2.4. Analytical procedure
Firstly, the sample preparation and the reagent concentrations were optimized for analysis by CV-GF AAS and then adopted for the pre-concentration system using the gold column. At this point, an aliquot of 3.0 mL of the sample solution or of the standard solution, both spiked with a mercury concentration of 6.0 μg L− 1, was transferred to the reaction flask together with 1.5 mL of 1.5 mol L− 1 HCl. A 3.0% m/v NaBH4 solution, stabilized with 1.0% m/v NaOH, was used as the reducing agent for the determination of mercury. This solution was injected for 15 s into the reactor, resulting in an approximate reducing agent volume of 1.5 mL. This procedure was repeated three times and the Hg vapors generated in the reaction flask were trapped in the gold column. In the final step, the thermal desorption system was activated at the same time than the reaction third cycle started, and the released Hg vapor was transported and trapped again in the graphite tube, coated with gold as permanent modifier, for posterior mercury determination by GF AAS.