NanoSIMS as a tool for characterizing soil model compounds and organomineral associations in artificial soils

Purpose: Secondary ion mass spectrometry at the nanoscale (NanoSIMS) is a new and promising technique in soil science, as it allows us to explore the elemental and isotopic composition of a solid sample with high sensitivity at a submicron scale. In this study, we demonstrate that it is possible to differentiate the major components of soils by this technique. Materials and methods: For this purpose, we employed samples from incubated mixtures of soil components of known composition (clay minerals, Fe oxide, organic material, and quartz), so-called artificial soils. Samples were prepared from particle size and density fractions of soils of various compositions and investigated with reflected light and electron microscopy in combination with energy dispersive X-ray spectroscopy prior to NanoSIMS analysis. Results and discussion: Our results show that we were able to show the submicron arrangement of the various components and to differentiate between charcoal and soil organic matter. Attachment of organic material to mineral surfaces was predominantly found to occur in patchy structures on the clay minerals, whereas only little sorption of homogeneously distributed organic material onto Fe oxides occurred. Although there are several reasons conceivable why we did not detect more sorption of organic matter to Fe oxides, it is likely that this is caused by the neutral pH of the soils, hampering sorption to the variable-charged surface sites of the Fe oxide. Conclusions: Consequently, NanoSIMS enables the analysis of submicron processes in soil science-related research. However, the very heterogeneous matrix of soil particles leading to various ionization rates will make attempts for a quantitative analysis difficult, which is also due to a lack in the availability of suitable standards representing these complex matrices.