Information on the size dependence of the concentration of pollutants in fine-grained residues is required for the design of classification processes for their processing. For such an analysis, the Samples have to be classified prior to chemical analysis. Depending on the particle size range of the residue, a classification method has to be selected. For very fine material, air classification is a suitable method. In this work, the results of the classification of fly ash from a biomass combustion plant with an air classifier are discussed with respect to the observed Sample contamination. The chemical analysis of the produced size classes for heavy metals yielded unexpected results. For most heavy metals, the mass balance resulted in a recovery rate of about 100 %; however, for Cr and Ni, the recovery rate was way above 100 %. A more detailed analysis of the data revealed that the ratio of the excess of Cr to Ni in the fly ash was nearly the same as the ratio of Cr to Ni in the material of parts of the classifier. Therefore, erosion of some material from the classifier can be assumed to have caused the contamination of the Sample. For the classification of Samples, which have to be analysed for Cr and Ni, a classifier has to be used which is made of a material other than stainless steel.