Sample insertion without injection

This analytical methodology, described by Riley et al. [17], who original- 

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Weld lines (also known as knit lines) are a potential source of weakness in molded and extruded plastic products. These occur when separate polymer melt flows meet and weld more or less into each other. Knit lines arise from flows around barriers, as in double or multigating and use of inserts in injection molding. The primary source of weld lines in extrusion is flow around spiders (multiarmed devices that hold the extrusion die). The melt temperature and melt elasticity (which is mentioned in the next section of this chapter) have major influences on the mechanical properties of weld lines. The tensile and impact strength of plastics that fail without appreciable yielding may be reduced considerably by in doublegated moldings, compared to that of samples without weld lines. Polystryrene and SAN copolymers are typical of such materials. The effects of weld lines is relatively minor with ductile amorphous plastics like ABS and polycarbonate and with semicrystalline polymers such as polyoxymethylene. Tliis is because these materials can reduce stress concentrations by yielding [22]. 

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. 

Sample insertion relying on time-based introduction is inherent to segmented flow analysis and was the preferred approach before the advent of flow injection analysis. Manual injection using syringes with [50] or without [1] needles was used in the earliest flow injection systems (Fig. 6.7) but is now rarely used, except when very small sample volumes are available. 

The spectrophotometric determination of Fe(II), Fe(III), nitrate and nitrite ions in natural and wastewaters in a sequential injection system [298] is an interesting example of this innovation. In-line tangential filtration was performed prior to the sample insertion port as part of the sampling step. Particulate material was not aspirated towards the holding coil and several samples could be analysed without the need for frequent cleaning of the filtration unit. Analytical figures of merit were similar to those for the corresponding flow-based analytical procedure with manual sample filtration prior to flow analysis. 

Cool on-column injection is used for trace analysis. Ah. of the sample is introduced without vaporization by inserting the needle of the syringe at a place where the column has been previously stripped of hquid phase. The injection temperature must be at or below the boiling point of the solvent carrying the sample. Injection must be rapid and no more than a very few, usuahy no more than two, microliters may be injected. Cool on-column injection is the most accurate and reproducible injection technique for capihary chromatography, but it is the most difficult to automate. 

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]. 

To avoid this phenomenon, the sample is injected into a stream of water, which is then merged with the combined reagent stream of molybdate and ascorbic acid. Furthermore, to obtain effective mixing without excessive dispersion of the sample solution, either the streams are conflu-enced as shown in Fig. 4.9 or an imprinted meander (in a microconduit) is inserted immediately following the confluence point, permitting the use of a short reactor before the flowthrough cell. 

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