Segmented flow analysis

Limitations associated with carryover and large sample volumes became less severe when a washing solution was aspirated between successive samples. This led to the concept of the sample carrier stream. This stream (also called background solution [10]) was introduced after the sample aliquot and pushed it towards the detector, also acting as a wash solution. The required sample volume was thereby drastically reduced. This manifold architecture was a driving force for the inception of segmented flow analysis. 

Segmented flow analysis was conceived in the early 1950 s [11] by Skeggs1 (Fig. 2.2) as a logical consequence of the early developments in flow analysis. Automation at the Veterans Administration Hospital in Cleveland (Ohio, USA) was needed, and Skeggs considered robotic systems at that time too cumbersome to be practical. Therefore, he began to perfect the art of doing chemistry in flowing streams. To this end, his expertise in haemodialysis was also valuable [12], 

The development of air-segmented flow analysis was restricted to a single company (Technicon Corporation Inc.), owner of the main patents until the mid 1970 s. The first air-segmented system marked with the AutoAnalyzer trade name is shown in Fig. 2.3. 

The expression continuous flow analysis and the corresponding acronym CFA have often been used to specify this mode of flow analysis, in accordance with official recommendations [15]. A historical survey 

1Skeggs and Skeggs Jr are written interchangeably in the scientific literature. In this monograph, Skeggs is used. 

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FLOW INJECTION ANALYSIS VERSUS SEGMENTED FLOW ANALYSIS AND SEPARATION TECHNIQUES... 

Skalar Relatively low-cost, recorder output on data analysis by microprocessor also carries out segmented flow analysis Colorimeter, flow cells for fluorometer and ion-selective electrodes available... 

Segmented By aspiration Sequential Continuous Segmented flow analysis (SFA)... 

Kerouel, R., and Aminot, A. (1997). Fluorometric determination of ammonia in sea and estuarine waters by direct segmented flow analysis. Mar. Chem. 57, 265—275. 

Segmented flow analysis relies on three cornerstone features sample aspiration, stream segmentation and reproducible conditions due to precise timing. 

After the intensive development of air-segmented flow analysis, some successful analytical procedures without stream segmentation were proposed, mainly in connection with enthalpimetric [29,30], chem-iluminometric [31] or spectrophotometric [32,33] detection. In these systems the time domain involved was generally short, thus air addition and removal was not performed. Either the sample or the wash solution was continuously aspirated towards the main channel and measurements were made under an almost steady state situation. Without air-segmentation, however, sample throughput was impaired and sample changing was cumbersome. 

As a consequence, a tendency to disregard segmented flow analysis became evident in the early 1980 s. The dominance of unsegmented relative to segmented flow analysis became so pronounced that the phrase Flow injection analysis - the end of the beginning Segmented-flow... 

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, the presence of successive drops of a second immiscible phase inside the flowing sample leads to the formation of vortices that define a circulating flow pattern between two successive solution plugs (see also Fig. 5.2). These vortices improve the mixing conditions and minimise sample broadening. 

There are many variants of analytical flow systems, e.g., segmented flow analysis, flow injection analysis, sequential injection analysis, multisyringe flow injection analysis, batch injection analysis, mono-segmented flow analysis, flow-batch analysis, multi-pumping flow analysis, all injection analysis and bead injection analysis, all of which have acronyms [176]. In view of the existence of several common features, however, all flow analysers can be broadly classified as either segmented or unsegmented, with the most common example of the later mode being the flow injection analyser. 

S. Coverly, Segmented flow analysis, in P.J. Worsfold, A. Townshend, C.F. Poole (Eds.), Encyclopedia of Analytical Science, second ed., vol. 8 Elsevier, Oxford, 2005,... 

C. Riley, B.F. Rocks, Flow-injection analysis - The end of the beginning Segmented-flow analysis - The beginning of the end J. Autom. Chem. 5 (1983) 1. 

D.J. Malcome-Lawes, C. Pasquini, A novel approach to non-segmented flow analysis. Part 3. Nitrate, nitrite and ammonium in waters, J. Autom. Chem. 10 (1988) 192. 

Pulsating flows are beneficial with regard to segmented flow analysis, as the bubble additions are synchronised with the flow pulsation. In order to enhance the pulsating nature of the confluent stream of air to be added, some peristaltic pumps (including the widely used Technicon AAII) incorporate a lifting bar to successively pinch the air delivery tubing. 

Regarding segmented flow analysis, a relatively large de-bubbler is usually required and this device may act as a mixing chamber, thus impairing system performance. This limitation is typical of older instruments. The bubble-gating approach [23] can be exploited to circumvent this drawback, but it has not been universally implemented. 

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. 

This effect is more pronounced in sample regions associated with the rise and fall of the peak. In fact, at the top of the peak, there is very little difference between the instantaneous sample concentration (sample pulse) and the mean concentration inside the illuminated sample portion. The same is true on the baseline. This effect has not been reported in relation to segmented flow analysis, probably because the analytical signal is associated with the flat region of the recorded peak (Fig. 2.5). 

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