Sampling and analytical quality control of the determination of aluminium in soybean leaves

A pot trial was set up with nine soil types, each replicated five times, in randomized blocks, to investigate the effect of soil properties on the uptake of Al by soybean plants. The variance of the measurements was estimated using five replicates of each soil type. This then enabled the statistical significance of the difference between results for each soil type to be established. The analytical variance was determined by two analyses per pot. Systematic errors during the analytical procedure were estimated by the analysis of reagent blanks and CRMs. Analysis of variance (ANOVA) was used to estimate the proportions of total variance contributed by analysis, sampling and geochemistry. Robust statistics were required because of the high sensitivity of classical ANOVA to a small number of outlying values, due to either technical or geochemical causes. In the case study described here, traditional analytical quality control apparently gave a satisfactory result. However, the results from robust ANOVA showed that the sampling error was the limiting factor for this application. It is suggested that the environmental analyst must consider errors from procedures of both sampling and analysis of environmental samples.