Determination of trace metals in high-purity tungsten hexafluoride by electrothermal atomic absorption spectrometry after hydrolysis and selective removal of tungsten by liquid-liquid extraction

Abstract

A method was developed for the determination of aluminium, calcium, chromium, cobalt, copper, iron, lead, magnesium, manganese, nickel, potassium, sodium and zinc at trace levels in high-purity tungsten hexafluoride. The procedure is based on the prior hydrolysis of the samples of WF 6 in water and on the selective removal of most of the tungsten (which prevents the trace determination of most of the other elements when it is present at high concentration) by liquid-liquid extraction with a lipophilic amine (e.g., trioctylamine) or a lipophilic quaternary ammonium salt (e.g., Adogen 464). Such a preliminary treatment provides aqueous hydrofluoric acid solutions containing all the metal species initially present in tungsten hexafluoride as impurities (recoveries close to 100% were found for all the elements added at a concentration of 80 ng g −1 W), but less than 10 mg l −1 of tungsten. The various metal species can then be determined by electrothermal atomic absorption spectrometry. The limits of determination are mainly imposed by the efficiency of the purification of the reagent used for the liquid-liquid extraction operations. The following limits of determination (in ng g −1 W) were obtained: Al 30, Ca 60, Co 2, Cr 10, Cu 20, Fe 100, K 60, Mg 40, Mn 10, Na 60, Ni 20, Pb 8 and Zn 10.