Sequential injections as an alternative to gradient exploitation for implementing differential kinetic analysis in a flow injection system

Exploiting multivariate calibration for compensation of iron interference in the spectrophotometric flow-based catalytic determination of molybdenum

Multivariate calibration involving partial least squares was exploited in the flow-based spectrophotometric determination of molybdenum in river waters relying on the Mo(VI)-catalyzed iodide oxidation by H₂O₂ under acidic conditions. Two sample aliquots were simultaneously inserted into the carrier stream, and differential pumping was accountable for in-line addition of sulfuric acid to one of them. Pronounced gradients (acidity and reagent concentrations) were established along the complex sample zone formed, and the absorbance-time function was characterized by local maximum and minimum values. As these values were intrinsically more precise, they were used for implementing the PLS multivariate calibration. Mo(VI) and Fe(III) were jointly determined, and Fe(III) interference was straightforwardly circumvented. Influence of reagent concentrations, acidity, available time for reaction development, and nature of the acid was investigated, and this later parameter manifested itself as relevant for discriminating purposes. The calibration set consisted of 6.2 - 50.0μgL^-1 Mo(VI) plus 0.5 - 7.0mgL^-1 Fe(III) solutions. The PLS model was characterized by good prediction ability [RMSEP = 0.67μgL^-1 for Mo(VI)]. The innovation was applied to spiked river waters, and analytical precision, sampling rate, recovery, detection limit and reagent consumption were estimated as 0.5 - 2.4%, 31h^-1, 98-114%. 0.88μgL^-1 Mo(VI), and 54.0mg KI per determination, respectively. Results were in agreement with ICP OES.