This article reports the application of on-line dynamic chemical fractionation using a stirred-flow chamber for fast characterization of ash-forming metals, namely, K, Ca, Na and Mg, and Al as well in woody biomass residues (bark and twigs) and laboratory ashed bark. The chemical fractionation procedure is harnessed to discriminate with minimal operational maintenance those target species associated to the water-soluble, the exchangeable and the acid-soluble fractions as a result of their varying occurrence in the biofuels by resorting subsequently to chemicals of increasing leachability. The final goal is to assist in the evaluation of actual pools of ash-forming elements in industrial boilers. The proposed technique is able to handle up to 1.0 g of woody biofuels and provide, as opposed to conventional fuel characterization, relevant insight into the amount of leachable elements under worst case scenarios, and real-time quantification of potentially reactive species beside insight into the leaching kinetics of target elements. Screening results can be obtained in less than 3 h instead of more than one week in the batchwise counterpart methods. For elucidation of metal-biomass/ash associations and investigation of the actual selectivity of extractants, a novel approach has been undertaken by resorting to dynamic fractionation assays in combination with scanning electron microscopy with energy dispersive X-ray fluorescence spectrometry (SEM-EDX) and X-ray diffraction (RXD) assays. (c) 2013 Elsevier Ltd. All rights reserved.