(1 mol L−1), followed by extraction of OrgHg by toluene. The
extractions were always performed in triplicate. The OrgHg mercury
compounds retained in the toluene fraction were back extracted
into an aqueous solution of thiosulphate. Mercury in liquid aliquots
of the extracts (250–1000 μL) was quantified by thermal decomposition
atomic absorption spectrometry with gold amalgamation (LECO
model AMA-254). It was not possible to quantify OrgHg in 20 soil, 7
root and 13 shoot samples since their contents were below the limit
of quantification (0.03 ng Hg). To ensure the quality of results procedural
blanks (i.e. procedure with the reagents only) and reference
material TORT-2 (Lobster Hepatopancreas; 0.152 ± 0.013 mg kg−1 of
methylmercury) were always carried out in parallel with the samples
and analyzed. The procedural blanks were always below the detection
limit of the equipment (0.01 ng Hg). The recovery for OrgHg mercury
(assuming that the dominant form in this fraction is methylmercury
(Ullrich et al., 2007; Nam and Basu, 2011) was in the range 80–86%
(n = 11). Inorganic mercury (IHg) was obtained subtracting OrgHg
from THg.