Total-reflection X-ray fluorescence spectroscopy (TXRF) as a fast new analytical method for the chemical analysis of clays

Clay minerals are phyllosilicates originating from the weathering of primary silicates and aluminosilicates. Their importance in agricultural and environmental sciences is due to their physical and chemical properties, such as their capability to retain water, ions, pollutants and organic molecules in their lattice structure. Usually, a large amount of soil is needed to extract a sufficient amount of clay minerals for their study and characterisation. In addition, long and elaborate procedures are normally employed to extract soil minerals from soil, requiring in some cases several weeks. After extraction, chemical analyses are usually performed using ICP-OES or WDXRF spectroscopies, which require 1-5 g of sample for digestion or bead/pellet production, respectively. Both these procedures need a time-consuming and elaborate processing of samples, and the use of expensive and dangerous chemicals in the case of acid digestion. Moreover, in the case of specific applications such as clay micro-synthesis, bacteria-mediated clay synthesis and remediation or sorption studies, the production of a sufficient amount of sample is often a critical issue. In these cases, chemical analysis is usually performed by Scanning Electron Microscopy coupled to microanalysis (SEM-EDS) which, besides requiring the use of a complex and expensive instrumentation, cannot provide information about the bulk sample composition. In the last two decades, total-reflection X-ray fluorescence spectroscopy (TXRF) has proven to be a sensitive and fast technique for the elemental analysis of several types of matrices (both organic and inorganic), requiring only a low amount of sample (few mg) and a simple sample preparation. In this work, a simple, fast, green and reliable method for the elemental analysis of clays employing TXRF is proposed. Clays are directly analysed by TXRF as suspensions deposited by a micropipette on sample carriers (reflectors). The method has been optimized using a two level full factorial design, to obtain the best recovery rate for each investigated element. Selenium (Se) was employed as internal standard. In particular, the following factors have been evaluated: sample amount, surfactant volume and reflector material. The best results were obtained by suspending 50 mg of clay in 2.50 ml of surfactant and using quartz reflectors. The optimized method has been subjected to in-house validation to evaluate analytical performances such as linearity, quantification limits, trueness and precision. The validation study included the analysis of certified reference materials (SARM-CRPG-CNRS). Satisfactory performances were obtained for most of the major elements including Mg, Al, Si, K, Ca, Ti, Mn and Fe and some minor elements (Cl, Ni, Zn, Ga, Rb, Sr and Pb).