Solid-Surface Fluorescence Determination of Samarium(III) Using an Eosin-o-Phenanthroline-HTAB System: Application to Water and Sediment Samples

Samarium is a technologically relevant rare earth element whose trace determination in aquatic systems requires sensitive and accessible analytical alternatives. A solid-surface fluorescence method was developed for Sm(III) using eosin, o-phenanthroline, hexadecyltrimethylammonium bromide (HTAB), and cellulose filter paper as the retention and measurement substrate. Experimental variables, including pH, reagent volumes, and the order of addition, were optimized. The selected sequence was eosin → o-phenanthroline → Sm(III) → HTAB → phosphate buffer, at pH 6.5. The retained system was measured at excitation/emission wavelengths of 525/645 nm. Fluorescence intensity was linear from 1.31 to 375.9 ng∙L⁻¹, with R² = 0.9967. The calculated limits of detection and quantification were 0.39 and 1.31 ng∙L⁻¹, respectively. No significant interference was observed for the investigated species at the tested molar ratios. Recoveries were 92.71% - 105.42% in natural waters and 99.50% - 100.43% in sediment extracts. Sm(III) was detected in six of ten water samples, while acid-extractable Sm concentrations in sediments ranged from 509.76 to 1218.68 µg∙kg⁻¹. The aqueous fluorescence step avoided organic solvents and used small sample aliquots, low reagent concentrations, a cellulose support, and conventional instrumentation. These results demonstrate a simple and sensitive approach for Sm(III) screening in aquatic environmental samples, with potential advantages from a green analytical chemistry perspective that should be confirmed through formal assessment.