Behaviour of cationic arsenic compounds in a microwave system with nitric acid and hydrogen peroxide

The stability and oxidation pathways of arsenobetaine (AB), arsenocholine (AC), trimethylarsine oxide (TMAO) and tetramethylarsonium iodide (TMI) 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 by changing the microwave power (300-600 W). AB, TMAO and TMI were stable and remained intact up to 160 degrees C when digested at 400 W of microwave power for 15 min. With digestion at 300 W (145 degrees C) for 15 min AC was unstable and partially decomposed to TMAO (52 +/- 4.8%). On increasing the microwave power to 400 W (160 degrees C) AC was readily converted to TMAO (97 +/- 2.8%). With digestion at 500 W for 15 min (190 degrees C) AB was unstable and partially decomposed to TMAO (37 +/- 3.3%). With digestion at 600 W for 15 min (207 degrees C) AB was completely transformed to TMAO (99 +/- 5.0%). AC was not converted to AB during chemical oxidation. The separation of cationic arsenic compounds in the microwave digests was carried out on a reversed-phase HPLC column using HGAAS as an arsenic-specific detector. The proposed microwave-assisted digestion approach is promising and was applied to the dissolution and determination of total cationic arsenic compounds in environmental samples.