Total elemental analysis digestion method evaluation on soils and clays

The standard digestion method for total elemental analysis of soil material by the Soil Survey Laboratory (USDA Natural Resources Conservation Service) uses a 2-mL clay suspension with hydrofluoric acid (HF) in a closed Teflon digestion vessel (method HF-SUS). The accuracy and efficiency of elemental recovery by method HF-SUS was compared to: (a) modification of HF-SUS by use of a dried sample (method HF-DRI); (b) modification of HF-SUS by use of a dried sample and HF+aqua regia (method HF+AR); (c) sample digestion by Li metaborate fusion (method FUS); and (d) microwave digestion of samples with HF+aqua regia in Teflon bombs (method MICRO). Three replications of three standard reference materials (SRMs), fine-earth (<2 mm) from 12 soils, and the clay (<2 mu m) from 10 of those soils were analyzed. Method HF+AR shows the most consistent statistical agreement with the certified SRM values. Analysis of variance (ANOVA) indicates significant effects (alpha=0.05) for method of digestion, nonsignificant effects for method times SRM and method times clay, but significant effects for method times fine-earth. Composition and/or variability of material are significant factors in the method of digestion. Method HF+AR yields significantly higher experimental means of Al2O3, Fe2O3, and K2O contents and oxide recovery (summation of experimental means for oxides of all reported elements) than all other methods.