Total elemental analysis digestion method evaluation on soils and clays

Wilson, M. A.; Burt, R.; Lynn, W. C.; Klameth, L. C.

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 ( lt 2 mm) from 12 soils, and the clay ( lt 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 Al-2O-3, Fe-2O-3, and K-2O contents and oxide recovery (summation of experimental means for oxides of all reported elements) than all other methods.