Flow-injection analysis sample dispersion

Effect of dispersion on a sample s flow profile at different times during a flow injection analysis (a) at injection and when the dispersion is due to (b) convection ... 

This technique differs from flow injection analysis in the sense that whereas in the latter technique the sample plug is injected into a flowing stream of reagent, in the former technique plugs of reagent are injected into a continuous stream of the sample. Under these conditions the amount of sample in the zone of the reagent will increase as the dispersion increases. The sample will become well... 

A more recent development is a technique known as flow injection analysis, in which a discrete volume of a liquid sample is injected into a carrier stream. Reagents required for the development of the analytical property of the analyte, e g. colour developing reagents for spectrophotometry, are already present in the stream. The stream then flows straight to the detector and the technique depends upon the controlled and reproducible dispersion of the sample as it passes through the reaction zone. Thus the reaction does not necessarily need to develop to completion,... 

Gine et al. [29] has described a semi-automatic determination of manganese in plant digests using flow injection analysis. This technique utilises the introduction of the sample into a continuously flowing carrier stream of formaldoxime reagent. When injected, the sample is pushed by this stream and dispersed into the reagent stream, whereupon the required reaction takes place. The coloured complex is then carried into a spectrophotometric flow cell, where the absorbance is measured after an exactly defined time interval. 

Fig. 19. Schematic design of a flow injection analysis (FIA) system. A selection valve (top) allows a selection between sample stream and standard(s). The selected specimen is pumped through an injection loop. Repeatedly, the injection valve is switched for a short while so that the contents of the loop are transported by the carrier stream into the dispersion/reaction manifold. In this manifold, any type of chemical or physical reaction can be implemented (e.g. by addition of other chemicals, passing through an enzyme column, dilution by another injection, diffusion through a membrane, liquid-liquid extraction, etc. not shown). On its way through the manifold, the original plug undergoes axial dispersion which results in the typical shape of the finally detected signal peak...

In flow injection analysis [32] with electrochemical detection a sample is injected into an electrolyte carrier stream dispersion of the sample plug into the carrier stream occurs so that electrolyte is effectively added to the sample—with consequent sample dilution—before reaching the electrode. Even so, by using a capillary flow injection system nanolitre sample volumes can be investigated [33]. In continuous flow systems, electrolyte often has to be added to the sample beforehand, also leading to sample dilution. 

Flow injection analysis (FIA), which was introduced by Ruzicka and Hansen (iz ) and by Stewart et al (iQ), is based on the concept of controlled dispersion of a sample zone when injected into a moving and nonsegmented carrier stream. In continuous flow analysis (CFA), successive samples are mixed and Incubated with reagents on the way toward a flow through detector. The greatest difficulty to overcome in CFA was intermixing of adjacent samples during transport from the injection valve to the detector. In the past, it was widely believed that there are only two ways to prevent carryover in CFA either by the use of turbulent flow or by air... 

Flow analysis has often been referred to as an analytical technique, but this is not strictly true, as it is an advanced procedure for carrying out automated chemical assays. The cornerstone features inherent to flow injection analysis, namely sample insertion, controlled dispersion and reproducible timing [5], are considered here in a broader context, in order to encompass the different modes of flow analysers. 

One can then infer that flow injection analysis relies on three cornerstone features sample injection, controlled dispersion and reproducible timing [50]. A typical flow injection analyser and the related recorder output are shown in Fig. 2.7. 

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 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]. 

In single-line (also called straight or single channel) flow systems, the required reactants are present in the sample carrier stream and are added to the sample zone as a consequence of dispersion. The configuration is associated with the inception of flow injection analysis and is characteristic of the sequential injection analyser. Flow injection systems comprising two or more streams that converge to form the main carrier stream into which the sample is inserted [134] are also considered as single-line flow systems. 

T. Korenaga, F.H. Shen, H. Yoshida, T. Takahashi, K.K. Stewart, Study of sample dispersion in flow injection analysis, Anal. Chim. Acta 214 (1988) 97. 

As the sample plug starts being pushed forward, axial diffusion is the main component of the dispersion process, due to the high concentration gradients at the sample/carrier stream interface. The hypothetical peak shape associated with the flowing sample is shown in Fig. 5.9b, which corresponds to the first theoretical Taylor solution [28,29] for the diffusive-convective equation (Eq. 3.4). Situations associated with Fig. 5.9a,b never occur in practice in flow injection analysis. 

The injected sample volume is by far the most important parameter affecting dispersion in flow injection analysis, as it has a pronounced influence on the height, width and area of the recorded peak (Fig. 5.11). 

Another parameter affecting sample dispersion in flow injection analysis is the way that the sample is inserted. The influence of Vs on the shape of the recorded signal is, however, relatively independent of the mode of sample insertion, and this is perhaps the main reason why this aspect is rarely reported. 

Most flow injection analysers exploit either loop-based or time-based injections (see 6.2.2), and the main difference between these injection modes is the shape of the initial sample plug. A critical comparison of these strategies is given elsewhere [85]. This aspect is of minor concern as a parameter affecting sample dispersion in flow injection analysis. 

The length of the analytical path plays an important role in the extent of sample dispersion in flow injection analysis. Increasing this length decreases the recorded peak height and increases sample broadening, with these effects being more evident for lower path lengths (Fig. 5.13). 

The presence of artefacts in the analytical path, such as mixing chambers, tubing connections, de-bubblers and other chamber-like components, can also affect sample dispersion in flow injection analysis. The effects of a mixing chamber and the detector inner volume are discussed in 3.1.2.2 and 6.3.2, respectively. The presence of devices for liquid—liquid extraction and gas diffusion (or dialysis) alters dispersion, and is dealt with in Chapter 8. 

Finally, it should be stressed that there are some specific strategies that can strongly influence sample dispersion in flow injection analysis, e.g., merging zones, zone sampling, stream splitting and closed-loop arrangements. These strategies are discussed in Chapter 7. 

After the advent of flow injection analysis, an alternative titration procedure based on gradient exploitation was proposed [322], The sample was inserted into a titrant carrier stream, underwent high dispersion inside a mixing chamber placed in the analytical path and thereafter... 

Karlberg B 1986 Flow injection analysis—or the art of controlling sample dispersion in a narrow tube Anal. Chim. Acta 180 16-20... 

Process control—continuous and discrete analyzers, p. 661 Automatic instruments, p. 664 Flow injection analysis, p. 665 Dispersion coefficient (key equation 23.1), p. 667 Sample volume, Sy2 (key equation 23.3), p. 670 Sequential injection analysis, p. 673 Microprocessors and computers in analytical chemistry, p. 674... 

The flow injection analysis (FIA) response curve is a result of two processes, both kinetic in nature the physical process of dispersion of the sample zone within the carrier stream and the chemical process of formation of a chemical species. These two processes occur simultaneously, and they yield, together with the dynamic characteristics of the detector, the FIA response curve. Simultaneous dispersion and chemical reaction have been studied in flow systems as used in chemical reaction engineering and in chromatography, and, therefore, the theories of these two areas are related to the theory of FIA. This is why most papers about FIA theory have adopted, as a starting point, the classical theory of flow in tubular conduits, with the intention of developing mathematical expressions for peak broadening, mean residence time, and fractional conversion of the analyte to a detectable product. 

Figure 3.4. Dispersion of a sample plug (width 6) by laminar flow in the absence of diffusion, exhibiting the progressively increasing dispersion due to Pois-. seuille flow. (According to Ref. 1354, by courtesy of the Journal of Flow Injection Analysis Japan.)...

B. Karlberg, Flow Injection Analysis—Or the Art of Controlling Sample Dispersion in a Narrow Tube. Anal, Chim, Acta, 180 (1986) 16. 

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