Radiogenic isotope signatures of Sr and Pb in soils are important tracers in the determination of e.g., food provenance, animal migration, and contamination sources and therefore widely applied in numerous fields of research spanning from environmental science to archaeology, ecology, and forensics. One common sampling strategy is to use equilibrium-based soil extraction methods to assess labile fractions of Sr and Pb available for biological uptake. Critically, these methods produce complex sample matrices which make subsequent sample preparation for isotope ratio analysis by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) resource- and time-consuming.
Diffusive gradients in thin films (DGT) provides an alternative sampling strategy for assessing labile Sr and Pb fractions in soil. DGT mimics diffusion-based element supply and uptake by biota such as plants and enables concomitant matrix separation and analyte preconcentration during sampling. Yet, simultaneous sampling of Sr and Pb by DGT in soil has proven especially challenging given the conditions of high ion competition with major matrix cations (Mg, K, Ca) and typically trace- to ultra-trace levels of Sr and Pb present in soil porewater. Here, a novel DGT method was evaluated under laboratory conditions for MC-ICP-MS-based assessments of Sr and Pb isotope ratios in soils and compared to equilibrium-based extraction methods when applied to natural soil samples. (1) Moreover, the transfer of Sr and Pb isotope signatures from different soil types (n = 5) to different plant species (n = 3) was studied in greenhouse experiments.
Results showed that DGT enables quantitative sampling of labile Sr and Pb under natural conditions (pH = 4.8-8.2; I(NaNO3) = 0.01-0.1 mol L-1; γ(Ca2+) = 50-160 mg L-1 in synthetic soil solution matrix), permitting the accurate assessment of isotopic variations without significant isotopic fractionation and with low uncertainty (uc,rel = 0.01-0.03 %). Isotope ratios determined by DGT showed excellent correlation with those measured in soil extracts (R2 ≥ 0.99) and plant tissues (R2 ≥ 0.99). Implications for environmental analysis and source tracing of Sr and Pb, along with future possibilities of DGT in isotope research will be discussed.