Continuous extraction with acidified subcritical water of arsenic, selenium and mercury from coal prior to on-line derivatisation-atomic fluorescencedetection

Acidified subcritical water is proposed for the continuous extraction of mi nor pollutants (namely, selenium, arsenic and mercury) from coal prior to c ontinuous derivatisation (by hydride formation for Se and As, and cold vapo r formation for Hg) and determination by atomic fluorescence. Coal samples (3 g) were subjected to a 15 min static extraction followed by a 90 min dyn amic extraction with water modified with 4% (v/v) HNO3 for both steps. An i n-depth study of the variables affecting the continuous leaching step, as w ell as those referring both to preconcentration (for mercury), derivatisati on and detection (all of them) was performed. The linear ranges of the cali bration curves for all analytes were at the ng ml(-1) level, with correlati on coefficients, r(2), better than 0.999 for Se and As and better than 0.99 for Hg. The method was validated by using a bituminous coal reference mate rial (NIST SRM 1635). The good precision of the method [RSDs (n = 6) of 12. 0, 4.7 and 6.5% for As, Se and Hg, respectively], together with its safety and rapidity make it a good alternative for the determination of these anal ytes in coal.