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

Changes to a commercial Soxwave-100 focused microwave device were made in o rder to improve both safety and speed of the conventional acid pre-treatmen ts. The changes allowed the conventional procedure (in which the sample is mixed with the appropriate reagent and the mixture is subjected to high tem peratures) to be replaced with a hybrid procedure which involves cycle perf ormance similar to a conventional Soxhlet extraction, in which contact betw een the sample and fresh acid solution is established in each cycle, thus d isplacing the equilibrium to total removal of the target analytes. Each cyc le is equivalent to a conventional microwave wet digestion. The optimisatio n of the most relevant variables of the leaching step was studied by a mult ivariate method involving response surface methodology. Total leaching of m inor elements from coal (namely selenium, arsenic and mercury) was achieved in 45-50 min using ultrapure water with 6-8% nitric acid as digestion agen t. The procedure was applied to four coals and two CRMs with recoveries clo se 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 x 10(- 4) mu g g(-1) for selenium, arsenic and mercury, respectively.