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The single well stirred tank model in which dispersion is modelled by the passage of a slug of fluid through the tank has been used to compare the sensitivity that can be obtained by the use of three types of flow injection manifold, which incorporate on-line chemical reaction. These manifolds are the single-line manifold, the double-line manifold and the single-line manifold used in the reverse mode (i.e., the reagent is injected into the sample which constitutes the carrier stream). The model indicates that each manifold type will give the same sensitivity, but that operating conditions and throughput will be different for each. The model calculations for the determination of phosphate, based on parameter values from the literature, suggest that the commonly applied guideline of designing "medium dispersion" manifolds for on-line chemical derivatisations is sub-optimal in terms of maximising sensitivity and that the guideline should be that the dispersion coefficient has a value of <2. Practical problems related to refractive index and base-line absorbance effects mean that the double-line manifold is the most suitable for trace analysis and the design of such a manifold is illustrated for the determination of chloride with a detection limit of 11 p.p.b.






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