Segmented flow analysis characteristics

In 1985, mono-segmented flow analysis was proposed [64] as a means of achieving extended sample incubation times without excessive sample dispersion. The sample was inserted between two air bubbles into an unsegmented carrier stream therefore the innovation combined the favourable characteristics of both segmented and unsegmented flow systems. Further development revealed other potential applications, especially with regard to relatively slow chemical reactions, flow titrations, sample introduction to atomic absorption spectrometers, liquid-liquid extraction and multi-site detection (Chapters 7 and 8). This innovation was also referred to as segmental flow injection analysis [65]. 

In segmented flow analysis, axial sample dispersion is not pronounced, being influenced mainly by the characteristics of the thin liquid film established at the tubing inner wall and by the number of segments per sample (see 5.1.2). Reduction in sample concentration is therefore strongly dependent on the addition of confluent streams. Hence, the flow rates of the sample/wash and confluent streams are the main parameters determining sample dispersion, and application of Eqs 3.10 and 3.13 can then make this practical index readily available. 

The classical manifold architecture is fully compatible with segmented flow analysis. It has also been exploited in flow injection analysis, but the inherent segmentation alters the characteristics of the analyser [163]. 

Multi-commuted flow analysers are characterised by enhanced flexibility and easy operation and combine some of the attractive characteristics of both segmented and unsegmented flow analysis. In this way, segmented flow, flow injection and sequential injection analytical procedures are efficiently implemented in multi-commuted flow manifolds. 

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. 

An overview of the different types of discontinuity used in automatic methods and their characteristics is presented in Table 7.1. The most common discontinuity in discrete and robotic methods is the absence of flow, which involves keeping the samples in separate vessels for measurement. On the other hand, automatic continuous methods use very different kinds of discontinuity or do not use one at all. The discrete nature of segmented methods is determined by the presence of bubbles and wash cycles as a means of avoiding carryover, whereas that of unsegmented methods is dictated by the manner in which the sample —and reagent— is introduced into the system. There is only a single type of method using no discontinuity completely continuous flow analysis (CCFA). 

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