A new protocol to detect light elements in estuarine sediments by X-ray microanalysis (SEM/EDS)

The analytical scanning electron microscope (SEM) has been used to determine the presence and distribution of atomic elements in mineralogy. However, the detection of light elements such as carbon is difficult to obtain with standard energy-dispersive X-ray spectrometry (EDS) and usual proceedings for SEM. This study proposes a new protocol to detect calcium carbonate by SEM/EDS using sediments from the Jaguaribe River estuary, NE Brazil, as a model. Handmade gold mounting discs (Au stubs) were used as sample support and samples were adhered with inexpensive glue (Loctite Super_Bonder) or directly disposed on the Au stubs. CaCO3 and NaCl for chemical analysis were used as control and counterproof to the carbon adhesive tape. Control salts EDS analyses indicate that the method was efficient to detect light elements. Sediments obtained from different depths in the core sampled at the Jaguaribe River estuary consist of particles and aggregates with diverse morphology that covers a wide range of particle or aggregate size. Morphology and dimensions were similar for all core depths. Analysis of samples disposed on gold mounting disc without glue showed that sediment bulk particles usually presented small particles adhering on the surface. Clay minerals were predominant but silica was also often identified. Calcium was a trace element in a small number of sediment bulk particles. Biological and non-biological calcium carbonates, including nanoparticles, were identified in all core depths. X-ray emitted from Au stub did not interfere in the CaCO3 EDS analysis. Calcium carbonate particles from sediments were identified using this novel approach.