Separation of niobium(V) and tantalum(V) as ternary complexes with citrate and metallochromic ligands by capillary electrophoresis

A method was developed for the determination of Nb(V) and Ta(V) as anionic ternary complexes of a metallochromic ligand and citrate as the auxiliary ligand by capillary electrophoresis. Three metallochromic ligands were evaluated, namely 4-(2-pyridylazo) resorcinol (PAR), 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) and 2-(5bromo-2-pyridylazo)-5-[N-propyl-N-(3-sulfopropyl)amino]-phenol (Br-PAPS). Each complex was detected spectrophotometrically using a light-emitting diode (LED) at 568nm. The separation of Nb(V) and Ta(V) was optimised for all three ligands, of which PAR displayed the best analytical method performance parameters, with detection limits of 9.1 and 25.2µg/l for Nb(V) and Ta(V), respectively, using a 20s injection time and linear ranges of 25-1000 and 50-4000µg/l for Nb(V) and Ta(V), respectively. The method was applied to the analysis of representative rock samples. Cleanup of the digested rock samples with a C18 pre-column coated with tetrabutylammonium hydroxide prior to analysis was used to remove excess salts which otherwise caused electrophoretic de-stacking. It was found that Nb(V) could be determined satisfactorily at natural levels in the tested reference rock materials (granite and basalt), but Ta(V) was below the detection limit for these samples. The values of Nb(V) found by the proposed method agreed well with the certified values and with results obtained by inductively coupled plasma-mass spectrometry (ICP-MS).