The Flowing Sample

After the introduction of a selected sample volume into a flow system, it is transported through the analytical path, and subjected to several physical and chemical processes such as reagent addition, chemical reaction and dispersion. Understanding how these processes affect the flowing sample and the formation of the chemical species to be monitored is essential for optimising manifold design and, hence, analytical performance. To this end, it is important to consider the flow pattern and how to modify it by varying experimental parameters. 

In this section, the flow pattern is discussed in terms of the flow regime, composition of the flowing stream and flow rate. Strategies for modifying the flow pattern are also presented. 

In 1883, Reynolds demonstrated that the transition from laminar to turbulent flow in a tube is associated with a dimensionless quantity called the Reynolds number, Re [1]  

FIGURE 3.1 Velocity distribution in laminar (upper) and turbulent (lower) flow. The 

Under laminar flow conditions, Poiseuille flow must be considered [5]. This means that the linear velocity of the fluid element at the centre of the tube is about twice the average linear flow velocity ( I ), whereas the velocity of fluid elements adjacent to the inner walls of the tubing 

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Departures from laminar flow, which are attributed to inertia and/or viscoelasticity, result in turbulences, i.e., an uneven flow pattern with locally clear deviations from the flow direction. In the extreme, the flowing sample can start to circulate locally, which is known as Taylor vortices and mainly observed in concentric cylinder instruments, where the inner cylinder rotates,i.e., in cup and bob viscometers. ... 

In this test, water quality was analyzed along the flow path, upon contact with dechlorinating chemical. A flow rate of 100 gpm was maintained, and one or four tablets were placed across the flow. Samples were collected at the point of release and 40, 80, 120, and 160 ft downstream of the tablets. The residual chlorine concentration in the water decreased with distance (Fig. 4). One tablet was sufficient to remove chlorine to below 0.1 mg/L after 120 feet of travel under the test conditions. 

As a consequence of dispersion, interaction of the sample with the reagents is improved, allowing precise analytical results to be obtained. Moreover, concentration gradients are established along the flowing sample, and their exploitation expands the application range of analytical... 

Stream sectioning into a number of small segments reduces broadening of the flowing sample, and plays a beneficial role in the mixing process, reducing intermixing of the sample with the carrier stream and between successive samples. 

The maximum signal is associated with the less dilute portion of the flowing sample and is often referred to as the plateau region. The recorded peak shape shows a tendency towards this steady state situation, as well as a slight axial dispersion of the sample zone. As segmentation is involved, a small sample aliquot is enough for the sample to reach the detector with its central portion almost undispersed. All of the above-mentioned features have a positive influence on sample throughput. 

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. 

FIGURE 3.3 Didactic representation of unsegmented (a), segmented (b), tandem (c) and mono-segmented (d) streams. The figure refers to the front portion of the flowing sample, with exception of (d) which refers to the whole sample. Black = sample white = carrier/wash stream oval shapes = air phase arrows = flow direction. 

Sample dispersion is altered when the flowing sample merges with a confluent stream [27]. At the vicinity of the confluence site, convective mass transport is strongly altered by the sudden change in concentrations caused by the convergence of the confluent and sample carrier streams. The laminar flow regime tends to be maintained therefore, interactions between the sample carrier stream and the confluent stream are mainly dictated by radial diffusion. 

In flow injection analysis, the first index proposed for this purpose was the dilution factor, D [7], further defined as "the ratio of concentrations before and after the dispersion process has taken place in the element of fluid that yields the analytical readout" [114]. This index has also been called dispersion number, dispersion D, dispersion value, dilution ratio and dispersion coefficient [115], and the latter term has been generally accepted. Its reciprocal was called the dispersion factor [116]. The dispersion coefficient also holds for flow injection systems with reagent injection into the flowing sample [117]. 

The concept of temporal variations in concentration at the flow-through detector explains why pronounced skewed peaks are often observed in flow analysis, especially with loop-based sample introduction. Taylor assumed that dispersion is symmetric in relation to an observer located at the dispersing zone [55,56], but in practice the recorded peaks are usually characterised by a rise time much shorter than the fall time (see also Fig. 1.3e). This skew effect is explained by the fact that the front and trailing portions of the flowing sample, which relate to the rise time and the fall time, respectively, have different residence times in the manifold and are therefore subjected to different extents of dispersion. 

The shape of the recorded peak does not match the true axial distribution of the analyte along the flowing sample. Therefore, expressions such as peak profile, analyte distribution and concentration gradients (typically associated with spatial variations) should be avoided. 

Recently, webcam images of the flowing samples were obtained [133] in order to emphasise the main differences between sample dispersion in laminar or pulsed flows (Fig. 3.14). 

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