Double peaks may be obtained in a single-line flow injection manifold if, by the time the reaction zone reaches the detector, there has been insufficient interspersion of the sample and reagent so that the sample is in excess over the reagent in the center of the reaction zone. The time between the peaks (Δ<i>t</i>) in a doublet profile in flow injection analysis (FIA) can be related to other experimental parameters by the equation Δ<i>t</i> = (<i>V/Q</i>)ln{<i>C</i><sup>s</sup>[exp(<i>V</i><sub>i</sub>/<i>V</i>) - 1]/<i>C</i><sup>R</sup>} (1) for the "well-stirred tank" model. Variation of the volume of the mixing chamber, <i>V</i>, the flow rate, <i>Q,</i> the injected volume of sample, <i>V</i><sub>i</sub>, the concentration of sample injected, <i>C</i><sup>s</sup>, or the reagent concentration in the carrier stream, <i>C</i><sup>R</sup>, affects the experimental value of Δ<i>t</i>. The advantage of doublet peak measurements is the extension of the linear working range to higher values of concentration. If <i>V, Q, V</i><sub>i</sub>, and <i>C</i><sup>R</sup> are chosen appropriately, the product profile will be a doublet with Δ<i>t</i> proportional to ln(<i>C</i><sup>s</sup>) over a large range of analyte concentration.

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