Amplitude modulated flow analysis for speciation—Proof of concept by quantification of Fe2+ and Fe3+ ions

Abstract We propose a novel concept of flow-based analysis for spectrophotometric speciation based on flow rate modulation and fast Fourier transform (FFT). A redox reagent solution's and a sample solution's flow rates are varied by sinusoidal control signals with periods of T and 0.5T, respectively. Both solutions are merged with a color reagent, while the total flow rate is held constant. Downstream, the absorbance of the mixed solution is measured and acquired as the detector output voltage (Vd). The Vd is analyzed by FFT with the window's time length of T. One species that directly reacts with the color reagent contributes only to the amplitude (A2) of the second harmonic wave component in Vd. The other species that needs the redox conversion before the coloration contributes to the amplitude (A1) of the fundamental wave component, in addition to A2. The former species’ concentration can be estimated from A2 by taking the latter's contribution to A2 into account. The latter species’ concentration can be determined only from A1. The proposed concept was demonstrated by applying it to the speciation of Fe2+ and Fe3+ by an o-phenanthroline spectrophotometry, where Fe3+ was reduced to Fe2+ by L-ascorbic acid before the coloration.