Classical dry ashing of biological and agricultural materials. Part II. Losses of analytes due to their retention in an insoluble residue

Retention of selected analytes (Cd, Cr, Cu, Mn, Pb, Zn) in solid residue which remains undissolved in 1.5% HNO3 used as a leaching medium after classical dry ashing of ten materials (alfalfa leaves, NIST SRM 1569 Brewer's Yeast, blood meal, meat-bone meal, feather-bone meal, silage residue, litter, pond sediment, coal waste, IRM NSC-21 Industrial Compost Vitahum) was studied. The elements remaining in the residue were determined mainly by PLAS after dissolution of this residue in the mixture of HF + HNO3. In several instances, pressurized wet digestion, alkaline fusion, stripping voltammetry and instrumental neutron activation were also applied. Increasing concentration of mineral acid in leaching medium which does not contain HF, plus increasing final volume of the solution, suppresses significantly this type of analyte losses in the majority of matrices tested. However, for industrial compost and standard reference material NIST 1569 Brewer's Yeast, application of an HF step is necessary for quantitative release of the analytes (in particular chromium) into solution.