have been used (Fig. 6.13). In the load position, specified in the figure, the first and second loops are simultaneously filled with the sample and with the reagent (or air, if a mono-segmented flow analyser is used). Switching the injector inserts the selected sample volume between two reagent (or air) plugs into the carrier/wash stream, and the complex zone established is directed towards the detector. 

FIGURE 6.13 Schematic representation of nested injection. The Figure refers to the load position. S = sample L = sampling loop R = reagent (or air) C = carrier/wash stream W = waste V = reagent recovery vessel M = towards manifold IC = injector-commutater shaded area = alternative position of the central sliding bar. For details, see text. 

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. 

Fig. 6 FI manifold with gas-diffusion separator nested in sample loop of the injection valve used for preconcentration of volatile species by time-bas sampling (sample loading sequence). AS, autosampler, T, heating thermostat (optional) CDS, gas-diffusion separator, V, injection valve Ri, reagent for generation of volatile species R2. acceptor reagent stream R3. derivatization reagent (optional) D, detector W, waste a, valve position in sample injection sequence. Crossed circles in valve represent blocked channels [20].

Fig.6J Schematic diagram of a FI dialysis system with sample circulation (valve in loading position). PI. P2 pumps DS, membrane dialyzer nested in sample loop A, acceptor stream R, reagent D, detector W, waste 111].

Like with continuous aleatory uncertainties, the influence of epistemic uncertainties is considered by Monte Carlo (MC) simulation (see chapter 2). The approach applied to account for both types of uncertainties is a double loop nested MC simulation Sets of values of the parameters subjected to epistemic uncertainty (epistemic variables) are sampled in the outer loop. For each of these sets, different sets of values for the continuous aleatory variables are sampled and contribute to the generation of different Dynamic Event Trees... 

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