Confluence point

Distance from the confluence point of sample and reagent (mm)... 

Active stream sediment represents the fine- to medium-grained bed load material (silty-clayey-sandy), which is transported by running water. The active stream sediment is collected from the small, second order, drainage basin (< 100 km2) at a suitable site above its confluence point with the main, third order, channel of the large drainage basin. 

Fig. 6.15. Experimental set-up for high-pressure liquid-liquid extraction. LS liquid sample, SR solvent reservoir, PP piston pump, HPP high-pressure pump, CP confluence point, EC extraction coil, R restrictor. (Reproduced with permission of Springer-Verlag.)...

A quantitative description of the influence of a confluent stream on sample dispersion is given below. For simplicity, radial mixing is considered as a very fast process, thus the slices perpendicular to the tubing walls can be considered as the fluid elements. Under ideal mixing conditions, the concentration Ci of a chemical species in a fluid slice inside a flowing solution (Fi) immediately before a confluence point (Fig. 3.8) is modified to C according to [27,59] ... 

Accordingly, the axial length (mean thickness) of the slice, X, is enlarged to x after passing the confluence point [27,59] ... 

The passage of a moving slice through a confluence point results in a decrease in its concentration and an increase in its mean thickness. Ideal flow-through detectors placed immediately before and after the confluence point would register peaks with widths at half the peak height equal to a and o, respectively. Without dispersion, Eq. 3.12 can be converted to [27] ... 

The continuous sample dispersion process occurring while it passes through the confluence point should also be taken into account. However, expansion of Eqs 3.12 and 3.13 to consider all the slices along the entire flowing sample, as well as those in the confluent stream that interact with the sample, has not yet been reported. 

PASSING A CONFLUENCE POINT The X pie-chart permits a rapid evaluation of the effects of a confluent stream addition. Analysis of... 

FIGURE 3.11 Passing a confluence point. Volumetric pie charts showing the situations before (left) and after (right) a 1 1 (v/v) confluence. Symbols as in Fig. 3.10. 

The reagent is added to all portions of the sample zone at the confluence point consequently, no time interval is required for the reagent to reach the central portion of the sample zone. The confluence configuration is therefore attractive for analytical procedures based on relatively slow chemical reactions. Moreover, the configuration is also useful in relation to very fast processes such as those inherent to chemi-and bio-luminescence reactions, where the confluent reagent is usually added very close to the detector. 

Several reagent solutions can be added at different confluence points. 

Intermittent stream addition is often exploited to achieve merging zones [106], to speed up the flushing time in a flow analyser [107] or to implement multi-analyte determinations [108]. At the confluence point, the sample/carrier stream concentrations undergo sudden variations caused by the addition of the merging stream (Eq. 3.10). Consequently, the Schlieren effect can manifest itself as a spurious transient signal, usually observed before sample arrival in the detector flow cell. This effect is more likely to occur in highly versatile flow systems such as multi-commuted flow systems [109]. 

The key to controlling sample dispersion in air-segmented flow analysis is the addition of confluent streams. After the confluence point, an increase in the length of each plug (Eq. 3.12) leads to an increase in the length of the entire sample zone. This does not result in a broader recorded peak, however, because the sample linear speed is... 

A. Fernandez, M.A. Gomes-Nieto, M.D. Luque de Castro, M. Valcarcel, A flow-injection manifold based on splitting the sample zone and a confluence point before a single detector unit, Anal. Chim. Acta 165 (1984) 217. 

A, single line B, two-line with a single confluence point C, reagent premixed into a single line D, two-line with a single confluence point and reagent premix E, three-hne with two confluence points. 

Another area worth attention is the part of the A-i/r signature near the starting point, which marks a confluence point for all the RIs, where discrimination of this property is low (Fig. 8). In this case, further information is... 

The theory of physical kinetics relevant to FIA allows in its present form identification of the major parameters governing the sample zone dispersion, but its limitations are still serious. It does not describe exactly even the simplest one-line FIA system and is limited to description of a zone that moves at constant speed. Acceleration of the sample zone during injection, changes of flow velocity and dispersion patterns at confluence points, where additional streams are joining the main stream, or dispersion when the flow is stopped and accelerated, have to be investigated in greater detail (cf. Section 3.2). 

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 ... 

Figure 4.12. (a) Flow-injection manifold for the enzymatic stopped-flow determination of alcohol. Samples (30 jlL) are injected into a carrier stream of pyrophosphate buffer of pH 8.7. The second channel delivers the same solution to which alcohol dehydrogenase has been added. The broken lines indicate those parts of the manifold that are thermostated. The coil between the confluence point of the reagent streams and the detector allows for a suitable dispersion of the sample zone prior to the stop sequence, (b) Recorder output for the stopped-flow determination of alcohol in blood. To the left is shown a series of six aqueous standards (STD) (0-32.0 mg/L) followed by 10 blood samples and a repetition of the six standards, all analyzed in duplicate stop time, 14 s delay time, 13 s total measuring cycle, 40-45 s. 

To avoid this phenomenon, the sample is injected into a stream of water, which is then merged with the combined reagent stream of molybdate and ascorbic acid. Furthermore, to obtain effective mixing without excessive dispersion of the sample solution, either the streams are conflu-enced as shown in Fig. 4.9 or an imprinted meander (in a microconduit) is inserted immediately following the confluence point, permitting the use of a short reactor before the flowthrough cell. 

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