Use of a certified reference material for extractable trace metals to assess sources of uncertainty in the BCR three-stage sequential extraction procedure

Various potential sources of irreproducibility in the BCR three-stage sequential extraction procedure have been investigated using the lake sediment CRM 601. Of the variables considered, the pH of the hydroxylamine hydrochloride in Step 2 proved most important. Factors such as the type of acid used in pH adjustment, the temperature and duration of extraction, and working under nitrogen did not affect precision, although they did alter the amounts of metals extracted. Improved precision was obtained when the hydroxylamine hydrochloride concentration was increased from 0.1 to 0.5 mol l(-1) and when the speed of the centrifugation was increased from 1500 to 3000 x g. The addition of a MgCl2 wash between the steps of the extraction procedure gave rise to increased uncertainty. Although it did not adversely affect reproducibility, the use of filtration to separate solid and liquid phases was not recommended since it promoted dissolution of non-target phases. Neither ammonium hydrogen oxalate nor oxalic acid proved suitable alternatives in Step 2 owing to precipitation of insoluble lead salts, particularly in the presence of calcium. (C) 1999 Elsevier Science B.V. All rights reserved