Two- and Multiline FIA Manifolds

The greatest asset of the single-line manifold is its simplicity, which permits the use of simple means of propelling the carrier stream. Therefore, even a constant head or a gas pressurized reservoir [102, 253], which both yield a pulse-free stream allowing excellent reproducibility of measurement, may be employed. However, for all other FIA systems, two or more well-controlled channels are required and therefore a peristaltic pump is the best choice when designing a flexible FIA system. 

Sequential use of only two reagents is a simple matter because a manifold for this purpose involves only one confluence point at which a second reagent is added to the sample zone when carried past by the stream of the first reagent. An example of a more complicated procedure, where three reagents are used sequentially, is the determinations of urea, based on the following reaction sequence  

Two-line and multiline manifolds are, of course, now commonplace for FIA methods. In fact, most of the procedures described in the FIA literature (Chapter 7) utilize this approach. Thus, in Ref. 52 is described a turbidimetric procedure for the determination of ammonia in low concentrations with the use of Nessler s reagent, while Ref. 253 recounts the spectrophotometric determination of chromium(VI). Besides being based on one-phase equilibria, multiline manifolds may also involve gas diffusion, solvent extraction, and liquid-liquid phase reactions in packed reactors (see the following sections). It should be emphasized, however, that a FIA system should always be kept as simple as possible, and that a well-designed chemical analysis will often require only the use of a two-line manifold. 

As discussed in Sections 2.2 and 2.3, any FIA readout is the result of two simultaneously occurring kinetic processes, that is, that of the physical dilution process and that of chemical reactions. Provided that the reaction rates of the chemical processes taking place vary significantly, these might be exploited for the purpose of kinetic discrimination. One such example is the determination of chlorate by reduction with tita-nium(III) in the presence of leukomethylene blue (LMB), according to the following reaction scheme [986]  

The assay is performed by injecting a sample of chlorate into an acidic carrier stream of titanium(III), which is subsequently merged with a second stream of leukomethylene blue. While the first two of these reactions are very fast, the reduction of the blue colored species MB by the third reaction is slow, that is, the chlorate concentration might readily be quantified via the absorbance due to the color of the MB species generated by reaction 2. 

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