Total-reflection X-ray fluorescence analysis (TXRF) of plant's guttation fluids as a new, fast, and non-invasive strategy for the assessment of the bioavailability of Zn, Cd and Pb in contaminated soils

The assessment of potentially toxic elements (PTE) concentration in biofluids is often used for the evaluation of their bioavailability in polluted environments. In the soil-plant system, the analysis of the composition of the xylem fluid can provide a real snapshot of the elements taken up from the soil by the plant. However, xylem collection is often difficult, and, for herbaceous plants, it requires cutting the plant. Alternatively, xylem can be collected through leaves as naturally exuded drops (i.e. guttation), thus in a fully non-destructive way. The guttation phenomenon is yet limited to micro-volumes, therefore the analysis with most techniques is challenging. For the first time, the capability and reliability of total-reflection X-ray fluorescence (TXRF) spectrometry for the quantification of PTE in plant's guttation fluids was tested in this work. In particular, the study was led on fluids sampled from Lolium rigidum plants grown in a soil contaminated with Zn, Cd, and Pb. Two different TXRF spectrometers were used and compared, equipped either with Mo- or W-anode-based X-ray sources; inductively coupled plasma optical emission spectroscopy (ICP-OES) and graphite furnace atomic absorption spectrometry GF-AAS were used as reference techniques. Differently from these latter, approximately 30 μL of sample were sufficient for the quantification of Zn, Cd, and Pb through TXRF (along with the determination of other essential plant nutrients). Furthermore, the comparison with ICP-OES and/or GF-AAS showed various improvements in using TXRF, including a fast sample preparation, a reduced use of chemicals, the multi-elemental capability. These results suggest that TXRF analysis of plant guttation fluids could represent a novel non-destructive, expeditious and “green” analytical approach for the study of Zn, Cd, Pb and other PTE availability in polluted soils. • TXRF allows to quantify Cd, Pb and Zn in 10 μL of plant's guttation fluids (PGF). • The best LOD obtained for Zn, Pb and Cd are 2.7, 0.6 and 16.2 μg L-1, respectively. • PTEs' concentrations in PGF vary according to soil characteristics. • Along with PTE, several mineral nutrients were simultaneously detected with TXRF. • The method can be potentially applied for the assessment of PTE bioavailability.