As MMHg is of greatest concern to organism health due to its
toxic and bioaccumulative nature in marine food-webs it was
determined for selected muscle tissue samples for 9 of 16 species
of shark to confirm approximate percentage of THg. The method
for the determination of MMHg in fish tissue is an adaptation for
biota of the US EPA method No. 1630 described for waters. Briefly,
tissue samples are digested using a NaOH/methanol solution. An
ethylating agent is then added to the aqueous sample to form a
volatile methyl-ethyl-mercury derivative. The ethylated species
are then purged onto Tenax traps as a means of preconcentration
and interference removal. The ethylated mercury species are then
removed from the traps by heating (250 C) and separated using
isothermal chromatography (70 C). The mercury species evolving
from the chromatography column are destroyed by pyrolysis
(800 C), releasing elemental mercury for detection by cold-vapour
atomic fluorescence spectroscopy (CVAFS) as described in Bloom
and Fitzgerald (1988). We used an AFS Tekran, Model 2500 for
the detection. The detailed protocols are described in Cossa et al.
(2002). The method of determination includes the use of the standard
addition technique with an MMHg solution in isopropanol.
Method precision of MMHg determinations, estimated from 4 replicates
of the CRM DORM-2, ranged between 8% and 15% (mean
14%) and its mean recovery was around 90% (Table 1). The detection
limit is 0.004 mg kg1.