Extraction of arsenic as the diethyl dithiophosphate complex with supercritical fluid and quantitation by cathodic stripping voltammetry

The separation of arsenic based on in situ chelation with ammonium diethyl dithiophosphate (ADDTP) has been carried out using methanol-modified supercritical CO2. Aliquots of extract were added to an electroanalytical cell and arsenic was determined by square wave cathodic stripping voltammetry (SWCSV) at a hanging mercury drop electrode (HMDE). Quantitative extractions of As(DDTP)3 were achieved when the experiments were carried out at a pressure of 2500 psi, a temperature of 90 °C, 2.0 mL of methanol, 20.0 min of static extraction and 5.0 min of dynamic extraction in the presence of 18 mg of ADDTP. Analysis of arsenic was made using 150 mg L−1 of Cu(II) in 1 M HCl solution as supporting electrolyte in the presence of ADDTP as ligand. Preconcentration was carried out by deposition at a potential of −0.50 V and the intermetallic compound CuxAsy was reduced at a potential of −0.77 to −0.82 V, depending on ligand concentration. The results showed that the presence of ligand plays an important role, increasing the method's sensitivity and preventing the oxidation of As(III). The calibration graph of the As(DDTP)3 solution was linear from 0.8 to 12.5 μg L−1 of arsenic (LOD 0.5 μg L−1, R = 0.9992, tacc = 60 s). The method was validated using carrot pulp spiked with arsenic solution. This method was applied to the determination of arsenic in samples of carrots, beets and irrigation water. Arsenic in beets was: skin 4.10 ± 0.18 mg kg−1; pulp 3.83 ± 0.19 mg kg−1 and juice 0.71 ± 0.09 mg L−1; arsenic in carrots was: skin 2.15 ± 0.09 mg kg−1; pulp 0.59 ± 0.11 mg kg−1 and juice 0.71 ± 0.03 mg L−1. Arsenic in water were: Chiu-Chiu 0.08 mg L−1, Inacaliri 1.12 mg L−1, and Salado river 0.17 ± 0.07 mg L−1.