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.

Methodology for the accurate determination of number concentration of inorganic nanoparticles (NP) by single particle inductively coupled plasma mass spectrometry (spICP-MS) using the novel dynamic mass flow (DMF) approach is systematically described. Using this method the determination of transport efficiency (TE) is achieved without the need for a reference nanomaterial. The impact of key parameters on the accuracy and uncertainty of the number concentration data obtained with this approach was evaluated. In particular the number of detected NP in the time scan is the major contributing factor to the overall measurement uncertainty. For Au NP of spherical shape with number based concentration in the range of 4.0 × 10¹² to 2.0 × 10¹⁴ kg⁻¹ (depending on the particle size) a relative expanded uncertainty (k = 2) of less than 10% was achieved. This reference methodology was also evaluated for the accurate determination of number concentration of more complex NP namely triethanolamine (TEA)-stabilised TiO₂ NP, for which like-for-like NP reference materials are not available. Using a sample mass flow of 0.3578 g min⁻¹ (with an associated uncertainty of 0.0002 g min⁻¹, k = 1) the average transport efficiencies for Au NP (in trisodium citrate) and TiO₂ NP (in TEA/NaOH) were very similar (7.57 ± 0.13% and 7.77 ± 0.22%, k = 1, respectively). Finally the number concentration values for both NP types agreed well with those obtained using particle tracking analysis (PTA), providing evidence for the good agreement between mass-based TE of the sample and NP-based TE with the newly proposed method.

The development of a novel and straightforward procedure for the preparation of matrix-matched calibration standards for the quantitative imaging of iron (Fe) in biological tissues by laser ablation (LA)-ICP-MS with on-tissue internal standard addition is described. This simple approach enabled on-tissue addition of Rh as internal standard to samples (with heterogeneous Fe distribution) and calibrants (with homogeneous Fe distribution). This is achieved without altering the original Fe distribution of the sample. Calibration standards were prepared by full horizontal immersion of slides with mounted homogenised sheep brain tissue section into the corresponding solution containing 0.5, 0.75, 1, 5, 10 and 20 mg kg⁻¹ Fe (each also containing 250 µg kg⁻¹ Rh as IS) in pure methanol for 30 minutes (6 immersions, each for 5 minutes). Subsequent air-drying (bench drying at room temperature) for approximately 5 minutes was undertaken in between consecutive immersions, to prevent long-term exposure of the tissue to lipid degradation. Tissue-matched standards were characterised in-house for Fe composition, homogeneity and stability (at storage temperatures of −80 °C, −20 °C, 4 °C and 25 °C for up to 2 months) in order to investigate their suitability as calibrants for quantitative LA-ICP-MS. The homogeneity data suggested that the materials are homogeneous in terms of Fe and Rh distribution with RSDs (n = 30) of 8.3% and 4.7%, respectively. The Fe measurement precision was improved by approximately a factor of 2 when normalising ⁵⁶Fe intensities to ¹⁰³Rh intensities; the RSD (n = 30) for ⁵⁶Fe/¹⁰³Rh was 3.6%. The produced calibration standards were found to be stable when stored at room temperature for approximately 50 days, suggesting that they can be reused for multiple batches. Using LA coupled to double-focusing sector field ICP-MS in medium resolution mode (m/Δm = 4000), linear calibration over a range of 107 to 1519 mg kg⁻¹ Fe (R² = 0.99) was achieved with a limit of detection of 1.84 mg kg⁻¹ Fe. Assessment of the accuracy of the method for the quantitative imaging of Fe in tissues was undertaken by comparison of the LA-ICP-MS data with that obtained by micro-XRF; the average Fe concentrations in selected tissue regions obtained by using XRF fell within the window defined by the LA-ICP-MS values and their associated standard deviations.

Analytical chemistry is crucial for understanding the complex behavior observed for engineered nanomaterials (ENMs). A variety of analytical chemistry techniques and methodological approaches are used for isolation/purification and determination of the composition of pristine nanomaterials and for the detection, identification, and quantification of nanomaterials in nano-enabled consumer products and the complex matrices found in cosmetics, food, and environmental and biological samples. Adequate characterization of ENMs also requires physicochemical characterization of number of other properties, including size, shape, and structure. The requirement for assessment of a number of ENM properties frequently requires interdisciplinary approaches and multi-modal analysis methods. This technical report starts with an overview of ENMs definitions and classification, their properties, and analytical scenarios encountered with the analysis of both pristine nanomaterials and complex matrices containing different nanomaterials. An evaluation of the current status regarding nanomaterial identification and characterization for regulatory purposes and legislation, including emerging regulations and related scientific opinions, is provided. The technical report also presents a large and critical overview of the metrology of nanomaterials, including available reference materials and the development and validation of standardized methods that are currently available to address characterization and analysis challenges. The report focuses mainly on chemical analysis techniques and thus it is complementary to previous IUPAC technical reports focused on characterizing the physical parameters of ENMs and on nanotoxicology.

A novel strategy is reported for the quantitative analysis of the Fe spatial distribution in biological tissue using laser ablation with ICP-MS and on-line double isotope dilution analysis (LA-ICP-IDMS). The proposed on-line IDMS method involves post-ablation introduction of an isotopically enriched ⁵⁷Fe spike solution using a total consumption nebuliser. To investigate the potential applicability of the developed method to biological tissue with varying Fe concentrations (akin to those observed in bio-imaging), the effect of sample-to-calibration standard blend ratio on the accuracy of the Fe data was investigated over a range of 1 : 0.2 to 1 : 10. To achieve this, homogenised sheep brain tissue doped with Fe (251 μg g⁻¹) was used as the model sample. Recoveries of 80–109% of the expected Fe concentration in the model tissue sample (as determined by ID-ICP-MS of the tissue digest) were obtained over a sample-to-standard ratio range of 1 : 1 to 1 : 5. A systematic estimation of measurement uncertainty for LA-ICP-IDMS was undertaken and for the first time the mass flow rate of the material was determined via single-IDMS. An overall combined expanded uncertainty (k = 2) of 15–27% was achieved for ratio matching of 1 : 1 to 1 : 5. The factors with greatest contribution to the overall uncertainty were the mass of spike, the measured ratio of the standard blend and the mass of calibrant. External calibration with internal standardisation was performed on the same model sample for the purpose of comparison. The measurement uncertainty associated with this calibration approach was for the first time estimated for LA bio-imaging by taking into account the contributions from the signal intensity variance, the errors from least squares regression and concentration of the standards. For external calibration the overall relative expanded uncertainty was approximately 50% (k = 2), with the uncertainty in the linear least squares regression (R² of 0.9833) and the signal variation being the main contributing factors. The results for Fe in the model sample agreed well with those determined via LA-ICP-IDMS. For the first time, the potential of a LA-ICP-MS isotope dilution calibration strategy to validate higher throughput calibration methodologies (e.g. matrix-matched external calibration with internal standardisation), as would be required for routine medical applications, has been demonstrated.

The industrial exploitation of high value nanoparticles is in need of robust measurement methods to increase the control over product manufacturing and to implement quality assurance. InNanoPart, a European metrology project responded to these needs by developing methods for the measurement of particle size, concentration, agglomeration, surface chemistry and shell thickness. This paper illustrates the advancements this project produced for the traceable measurement of nanoparticle number concentration in liquids through small angle X-ray scattering (SAXS) and single particle inductively coupled plasma mass spectrometry (spICPMS). It also details the validation of a range of laboratory methods, including particle tracking analysis (PTA), dynamic light scattering (DLS), differential centrifugal sedimentation (DCS), ultraviolet visible spectroscopy (UV-vis) and electrospray-differential mobility analysis with a condensation particle counter (ES-DMA-CPC). We used a set of spherical gold nanoparticles with nominal diameters between 10 nm and 100 nm and discuss the results from the various techniques along with the associated uncertainty budgets.