Multivariate assessment of extraction conditions for the fractionation analysis of mercury in oily sludge samples using cold vapor atomic fluorescence spectrometry
This work focused on the evaluation of extraction protocols using convective heating and HNO3 or HCl solutions regarding their performance towards fractionation analysis of mercury (inorganic and organic fractions) in oily sludge samples using cold vapor atomic fluorescence spectrometry (CV-AFS). Sample mass, volume of extraction, temperature, type and concentration of the extractant solution (HNO3 or HCl) and time of extraction were evaluated. Doehlert and Box–Behnken designs were employed to generate response surfaces that would allow multivariate evaluation of experimental conditions to be carried out. After statistical treatment and response surface evaluation, the conditions 100 mg of sample, 90 °C, 10 mL of 8.0 mol L−1 HNO3 and 10 minutes of extraction were selected and applied in a desirability function to combine the maximum extraction efficiency of Hg species and the least acceptable CH3Hg+ conversion rate, as well as the highest recovery of Hg2+ and CH3Hg+. Three different oily sludge samples were analyzed, resulting in 45–66% of Hg2+ fraction, negligible CH3Hg+ to Hg2+ conversion and recoveries of 49–102% for Hg2+ and 89–106% for CH3Hg+. The analytical results were compared to those obtained by high-performance liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS) and no statistical differences were observed at a 95% confidence level. Limits of detection (LOD) and quantification (LOQ) achieved for CV-AFS were 0.07 and 0.2 ng g−1, respectively, whereas the LOD and LOQ achieved using HPLC-ICP-MS were 0.01 and 0.03 μg g−1. The concentration of organic species of Hg in oily sludge samples was below the LOD of both techniques.