Determination of Niobium(V) and Tantalum(V) as 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP)-citrate ternary complexes in geological materials using ion-interaction reversed-phase high-performance liquid chromatography

Abstract

A method for the simultaneous separation and determination of Nb(V) and Ta(V) as ternary complexes with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) and citrate using ion-interaction reversed-phase high performance liquid chromatography on a C 18 column has been developed. The mobile phase compositions and pre-column complex formation conditions were optimised. Under the optimum conditions, the Nb(V) and Ta(V) complexes were eluted within 4 min with a mobile phase of methanol-water (50% v/v) containing 12 mM tetrabutylammonium bromide and 12 mM citrate buffer at pH 7, with absorbance detection at 600 nm. The resulting % relative standard deviation of retention times, peak area and peak height for Nb(V) and Ta(V) complexes were 0.007, 0.000, 0.67, 2.69, 0.44 and 2.17, respectively. The detection limits (signal to noise ratio = 3) for Nb(V) and Ta(V) were 0.03 ppb and 0.41 ppb, respectively. The method was applied to the analysis of geological samples (granite, basalt and andesite) and the results for Nb(V) obtained by the HPLC method using standard addition calibration agreed well with ICP-MS analysis and with certified values for all materials. For Ta(V), good agreement with certified values was obtained for granite samples using standard addition, but interferences were observed for basalt and andesite which prohibited the determination of Ta(V) in these samples.