Evaluation of the precision of the flow injection doublet peak method

The factors affecting the precision of the flow injection doublet peak method are discussed with particular reference to the contribution from the uncertainty associated with the calibration procedure. A single-line flow injection manifold was used in which the concentration gradients were generated by alternating helical reactors and the reaction products were monitored by a simple detector incorporating diodes as light source and intensity transducer. The reactions between lanthanum and Methyl Thymol Blue and between sodium hydroxide and hydrochloric acid in the presence of Bromothymol Blue were used. Optimization studies involved the method of steepest ascent, two figures of merit (the slope of the calibration and the standard deviation of a measured concentration) and two parameters (pump speed setting and the number of segments in the alternating helical reactor). The resulting minimum uncertainties, which ranged from 3 to 9%, were found to be insensitive to changes in experimental parameters. The major contribution to the uncertainty in a determined concentration was found to be the scatter of the points about the regression line fitted to the calibration data. Alternative approaches to flow injection peak width methods are discussed and it is pointed out that the doublet peak method has the unique feature of giving rise to an inherently linear calibration function, because the reference points at which the time interval is measured are not fixed at a particular value of the detector response for all concentrations.