Clay minerals are phyllosilicates originating from the weathering of primary silicates and aluminosilicates. Due to their mineralogical and chemical characteristics they are largely employed in different fields such as chemical, pharmaceutical and food industry, ceramics production, crude oil extraction, etc. In order to assess the suitability of a certain clay material for a particular use, both mineralogical and chemical analyses are usually performed. In particular, for the elemental analysis of clays, standard procedures are based on WDXRF or ICP-OES analyses, which usually require 1-5 g of sample (UNI EN ISO 21587-3:2007, EN ISO 12677:2011). In addition, both the production of beads for WDXRF and the sample digestion for ICP-OES analyses need long and elaborate processing of samples, often requiring costly chemicals which can be also dangerous for the environment and the operator. Furthermore, in case of specific applications (e.g., clay micro-synthesis, bacteria-induced clay synthesis, remediation or sorption studies, etc.) the production of enough amount of sample is often a critical issue. In such cases, chemical analysis is usually performed with 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 and reliable method for the elemental analysis of clays employing TXRF is proposed. Clays are analysed by TXRF as suspensions deposited on sample carriers (reflectors). The method has been optimized using a two level full factorial design, using selenium as internal standard, to obtain the best recovery rate for each focused element. In particular, the following factors have been evaluated: sample amount, surfactant volume and reflector type. The best results were obtained by suspending 50 mg of clay in 2.50 ml of surfactant and using quartz reflectors for TXRF analyses. 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).
A FAST NEW METHOD FOR THE CHEMICAL ANALYSIS OF CLAYS BY TOTAL-REFLECTION X-RAY FLUORESCENCE SPECTROSCOPY (TXRF)
Ignazio Allegretta
;Roberto Terzano
2018-01-01
Abstract
Clay minerals are phyllosilicates originating from the weathering of primary silicates and aluminosilicates. Due to their mineralogical and chemical characteristics they are largely employed in different fields such as chemical, pharmaceutical and food industry, ceramics production, crude oil extraction, etc. In order to assess the suitability of a certain clay material for a particular use, both mineralogical and chemical analyses are usually performed. In particular, for the elemental analysis of clays, standard procedures are based on WDXRF or ICP-OES analyses, which usually require 1-5 g of sample (UNI EN ISO 21587-3:2007, EN ISO 12677:2011). In addition, both the production of beads for WDXRF and the sample digestion for ICP-OES analyses need long and elaborate processing of samples, often requiring costly chemicals which can be also dangerous for the environment and the operator. Furthermore, in case of specific applications (e.g., clay micro-synthesis, bacteria-induced clay synthesis, remediation or sorption studies, etc.) the production of enough amount of sample is often a critical issue. In such cases, chemical analysis is usually performed with 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 and reliable method for the elemental analysis of clays employing TXRF is proposed. Clays are analysed by TXRF as suspensions deposited on sample carriers (reflectors). The method has been optimized using a two level full factorial design, using selenium as internal standard, to obtain the best recovery rate for each focused element. In particular, the following factors have been evaluated: sample amount, surfactant volume and reflector type. The best results were obtained by suspending 50 mg of clay in 2.50 ml of surfactant and using quartz reflectors for TXRF analyses. 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).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
