Arsenic speciation was determined by HPLC-ICP-MS (7500a;
Agilent Technologies) as described in the literature (Zhu et al.,
2008). Chromatographic columns were obtained from Hamilton
and consisted of a precolumn (11.2 mm,12e20 mm) and a PRP-X100
10-mm anion-exchange column (250 4.1 mm). The mobile phase
consisted of 10 mM diammonium hydrogen phosphate
((NH4)2HPO4) and 10 mM ammonium nitrate (NH4NO3), adjusted
to pH 6.2 using nitric acid. The mobile phase was pumped through
the column isocratically at 1 ml min1. Arsenic species in the
samples were identified by comparing their retention time with
those of the standards, including arsenite (As(III)) and arsenate
(As(V)), and quantified by external calibration curves with peak
areas.