Focused microwave Soxhlet device for rapid extraction of mercury, arsenic and selenium from coal prior to atomic fluorescence detection

Abstract

Changes to a commercial Soxwave-100 focused microwave device were made in order to improve both safety and speed of the conventional acid pre-treatments. The changes allowed the conventional procedure (in which the sample is mixed with the appropriate reagent and the mixture is subjected to high temperatures) to be replaced with a hybrid procedure which involves cycle performance similar to a conventional Soxhlet extraction, in which contact between the sample and fresh acid solution is established in each cycle, thus displacing the equilibrium to total removal of the target analytes. Each cycle is equivalent to a conventional microwave wet digestion. The optimisation of the most relevant variables of the leaching step was studied by a multivariate method involving response surface methodology. Total leaching of minor elements from coal (namely selenium, arsenic and mercury) was achieved in 4550 min using ultrapure water with 68% nitric acid as digestion agent. The procedure was applied to four coals and two CRMs with recoveries close to 100%. The precision, expressed as relative standard deviation, ranged between 2.7 and 3.7%. The detection limits were 0.035, 0.02 and 7.5 × 10⁻⁴ µg g¹ for selenium, arsenic and mercury, respectively.