Catalytic determination of molybdenum (VI) in plants using mono-segmented continuous-flow analysis and spectrophotometric detection

Abstract

A mono-segmented continuous-flow analysis (MCFA) procedure has been developed for the determination of molybdenum in plants, based on the catalytic oxidation of l^– by H₂O₂ in acid medium as the indicator reaction, with spectrophotometric detection at 350 nm. High sensitivity was achieved using multivariate and univariate screening experiments, optimized reagent concentrations, a reaction time of 130 s and a sampling-loop volume of 300 µl or higher. The reagents were pumped at 1.2 ml min^–1 and the sample at 4.7 ml min^–1, yielding a sampling rate up to 120 h^–1, with negligible carryover. The effects of Fe^III, Ti^IV, W^VI, Cr^VI, V^V, PO₄^3– and ionic strength on the analytical signal were evaluated under these conditions. Only Ti^IV does not interfere significantly. Hence, a prior separation step is necessary, consisting of an extraction of Mo^VI into HCI-saturated chloroform as an α-benzoin oxime complex. For a reaction time of 130 s and an ambient temperature of 22 ± 2 °C the calibration graph, obtained by submitting Mo^VI standards to the extraction procedure, follows the equation ΔA=–0.0018 + 0.0172 C_Mo(r²= 0.998), where ΔA is the net analytical response, taken as the difference between the catalysed and non-catalysed signals, and C_Mo is the molybdenum concentration, in ng ml^–1. Under these conditions, the linear range extends to 50 ng ml^–1, the calculated detection limit is about 1.0 ng ml^–1 and the relative standard deviation is close to 1.0% over the entire linear range. The sensitivity and linear ranges of the MCFA procedure can be easily modified by simply changing the sampling volume and the reaction time. The accuracy of the method was evaluated by means of plant samples supplied by the ‘International Plant-analytical Exchange’(IPE), from the Wageningen Agricultural University, The Netherlands. The results obtained for the determination of Mo^VI in these samples were acceptable under the recommended IPE criteria.