Size-resolved analysis of trace elements in the dissolved fraction (<0.45 μm) of soil solutions using a novel lysimeter and asymmetrical flow field-flow fractionation coupled to ultraviolet absorbance and inductively coupled plasma mass spectrometry

The distributions of dissolved (<0.45 μm) trace elements (TEs) amongst major colloidal forms in soils have implications for their availability, accessibility, and toxicity to plants and animals. The size-resolved distributions of TE species in soil solutions were collected using lysimeters and were measured using asymmetrical flow field-flow fractionation (AF4) coupled to ultraviolet absorbance (UV) and inductively coupled plasma mass spectrometry (ICP-MS). Using this AF4-UV-ICPMS system, dissolved TEs were separated by size, and concentrations in major forms were quantified: “truly dissolved” primarily ionic and small molecules <ca. 1 kDa, organic-dominated colloids, and primarily inorganic colloids. The soil solutions were collected under vacuum using a novel surgical (316L) stainless steel (SS) lysimeter with a 5 μm pore size. Analyses were performed in the metal-free, ultraclean SWAMP laboratory. The acid-cleaned lysimeters yielded excellent blank values for TEs of environmental interest (i.e., Li, Al, V, Mn, Co, Cu, As, Mo, Ag, Cd, Ba, Pb, Th, and U). Lysimeter sampling offers the major advantage that it can minimize disturbances to the natural TE concentrations and distributions amongst major dissolved colloidal forms in soil solutions and thus provides information that is relevant to plant uptake.