Hydride generation from slurry samples after ultrasonication and ozonation for the direct determination of trace amounts of As(iii) and total inorganic arsenic by their in situ trapping followed by graphite furnace atomic absorption spectrometry

Abstract

A slurry sampling hydride generation method for As(III) and total inorganic As, without total sample digestion, has been developed using a continuous flow mode generation system coupled with atomic absorption spectrometric determination from environmental (marine sediment, soil, rock salt, waste water) and biological (human hair and urine, lobster, liver, muscle, beer and wort, tablets) samples. It involves trapping of the arsenic vapor on a pre-heated graphite furnace inner wall at 300 °C, treated with 150 µg of iridium as chemical permanent modifier, and determination by graphite furnace atomic absorption spectrometry (GF-AAS). The same graphite tube could be used for at least 200 cycles without any re-treatment. It was shown that this relatively simple and nearly direct procedure could give total arsenic concentrations very close to those obtained by analytical procedures requiring total sample decomposition and dissolution of organic and inorganic matter prior to the determination of analyte. Pretreatment of samples (slurried in HCl with addition of ozone) by ultrasonic agitation enabled the extraction of more than a 95% confidence level of the total arsenic from the certificate or close to the information value of the reference materials investigated. For the estimation of As(III) and As(V) concentrations in samples, the difference between the analytical sensitivities of the absorbance signals obtained for arsenic hydride without and with previous treatment of samples with thiourea can be used. The concentration of arsenate (As(V)) was calculated by the difference between total As and As(III). Reference and real sample materials were analyzed. The accuracy of the developed slurry sampling hydride generation method has been verified by analyzing several certified reference materials (TORT-1, DOLT-2, LUTS-1, DORM-1, BCSS-1, GBW 07601, NIST SRM 2709, LKSD-2, NIES CRM No. 18). The good agreement with the certified value confirms the validity of such a method for As determination instead of wet digestion procedures. The advantage of the method was that only a minimum of reagents and sample handling were required, reducing the risks of contamination and/or analyte loss and that non-chromatographic speciation approaches have been developed. However, the calibration was achieved via the technique of standard additions. In optimized conditions by Simplex the method shows an absolute detection limit (3σ_blank) of 1.5 ng, obtained with a 60 s trapping time using NaBH₄ as reductant with Ir modifier, which corresponds to an LOD (10σ) of 4.8 ng g⁻¹ As in sample. The relative standard deviation (RSD) of 7.8% was obtained for As concentration at the level of 20 ng g⁻¹. The overall efficiency of the volatile species generation and trapping process estimated for arsenic was 89%. A particle size less than 20 µm is adequate to obtain total vapor generation of As content in the acidic slurry particles.