Improved sample preparation and quality control for the characterisation of titanium dioxide nanoparticles in sunscreens using flow field flow fractionation on-line with inductively coupled plasma mass spectrometry

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Titanium dioxide in nanoparticulate form is used in large scale in a variety of consumer products including sunscreens. There is an increasing need for methodology for the reliable characterisation of the particle size and size dependent elemental composition in these complex matrices. Such measurement capability is essential for underpinning safety assessments, for quality control of existing products and for correlation of nanoparticle characteristics with biological effects observed in toxicity tests. This work describes the first systematic comparison and optimisation of extraction methods for titanium dioxide nanoparticles in sunscreen samples. Sunscreens were selected because of their wide use, high fat content and matrix of high complexity. Defatting of the sample with hexane followed by bath sonication with an aqueous extractant was found to provide stable suspensions of secondary titanium dioxide particles for their size characterisation by flow field flow fractionation on-line with element selective detection by inductively coupled plasma mass spectrometry. Further addition of a small amount of hexane to the aqueous extractant resulted in particle disaggregation and thus allowed for characterisation of the primary particle size. A novel approach based on sample spiking with aluminium-labelled titanium dioxide reference particles of known size was used to study the effect of extraction and separation conditions on particle size distribution in the presence of the real sample matrix. The developed methodology was applied to analysis of commercial sunscreens with various sun protection factors. Titanium extraction efficiency, particle size distribution and titanium dioxide recovery from the FFF channel were determined for each product.