The use of secondary ion mass spectrometry coupled with image analysis to identify and locate chemical elements in soil minerals: The example of phosphorus

The mobility and bioavailability of elements in soils and sediments largely depends on their distribution on the diverse inorganic and organic constituents. This work addresses the example of phosphorus (P) associated to goethite and calcite, that is, to the major minerals involved in the retention of P in soils and sediments in calcareous environments. Synthetic goethite (FeOOH) and calcite (CaCO3) were reacted with P prior to being analysed by dynamic secondary ion mass spectrometry (SIMS). Powdery samples were embedded in resin, cut in thin sections, and imaged with a CAMECA IMS 4F ion microscope used in scanning mode with a primary ion beam of caesium that produced negatively charged secondary ions ((-)) (Cameca, Cedex, France). Carbon, O, P, and calcium (Ca) were directly imaged at m/z 12, 16 31, and 40, respectively, while Fe was imaged via the polyatomic ion FeO- ion at m/z 72. The SIMS data were treated by image analysis procedures. The visual comparison of images and the scatterplot method showed that P strongly interacted with goethite, probably following an adsorption process, and was thus evenly distributed at its surface. Conversely, P was not evenly distributed at the surface of calcite which rather suggests a precipitation process, and the scatterplot method confirmed a poor relationship between P and Ca. For the goethite-calcite mixture, visual examination suggested that P occurred as clusters which were largely associated with calcite, whereas a statistical analysis of the various images showed that the distribution of P was largely related to that of goethite particles. This work confirms the potential contribution of iron oxides in the retention of P in calcareous environments and shows that coupling image analysis to sensitive analytical techniques such as SIMS is a powerful approach for providing quantitative information on the location of elements at low bulk concentrations.