Non-chromatographic speciation of inorganic arsenic by atomic fluorescence spectrometry with flow injection hydride generation with a tetrahydroborate-form anion-exchanger

Abstract

A method has been developed for the determination of arsenite and arsenate in natural water samples based on the generation of arsine (AsH₃) from the reaction between the arsenic species in the injected solution and tetrahydroborate immobilized on a strong anion-exchange resin (Amberlite IRA-400). Speciation was based on two different measurement conditions: (i) acidification to 0.7 M with HCl, and (ii) acidification to 0.1 M with HCl in the presence of 0.5% L-cysteine, which produced two calibration equations with different sensitivities for each species. The LOD for a 0.5 mL sample volume was 13 ng L⁻¹ and 15 ng L⁻¹ for arsenite and arsenate, respectively. The precision, expressed as % relative standard deviation of the measurement of 0.5 μg L⁻¹ As was 4.3% and 4.1% for determination of arsenite and arsenate, respectively, in 0.7 M HCl; and 3.8% and 3.6%, for the determination in 0.1 M HCl and 0.5% L-cysteine. Interferences from transition metals and hydride-forming elements were eliminated by the addition of L-cysteine. The method was evaluated by the analysis of spiked natural waters. The recoveries for 0.5 and 1 μg L⁻¹ arsenite were 92–108% and 88–112%, respectively; the recoveries for 0.5 and 1 μg L⁻¹ arsenate were 94–111% and 95–112%, respectively. This method was also validated by the accurate analysis of a seawater certified reference material, NASS-6, which contains 1.43 ± 0.12 μg L⁻¹ (total arsenic). The method was applied to the analysis of a number of real water samples, none of which contained arsenic below the method detection limit. The time required per measurement was less than 4 min and the procedure consumes about 100-times less hydrochloric acid that the conventional continuous-flow procedure.