Development of internal forest soil reference samples and testing of digestion methods

Abstract

Our research requires determinations of total elemental concentrations of forest soils. The lack of certified forest soil reference materials led us to develop internal reference samples. Samples were collected from three soil horizons (Oa, B, and C) at three locations having forested, acidic soils similar to those we commonly analyze. A shatterbox was used to homogenize the nine samples. The Radiation Laboratory of the University of Massachusetts, Lowell completed neutron activation analyses (NA) on our samples. Our laboratory performed five digestion procedures: microwave [nitric acid (HNO3), hydrochloric acid (HCl), fluoboric acid (HBF4), hydrogen peroxide (H2O2)], block [sulfuric acid (H2SO4), selenious acid (H2SeO3), and H2O2], hot plate ([hydrofluoric acid (HF) and perchloric acid (HClO4)], and flux [lithium borate (LiBO2) and lithium treborate (Li2B4O7)]. Concentrations of phosphorus (P), iron (Fe), copper (Cu), manganese (Mn), magnesium (Mg), sodium (Na), aluminum (Al), calcium (Ca), and potassium (K) were measured by direct current plasma spectrometry (DCP). The results of the digestion procedures were compared with results from the neutron activation analyses (NA) allowing us to choose the most effective digest procedure. Each digestion procedure obtained varied elemental recovery rates (RR) ranging from Al with <10% RR for a C horizon sample digested in a block digestor to Ca with > 125% RR for a B horizon sample digested on a hot plate when compared to the neutron activation analyses values. Overall, the maximum recovery rates were achieved by the flux, microwave, and hot plate procedures.