The analytical manifold

As in the manual determination methods, one or more reagents are added sequently to the sample and mixed with it. After formation of the coloured component, the solution passes through a spectrophotometer cuvette. 

The trick of the CFA is the insertion of air bubbles into the flow which separate the liquid phase into small segments. These liquid segments act as small subsample containers and considerably reduce the mixing. The mixing zone between conscutive samples is restricted to S-10 liquid segments. 

Recommended air/liquid ratios are about 1 4 for standard tubing systems with air being removed before entering into the spectrophotometer and about 1 10 for capillary systems and systems without bubble separators. The liquid segments must be long enough to fill the spectrophotometer cuvette bubble-free for a minimum spectrophotometer reading interval. 

The schematic manifolds presented in Sections 10.2.5-10.2.11 use symbols for the manifold components. The symbols are explained in Fig. 10-19. 

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The instrumentation used for FIA with CL detection is usually simple and is composed of the components depicted in Figure 2. These components are readily assembled to form the analytical manifold, although there are also commercially available flow injection systems with CL detection. Spectrophotometric or fluo-rimetric flow injection systems can often be used for CL measurements after some modifications. 

The TRAACS 800+ shown in Fig. 2.18 consists of one or more analytical consoles, a random-access sampler and personal computer (PC). The analytical console is fitted with two peristaltic pumps which can each handle up to 10 pump tubes. The analytical manifolds are located above the pumps and utihze glass parts 1 mm in diameter, in order to keep reagent consumption low. Heating baths, UV digesters and dialysers can be built into the manifold. 

The tubing material that connects CFA systems is either glass or Teflon tubing. Tubing is used for the mixing coils that make up the analytical manifold, which is the junction box that contains all the connections between the different streams. The output from the manifold is then passed through a flow-through detector. 

Analyser systems are available commercially but may also be constructed from single components according to individual requirements. A typical system for the automated analysis of seawater constituents consists of a sampler, proportioning pump, the analytical manifold (a delay and reaction system), a flow-through spectrophotometer and a data acquisition system. Fig. 10-13 shows a 4-channel CFA system built and used in the Institute of Marine Research in Kiel (Germany). One example manifold (nitrate) including a flow-through-spectrophotometer is displayed in Fig.10-14. 

Seawater samples are often collected at temperatures below the laboratory temperature. Consequently, small gas bubbles tend to separate out while warming up. These bubbles enter the analytical manifold and disturb the spectrophotometric detection. If the air segments are removed in front of the cuvette, the random micro-bubbles will also be removed. When a spectrophotometer is used which allows the air segments to pass through the cuvette, a bubble separator has to be inserted at the very beginning of the manifold just after the sample inlet. The bubble separator removes random and micro-bubbles before the defined air segmentation. 

A single-channel manifold also can be used for systems in which a chemical reaction generates the species responsible for the analytical signal. In this case the carrier stream both transports the sample to the detector and reacts with the sample. Because the sample must mix with the carrier stream, flow rates are lower than when no chemical reaction is involved. One example is the determination of chloride in water, which is based on the following sequence of reactions. ... 

The actual properties of this transformation combined with the convergence properties of molecular electron densities implies analyticity almost everywhere on the compact manifold. Consequently, this four-dimensional representation of the molecular electron density satisfies the conditions of a theorem of analytic continuation, that establishes the holographic properties of molecular electron densities represented on the compact manifold S3. 

Figure 2.5. Analytical manifolds for the determination of phosphate by flow injection analysis (a) and reverse flow injection (b). The symbols S, M, and A are the seawater, mixed reagent, and the ascorbic acid solutions. The pump injection valve and detector are represented by P, I, and D, respectively. W = waste. From [177]... 

FTA [5-7] is a version of continuous-flow analysis based on a nonsegmented flowing stream into which highly reproducible volumes of sample are injected, carried through the manifold, and subjected to one or more chemical or biochemical reactions and/or separation processes. Finally, as the stream transports the Anal solution, it passes through a flow cell where a detector is used to monitor a property of the solution that is related to the concentration of the analyte as a... 

In some applications, additional components acting as reactors for specific chemical pretreatment are incorporated within the flow manifold. Typical examples are ion-exchange microcolumns for preconcentration of the analyte or removal of interferences and redox reactors, which are used either to convert the analyte into a more suitable oxidation state or to produce online an unstable reagent. Typical examples of online pretreatment are given in Table 2. Apart from these sophisticated reactors, a simple and frequently used reactor is a delay coil (see also Fig. 4), which may be formed by knitting a segment of the transfer line. This coil allows slow CL reactions to proceed extensively and enter into the flow cell at the time required for maximum radiation. The position of the reactors within the manifold is either before or after the injection port depending on the application. 

The simplest flow injection arrangement is the single-line configuration shown in Fig. 13.9. As can be seen, the carrier stream is propelled by a pump through a narrow conduit to an injector, then via the reaction manifold to the detector and finally to waste. Each injection yields a single peak at the detector, with the height, area, or width proportional to the analyte concentration. 

A typical extraction manifold is shown in Figure 13.2. The sample is introduced by aspiration or injection into an aqueous carrier that is segmented with an organic solvent and is then transported into a mixing coil where extraction takes place. Phase separation occurs in a membrane phase separator where the organic phase permeates through the Teflon membrane. A portion of one of the phases is led through a flow cell and an on-line detector is used to monitor the analyte content. The back-extraction mode in which the analyte is returned to a suitable aqueous phase is also sometimes used. The fundamentals of liquid liquid extraction for FIA [169,172] and applications of the technique [174 179] have been discussed. Preconcentration factors achieved in FIA (usually 2-5) are considerably smaller than in batch extraction, so FI extraction is used more commonly for the removal of matrix interferences. 

In the continuous processing of discrete samples in the AutoAnalyzer system, the reaction-time is held constant by the manifold design, and because the rise-curve is exponential the degree of attainment of steady-state conditions is independent of concentration. Consequently it is unnecessary for the analytical reaction to proceed to completion for Beer s Law to be obeyed. This confers a considerable advantage upon the AutoAnalyzer approach and one which is frequently emphasized. The relationship between degree of attainment of steady state and IT,/, can be generaHzed in the semi-logarithmic plot of Fig. 2.16 [10], where time is expressed in units of IT,/,. 

Solvent extraction can be automated in continuous-flow analysis. For both conventional AutoAnalyzer and flow-injection techniques, analytical methods have been devised incorporating a solvent extraction step. In these methods, a peristaltic pump dehvers the hquid streams, and these are mixed in a mixing coil, often filled with glass ballotini the phases are subsequently separated in a simple separator which allows the aqueous and organic phases to stratify. One or both of these phases can then be resampled into the analyser manifold for further reaction and/or measurement. The sample-to-extractant ratio can be varied within the limits normally applying to such operations, but the maximum concentration factor consistent with good operation is normally about 3 1. 

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