Accurate quantification and transformation of arsenic compounds during wet ashing with nitric acid and microwave assisted heating

Abstract

Arsenous acid, dimethylarsinic acid (DMA), methylarsonic acid (MA), arsenic acid, arsenobetaine bromide (AB), trimethylarsine oxide (TMAO), arsenocholine iodide (AC), and tetramethylarsonium iodide (TETRA) were heated in a microwave autoclave with nitric acid to 100–300 °C. The arsenic compounds in the digests were separated with anion- and cation-exchange chromatography and determined with an inductively coupled plasma mass spectrometer as arsenic-specific detector. Arsenous acid was completely oxidized to arsenic acid at 100 °C. For a complete oxidation of MA and DMA to arsenic acid temperatures >220 °C and >280 °C were necessary. AB decomposed to arsenic acid via TMAO. Complete conversion was only obtained after heating the sample for 90 min to 300 °C. For a complete conversion of TMAO similar harsh conditions were necessary. AC was already substantially degraded to TMAO, TETRA and two unknown compounds at 100 °C. The unknown arsenic compounds were found only in the digests up to 160 °C. Quantitative conversion of AC to arsenic acid went also via TMAO. At temperatures above 220 °C TETRA started to convert to TMAO, which then was further converted to arsenic acid. To investigate whether the results obtained for the arsenic standards are transferable to real samples, the certified reference material DORM-2 was also heated in nitric acid with variable digestion temperatures and times. For an almost complete conversion of the AB present in DORM-2 90 min at 300 °C were necessary. Total organic carbon (TOC) was less <0.2% when DORM-2 was heated at temperatures ≥260 °C for 60 min. UV photo-oxidation of DORM-2 was investigated as an alternative sample decomposition. Only 6% of AB was converted to arsenic acid when DORM-2 was irradiated for 2 h at 1000 W. In contrast to microwave heating substantial amounts of MA were observed as degradation product.