Laser Ablation(LA)-ICP-MS on soil thin sections: a powerful tool for assessing trace element distribution, pedogenetic processes and soil pollution

We recently proposed a new methodological approach for investigating trace element distribution in soils, combined with traditional micromorphological and geochemical techniques. We performed compositional spot analyses on 100μm-thick thin sections prepared from undisturbed soil samples, using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). This method was integrated with conventional observations using a polarizing optical microscope and a scanning electron microscope equipped with chemical microprobe analysis (SEM-EDS). This study, based on chemical analysis of different subportions of soil horizons (illuvial pedofeatures, pedogenic matrix and skeletal grains), permits a detailed assessment of microscale spatial variability and particle/element retention and mobility. Chemical concentrations can be measured with a high degree of accuracy and precision, reaching detection limits of 1-10 to 0.1-0.01 mg/kg, which are considerably lower than those obtained using other microanalytical techniques (EDS, WDS). We focused on behaviour of trace elements (including rare earths, heavy metals and radionuclides) in different types of silty/clay coatings and mineral or organic matrix with respect to parent rock. Main trends of increase from rock to matrix to coatings observed for many trace elements revealed a clear pedogenetic control on element fractionation, related to weathering of primary components, neogenesis of clay minerals, metal adsorption onto reactive sites of clays or organic matter, and further concentration by translocation processes of the fine fraction. In situ laser ablation techniques can be used as detailed tracers of chemical element behaviour in soils, pedogenetic/morphodynamic processes and as potential tools for assessing microsite location and mobility of soil pollutants.