Behaviour of anionic arsenic compounds in microwave system with nitric acid and hydrogen peroxide - preliminary laboratory study

The stability and oxidation pathways of arsenate, arsenite, methylarsonic acid (MA), and dimethylarsinic acid (DMA) were studied in the presence of concentrated nitric acid (2.0 ml) and 30% hydrogen peroxide (0.20 ml) in a closed pressurized microwave digestion system for 15 min by changing the microwave power (from 300 to 600 W). MA DMA and arsenate were stable and remained intact up to 160 degrees C when digested at 400 W microwave power. The arsenite was unstable and converted to arsenate (97 +/- 3.6%) when 309 W (145 degrees C) microwave power was applied. At 500 W for 15 min digestion (up to 190 degrees C) MA and arsenite were fragile. MA was partially decomposed to arsenate (49 +/- 3.2%), but arsenite adequately converted to arsenate (96 +/- 2.2%). At 600 W (207 degrees C) MA was completely transformed to arsenate (97 +/- 3.6%) whereas DMA and arsenate were unaltered. The separation of arsenic compounds in the microwave digests, was carried out on an anion exchange HPLC column using HGAAS as an arsenic specific detector. This microwave digestion approach is very promising and may be applied for the dissolution and determination of anionic arsenic compounds forms in the environmental samples. (C) 1999 Elsevier Science B.V. All rights reserved.