A fast method for the chemical analysis of clays by total-reflection x-ray fluorescence spectroscopy (TXRF)

A new, fast and cheap method for the analysis of clays using total-reflection x-ray fluorescence spectroscopy (TXRF) has been developed following a full factorial experimental design to optimize the sample preparation conditions. The optimized method consists of the dispersion of 50 mg of clay into 2.5 ml of 1% Triton X-100 solution, the deposition of 5 μl of the suspension onto a siliconized quartz reflector and drying it at 50 °C on a heating plate. By using a commercial benchtop instrument, 15 major and trace elements were correctly quantified with 1000 s live time acquisitions. Thirty minutes were sufficient for both sample preparation and analysis. Validation experiments, performed using a certified reference material, showed recoveries in the range 80–120% for the main targeted elements, whereas the within laboratory reproducibility (RSDWLR) and the repeatability (RSDr) were lower than 20%, demonstrating the good precision and reliability of the method. Only in the case of Si, the RSDWLR and RSDr were higher than 20%, due to the variable contribution of the quartz reflector. Suitable LOD and LOQ values were estimated, varying from 0.1–0.4% for Al to 1–2 mg/kg for Sr, with higher sensitivity for elements with higher fluorescence energy (and high atomic number, Z). Finally, a good agreement was obtained between the results of the analysis of reference materials performed with the new TXRF method and a reference method, such as wavelength dispersive x-ray fluorescence spectroscopy (WDXRF). Based on the above performances, this method may represent a valuable and reliable alternative analytical tool when only small amounts of clay samples are available such as in the case of mineral synthesis, clays extraction from soils and sorption tests.