Certification of trace metal extractable contents in a sediment reference material (CRM 601) following a three-step sequential extraction procedure

Sequential extraction schemes have been developed in the past 20 years for the determination of binding forms of trace metals in sediment. The lack of uniformity of these schemes, however, did not allow the results so far to be compared worldwide nor the procedures to be validated. Indeed, the results obtained by sequential extraction are operationally defined, i.e. the ‘forms’ of metals are defined by the determination of extractable elements using a given procedure. Therefore the significance of the analytical results is related to the extraction scheme used. Another problem which hampered a good comparability of data was the lack of suitable reference materials which did not enable the quality of measurements to be controlled. Owing to this lack of comparability and quality control, the Community Bureau of Reference (BCR, now Standards, Measurements and Testing Programme) has launched a programme of which one of the aims was to harmonize sequential extraction schemes for the determination of extractable trace metals in sediment. This programme involved the comparison of existing procedures tested in two interlaboratory exercises, and it developed into a certification campaign of extractable trace element contents in a sediment reference material, following a three-step sequential extraction procedure duly tested and adopted by a group of 18 EU laboratories. This paper briefly describes the results of the interlaboratory studies and gives all details on the preparation of the sediment reference material, CRM 601, the homogeneity and stability studies and the analytical work performed for the certification of the extractable contents of some trace elements, following a standardized sequential (three-step) extraction procedure.