Judging Fitness for Purpose of a X-ray Fluorescence Based Method Investigating the Vertical Distribution of Metals and Metalloids in Soil Profiles

A column of soil, excavated from a contaminated landscape was evaluated by means of X-ray fluorescence analysis. The measurements were intended to assess the vertical distribution of heavy metals and toxic elements in the depth profile. To judge fitness for purpose of the analytical method used the element specific power functions were derived yielding the minimum detectable variations of analyte concentrations refer to the investigated soil profile. The required measurement uncertainty components caused by both the sampling procedure and chemical analysis were empirically estimated using a nested sampling design (duplicate method). For this purpose the full length of the soil core was divided into horizontal layers. From each selected layer (sampling target) two composite samples were taken by simple random sampling to represent the typical composition of the sampling target. The pool of measurement results, obtained for the nested sampling design finally was subjected to variance analysis. The evaluation of the estimated variance components in terms of the percentage of total variance confirmed fitness for purpose for the method used.