Arsenic speciation in terrestrial plant material using microwave-assisted extraction, ion chromatography and inductively coupled plasma mass spectrometry

Abstract

Determining arsenic speciation in terrestrial plants is necessary to understand how plants transform and metabolise arsenic. Ion chromatography in conjunction with inductively coupled plasma mass spectrometry can be used to measure arsenic speciation in plant material. However, the arsenic species need to be extracted quantitatively from the plant matrix before measurement. A method is therefore presented for the extraction of arsenic species from freeze-dried plant material. The method was optimised by using a variety of extracts (water, a modified protein extracting solution, and 10% (v/v) tetramethylammonium hydroxide solution), different microwave heating temperatures (50, 70 or 90 °C) and different heating times (5, 10 or 20 min). Microwave-heating the samples at 90 °C during 20 min in a modified protein extracting solution provided good extraction efficiency (98 ± 1%). The individual arsenic species remained intact during the extraction procedure, with fortification recoveries between 97 and 107% for all species (arsenite, arsenate, dimethylarsinic acid and monomethylarsonic acid) (added as 0.333 mg As kg⁻¹). The method was used to extract arsenic species from shoots and roots of canola (Brassica napus), velvet grass (Holcus lanatus), red brome grass (Bromus rubens), Arabidopsis thaliana and Senna planitiicola containing various amounts of total arsenic (0.2–2434 mg As kg⁻¹ dry weight). Extraction efficiency across all these diverse plant materials was 104 ± 16%. Most of the plant samples contained mainly arsenate and arsenite, but low levels (1 to 2%) of dimethylarsinic and monomethylarsonic acids were measured in shoots and roots of Holcus lanatus and Arabidopsis thaliana.