Temporal Stability of Extractable Contents

One of the major hurdles to be surmounted, before the preparation of a reference material certified for extractable trace metal content could be contemplated, was the question of temporal stability. This problem has been studied by Salomons and Scheltens [200] who repeated, in 1987, the sequential extraction analysis of freshwater sediments first carried out in 1975. While those authors were somewhat disappointed in that significant changes did occur in the measured extractable content after 12 years, it is likely that the temporal stability was sufficient for the sediments to be characterized on each occasion in such a way that decisions on the management or use of the sediments would be unchanged. 

A test of the stability of the extractable contents of a single soil was carried out for EDTA, acetic acid, ammonium acetate and calcium chloride extracts. With the exception of chromium, the EDTA extractable contents were shown to be stable over 3 year intervals within about 10% for the elements Cd, Cu, Fe, Mn, Ni, Pb and Zn [193]. For acetic acid extracts, one-year changes for Cr, Fe, Mn and Zn were poorer than for EDTA, but for Cd, Cu, Ni and Pb the extracts were reasonably stable. For ammonium acetate and calcium chloride extracts, however, the results, apart from Cd and Ni in ammonium acetate, were useless. These failures in ammonium acetate and calcium chloride were probably related to the fact that the solution concentrations measured were too low for reliable determination by the FAAS and ICP-AES methods used. 

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