Conduits,tubular,dispersion

Having established that a finite volume of sample causes peak dispersion and that it is highly desirable to limit that dispersion to a level that does not impair the performance of the column, the maximum sample volume that can be tolerated can be evaluated by employing the principle of the summation of variances. Let a volume (Vi) be injected onto a column. This sample volume (Vi) will be dispersed on the front of the column in the form of a rectangular distribution. The eluted peak will have an overall variance that consists of that produced by the column and other parts of the mobile phase conduit system plus that due to the dispersion from the finite sample volume. For convenience, the dispersion contributed by parts of the mobile phase system, other than the column (except for that from the finite sample volume), will be considered negligible. In most well-designed chromatographic systems, this will be true, particularly for well-packed GC and LC columns. However, for open tubular columns in GC, and possibly microbore columns in LC, where peak volumes can be extremely small, this may not necessarily be true, and other extra-column dispersion sources may need to be taken into account. It is now possible to apply the principle of the summation of variances to the effect of sample volume. 

To summarize, the dispersion of the sample zone increases with the square root of the distance traveled through the tubular conduit and the square of the radius of the tube. It generally increases somewhat with increasing flow rate but may behave in the opposite maimer at very slow flow rates where diffusion effects predominate. 

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. 

The geometrical form of the tubing from which the FIA reactor is made has a distinct influence on the solute dispersion, as seen in Fig. 2.10. The reason is that helical coiling, or irregular knitting of the tubular conduit. 

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