On-line preconcentration of niobium(V) and tantalum(V) as 4-(2-pyridylazo)resorcinol-citrate ternary complexes in geological samples by ion interaction high-performance liquid chromatography

4-(2-Pyridylazo) resorcinol (PAR) and citrate were used as pre-column compl exing agents for the determination of Nb(V) and Ta(V) as ternary complexes in geological samples. Aliquots of 2 mi of the standard and sample solution s containing the Nb(V) and Ta(V) complexes were loaded onto a concentrator column (C-18, 0.4 cm X 4.6 mm) with a carrier mobile phase comprising 20% ( v/v) methanol and containing 5 mM acetic acid, 5 mM citric acid and 10 mM t etrabutylammonium bromide (TBABr), pH 6.5 at 2 ml/min for 2 min, with the e ffluent being directed to waste. An automatic switching valve was then swit ched to flush both complexes from the concentrator column onto a C-18 analy tical column using a mobile phase comprising 32% (v/v) methanol and contain ing 5 mM acetic acid, 5 mM citric acid and 3 mM TBABr, pH. 6.5 for 2.5 min. The switching valve was then switched back to the original position, and c leaned with methanol for 7 min to eliminate unwanted species still adsorbed to the concentrator column. This procedure prevented later eluting compoun ds from reaching the analytical column, which reduced the overall run time. The detection limits of Nb(V) and Ta(V) (determined at a signal-to-noise r atio of 3, detection wavelength of 540 nm and a 2-ml sample volume) were 0. 012 and 0.039 ppb for Nb(V) and Ta(V), respectively. Recoveries of Nb(V) an d Ta(V) were 99.4 and 96.2%, respectively. The HPLC results obtained from t he reference granite and basalt samples agreed well with inductively couple d plasma MS and certified values, but the HPLC method yielded slightly low values of the Nb/Ta ratio. (C) 2000 Elsevier Science BN. All rights reserve d.