Report No DWI0230

Sept 1982

Sixty-three laboratories have taken part in an interlaboratory comparison to define the extent of analytical errors associated with routine metal determinations on a sample of soil. Average accuracy (± maximum probable deviation) was found to be 37 per cent for cadmium, 25 per cent for chromium, 14 per cent for copper, 21 per cent for nickel, 14 per cent for lead and 12 per cent for zinc. The results obtained for this exercise which used a contaminated soil with enhanced concentrations of metals were more accurate than for the previous survey* which used a sample of background soil of lower metal content. The accuracy and bias of groups of laboratories using different methods of sample preparation is reported, including the influence of filtration, background correction,and flame fuel for Cr determinations. A wet digestion procedure with nitric acid, used by about one third of the participating laboratories, produced "total" levels of Cd, Cr, Cu, Ni, Pb and Zn with comparatively high overall accuracy and little bias. In contrast, procedures involving dry ashing gave low results for Cd and high results for Cr, Ni, Pb and Zn, and were associated with high precision errors. Filtering of digests produced lower and less accurate results for Cd, Cr, Cu and Pb. The use of background correction reduced estimates of Cd and Ni concentrations by 25 per cent on average. The comparison of methods for determining extractable concentrations of Cu, Ni and Zn in soil showed that an EDTA method produced more accurate results than an acetic acid method for all three elements. Only between five and fourteen laboratories (according to element) attempted analysis of an extra six elements, arsenic, boron, fluorine, mercury, molybdenum and selenium. The methods used and results obtained varied widely suggesting that more work is needed on methods suitable for the routine determination of these elements in soils receiving sludge.

* See page 47 for details of first survey.

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.