Direct quantification of TiO2 nanoparticles in suspension by grazing-incidence X-ray fluorescence spectrometry: Influence of substrate pre-treatment in the deposition process

X-ray fluorescence at grazing incidence (GIXRF) was investigated as a method for the quantification of TiO2 nanoparticles in aqueous suspensions. One of the major advantages of this technique is the possibility to analyze the particles without pre-treatment, like harsh acid digestion, as required by most other conventional methods. However, reliable quantitative measurements require a number of precautions. Particularly, the deposition process of the sample on the flat reflecting substrate must maintain homogeneity in composition and concentration over the entire surface of the deposition residue once dried. Scanning electron microscopy showed that using an adhesive coating of the substrate significantly improves the morphology and chemical homogeneity of the residue, hence leading to better performance of the method from a quantitative point of view. Linear calibration curves using internal standardization were established with ionic Ti and with two different types of TiO2 nanoparticles. Low limits of detections of 18 μg L− 1 and 52 μg L− 1 at incident angles of 0.20° and 0.75°, respectively, were obtained. It was found that correlation factors of the calibration linear fits were particle-size dependent, which was assigned to sampling problems due to possible incomplete dispersion of the particles in suspensions. The measured fluorescence of the dried deposits changed within a 4-month timespan for both types of TiO2 nanoparticles, demonstrating the very peculiar behavior of these particulate samples.