The Segmented Flow Analyser

The segmented flow analyser, based on Skeggs original proposal [1], was the first automatic analyser dedicated to serial assays. The approach utilises air-segmented flowing streams and, in principle, permits the successful implementation of any manual protocol for wet chemical assays. 

In the sampling step, small volumes of air are introduced into the sampling tube during its displacement between the different cups of the 

FIGURE 5.1 Diagram of a simple segmented flow system incorporating dialysis. S = 

The presence of an immiscible gas phase in the analytical path results in the bubble pattern shown in Fig. 5.2. Consequently, establishment of 

FIGURE 5.2 Flow pattern inherent to segmented flow analysis. 1 = tube wall of part of a coiled reactor 2 = thin liquid layer adhering to it 3 = vortices inside the liquid phase 4 = air bubble outer large arrow = overall displacement of the segmented flow. Adapted from P.J. Worsfold, A. Townshend, C.F. Poole (Eds), Encyclopedia of Analytical Science, 2nd Edn, Oxford, 2005, v.3, p.24, E.A.G. Zagatto, PJ. Worsfold, Flow Analysis Overview, with permission from Elsevier (Ref. [4]). 

---

In the segmented flow analyser (Fig. 2.4), a sampling arm successively selects the sample or the carrier/wash solution to be aspirated towards the detector, thus establishing the main aqueous stream. A convergent stream of air is thereafter added to promote segmentation. 

The segmented flow analyser is very robust but lacks versatility because its only commuting element is the sampling arm. Moreover, the... 

Conversely, confluence flow injection systems rely on sample insertion into a chemically inert carrier stream and the required reagents are added by confluence. The configuration is characteristic of the segmented flow analyser. The carrier (or background) stream is a solution similar to the sample but without the chemical species under determination. Distilled water, soil extracting solution, ethanol and synthetic seawater are examples of chemically inert carrier streams for the analysis of natural waters, soil extracts, spirits and seawater, respectively. 

Basel et al. [23] have described methods of compensating for chloride, ammonia, and bicarbonate interferences in determining bromide in sabne waters with an automated segmented flow analyser utihsing the phenol red method. 

TTie major features of a determination carried out on an automatic segmented-flow analyser, namely precision and rapidity, are highly influenced by technical factors such as the extent of carry-over and mixing of reactants, and the time during which the reactingplug remains in the system. 

FIGURE 2.2 The first prototype of a single-channel segmented flow analyser from May 1951. Reproduced with permission of Hindawi Publishing Corporation from "R. Stanley, Journal of Automatic Chemistry, 6 (1984) 175". 

FIGURE 2.4 Flow diagram of a single-channel segmented flow analyser and the associated recorder tracing. S/C — sample/carrier wash stream Air = air R = reagent R( = coiled reactor DB — de-bubbler D = detector arrows = sites where pumping is applied. 

Passage of the sample through the detector causes a variation in the monitored radiation, and the related transient signal is recorded as a peak which is ideally proportional to the analyte concentration in the sample. Figs 2.5 and 5.5 show typical recorder tracings from a segmented flow analyser, where a tendency towards a plateau is observed as a consequence of the low sample axial dispersion involved. Important parameters related to the recorded peak shape are the lag phase, peak width at... 

FIGURE 5.6 The segments established in a segmented flow analyser. 1 = tube wall 2 — air bubble 3 = thin liquid film on the tube wall 4 and 5 = liquid segments with indication of the vortices (rounded arrows) large arrow = overall flow movement. 

Manual operation of the time-based flow analyser is also feasible, but the aspirated sample volume should always be in excess in order to guarantee the establishment of the flat peaked signal typical of segmented flow analysers (see e.g., Fig. 2.5). In this way, variations in the sample volume introduced do not affect the recorded peak height. Carryover should be kept to a minimum, as its compensation is not feasible. 

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. 

A U-shaped glass tube was used in the first segmented flow analysers with spectrophotometric detection but is now rarely used due to the pronounced radiation losses at the curved portions and in the cylindrical walls of the tube. A single tube axially traversed by the radiation incident beam can be used instead of a typical flow cell in situations requiring a short optical path, as originally demonstrated in the spectrophotometric determination of the major constituents of fertilisers in a flow injection system [84] this approach avoided the need for manual sample dilutions. 

When sensitivity is critical, sample dilution should be minimised moreover, sufficient time should be available for sample handling if the physico-chemical processes involved are relatively slow. These requirements are fulfilled in a segmented flow analyser with the confluent stream flow rates set as low as possible in order to minimise sample dilution at the confluent sites (cf. Eq. 3.10). Another option is the use of a mono-segmented flow analyser which permits long sample residence times to be achieved without excessive sample dispersion. 

The flow system is designed to permit different reagent streams to be successively added by confluence. This is the essence of segmented flow analysers and is inherent in unsegmented flow analysers designed in the confluent configuration. 

Initial attempts to perform in-line digestion of plant tissues were made inl960 [122]. The Kjeldahl digestion procedure was implemented in a segmented flow analyser by adding the solid sample and the digestion mixture (500 mL L 1 sulphuric acid + 3.0 mL L-1 perchloric... 

Exploitation of dialysis in flow analysis dates back to the 1950 s when the original air-segmented flow analyser was conceived (see 2.2). In 1963, a dialysis unit with a planar membrane was commercially available [279]. The first application of dialysis in unsegmented flow analysis dealt with... 

Segmented flow analysers using a bubble gating flow cell (Fig. 5.4) for monitoring the sample zone without the need for mechanical removal of the air phase [405] can also be considered as examples of flow systems exploiting a feedback mechanism that are not expert flow systems. 

---