2.5. Chemical analysis
The analytical method used to determine the total mercury concentration
in the water samples was based on thermal decomposition
detected by atomic absorption spectrometry using a milestone
DMA-80 (Direct Mercury Analyzer), with direct mercury analysis without the digestion or pre-digestion of the sample, according to
recommendations of the environmental protection agency (U.S
EPA, 1998).
The samples were weighed (400–600 mg) in a nickel panel and
promptly introduced in the catalysis oven through the automatic
sampler, they were initially dried and then thermally decomposed
under a continuous oxygen flow. The products of the combustion
were decomposed over a heated catalyzer, and mercury was displaced
as vapor which was retained by reacting with gold in the
amalgamator. By heating the amalgam, Hg was quantitatively
released and determined through atomic absorption at 253.7 nm.
The validation of the analytical method was performed through
mercury determinations in two certificated reference materials
(fish protein and lake sediment) from the National Research Council
Canada (NRC) (DORM-3) and the Canadian Certified Reference
Materials Project (CCRMP) (LKSD-1). Metal recoveries were of
about 96.3% and 97.8%, respectively. By considering the average
and the standard deviation of ten blank samples, method detection
limit (MDL) was calculated as three times this standard deviation,
and method quantification limit (MQL) was evaluated as ten times
the standard deviation. This procedure provided a MDL of
0.0518 lg kg1, and a MQL of 0.2072 lg kg1.