Microwave-assisted extraction of monomethyl arsonic acid from soil and sediment standard reference materials

Abstract

Arsenic-containing species were extracted from soil and sediment SRM by a mixture (1 + 1) of acetone and hydrochloric acid (10% v/v) in a sealed vessel in a microwave oven during heating to 160 °C at pressures up to 1150 kPa (160 psi). Following separation by anion-exchange HPLC those species which gave a volatile derivative on reaction with borohydride in acid were detected by plasma-source mass spectrometry. The procedure was used to determine the monomethyl arsonate species in SRM 2704 Buffalo River sediment (0.30 ppm), SRM 1944 New York–New Jersey waterway sediment (0.23 ppm) and in SRM 2710 highly elevated Montana soil (1.03 ppm). The method was developed by investigating the recovery of dimethylarsinate added to Buffalo River sediment as a function of various experimental parameters, including the composition of the solvent. For 1 + 1 mixtures of acetone and 5% HCl, methanol and 5% HCl, and isopropanol and 5% HCl, recoveries ranged from 91% to 112%. Similar recoveries were obtained for ultrasound-assisted extractions with the same solvents. The chromatographic eluent was not directly introduced into the mass spectrometer as, compared to the post-column hydride generation procedure, the sensitivity was too low for reliable quantitative measurements, although the chromatographic resolution was better. Problems with signal pulsations were overcome by incorporating pulse dampers into the reagent delivery lines.