A multimethod platform based on the combination of dynamic image analysis (DIA) and single particle inductively coupled plasma mass spectrometry (spICP-MS) has been developed, validated, and applied for the first time to reliably determine the particle number concentration of small microplastics. The ability of the DIA/spICP-MS platform to reliably detect and quantify microplastics was carefully studied using polystyrene (PS) microparticles in the size range from 1 to 10 μm. Critical instrumental parameters affecting the accuracy of the number-concentration measurements were identified for both techniques. In the case of DIA, the detection threshold (DT) was found to be the most important, while for spICP-MS, both the calibration of the transport efficiency (TE) using the frequency method and the choice of sample introduction system were critical. Under optimal conditions, the number-concentration values obtained for 5 μm PS-Latex microspheres using DIA and spICP-MS methodologies agreed well within their associated uncertainty (u, k = 1 of approximately 2.5% for DIA and u, k = 1 of approximately 10.2% for spICP-MS). Main contributing factors to the overall measurement uncertainty were evaluated for methodology based on both techniques, being the variability in the number of detected particles for DIA and the number of detected particles and transport efficiency calibration for spICP-MS. These accounted for approximately 94% (DIA) and approximately 45% (spICP-MS) of the overall uncertainty budget, respectively.

A multi-modal metrological approach for the quantification of lanthanide doped carbon dots (Eu and Yb) uptaken by HeLa cells using sc-ICP-ToF-MS.

Reference materials (RMs) are increasingly needed to support number-based characterisation of nanomaterials (NM) in a regulatory context for the purpose of method development, validation and measurement quality control. To date, RMs for number concentration in suspension with a directly assigned value that is SI traceable have been rather scarce, being the LGCQC5050 the only material commercialised so far. This could be attributed to the limited availability of metrologically validated measurement methods and stability challenges associated with long-term storage of NM suspensions. This paper describes development and characterisation of the first RM consisting of 30-nm colloidal gold nanoparticles and value assigned for particle number concentration using the dynamic mass flow (DMF) method with single particle ICP-MS (spICP-MS). Special attention is paid to systematic assessment of the DMF method's performance under operating conditions set outside recommendations described in ISO/TS 19590:2024, but that still comply with key requirements of this method (e.g. use of an ICP-MS system in equilibrium, a cooled spray chamber, etc.). The results of such investigations are reported here for the first time. The paper also discusses practical considerations for the production, storage and transport of nano RMs, and provides guidance on best practice for the production and certification of future nano RMs in accordance with ISO 17034:2016.

A multi-technique platform for the size-resolved quantification of TiO 2 particles in food.

The concentration of cell-type specific extracellular vesicles (EVs) is a promising biomarker for various diseases. However, concentrations of EVs measured by optical techniques such as flow cytometry (FCM) or particle tracking analysis (PTA) in clinical practice are incomparable. To allow reliable and comparable concentration measurements suitable reference materials (RMs) and SI-traceable (SI—International system of units) methods are required. Hollow organosilica beads (HOBs) are promising RM candidates for concentration measurements of EVs based on light scattering, as the shape, low refractive index, and number concentration of HOBs are comparable to EVs of the respective size range that can be detected with current optical instrumentation. Here, we present traceable methods for measuring the particle size distribution of four HOB types in the size range between 200 and 500 nm by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM), as well as the number concentration by single-particle inductively coupled plasma mass spectrometry (spICP-MS). Based on the size and shape results, traceable reference values were obtained to additionally determine the refractive index of the shell of the HOB samples by FCM. Furthermore, the estimated refractive indexes of the HOBs plausibly agree with the refractive indexes of EVs of corresponding size. Due to their narrow size distribution and their similar shape, and low refractive index, all HOB samples studied are suitable RM candidates for calibration of the measured sample volume by optical methods within the photon wavelength range used, and thus for calibration of number concentration measurements of EVs in the size range indicated. This was confirmed as the number concentration values obtained by PTA and two independent flow cytometric measurements agreed with the concentration reference values obtained by two independent spICP-MS measurements within the calculated uncertainty limits.

Biomedical analytical applications, as well as the industrial production of high-quality nano- and sub-micrometre particles, require accurate methods to quantify the absolute number concentration of particles. In this context, small-angle x-ray scattering (SAXS) is a powerful tool to determine the particle size and concentration traceable to the Système international d'unités (SI). Therefore, absolute measurements of the scattering cross-section must be performed, which require precise knowledge of all experimental parameters, such as the electron density of solvent and particles, whereas the latter is often unknown. Within the present study, novel SAXS-based approaches to determine the size distribution, density and number concentrations of sub-micron spherical silica particles with narrow size distributions and mean diameters between 160 nm and 430 nm are presented. For the first-time traceable density and number concentration measurements of silica particles are presented and current challenges in SAXS measurements such as beam-smearing, poorly known electron densities and moderately polydisperse samples are addressed. In addition, and for comparison purpose, atomic force microscopy has been used for traceable measurements of the size distribution and single particle inductively coupled plasma mass spectrometry with the dynamic mass flow approach for the accurate quantification of the number concentrations of silica particles. The possibilities and limitations of the current approaches are critically discussed in this study.

The analysis of large nanomaterials with only one dimension in the range of nanometers (nanoclays) by SP-ICP-MS is proposed.

Single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) refers to the use of ICP-MS as a particle counting technique. When ICP-MS measurements are performed at very high data acquisition frequencies, information about (nano)particles containing specific elements and their dissolved forms can be obtained (element mass per particle, size and number and mass concentrations). As a result of its outstanding performance, SP-ICP-MS has become a relevant technique for the analysis of complex samples containing inorganic nanoparticles. This review discusses the maturity level achieved by the technique through the methods developed for the detection, characterisation and quantification of engineered and natural (nano)particles. The application of these methods in different analytical scenarios is comprehensively reviewed and critically discussed, with special attention to their current technical and metrological limitations. The emergent applications of SP-ICP-MS in the field of nanoparticle-tagged immunoassay and hybridization methods are also reviewed.

A procedure for the size characterization and quantification of titanium dioxide (TiO2) nano- and microparticles by Asymmetric Flow Field-Flow Fractionation (AF4) coupled to Dynamic Light Scattering (DLS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is described. Different strategies for size characterization with size standards and the use of the DLS signal for the estimation of hydrodynamic diameters are evaluated. The procedure has been applied to the characterization of TiO2 nanoparticles in photocatalytic products and crab sticks (surimis), where TiO2 is present as E171 food additive. Sizes in the range of 50–90 nm and 160–170 nm were estimated in the different photocatalytic products by AF4-DLS, in good agreement with the sizes predicted by calibration versus SiO2 and polystyrene standards. In surimis, sizes between 140 and 350 nm were estimated by AF4-DLS, similar to those reported in literature for E171 additive. These results were also compared to those obtained by single particle ICP-MS, which allowed the detection of a nano-sized fraction of TiO2 present in the four surimis analyzed. Titanium contents in one of the photocatalytic products determined by AF4-ICP-MS was 16.86 ± 2.54 mg g−1, whereas the alkaline extraction followed by AF4-ICP-MS allowed the determination of TiO2 content in four surimis at concentration levels in the range of the μg g−1 (from 3.14 ± 0.10 to 14.55 ± 1.46 μg Ti g−1), with channel recoveries above 85% in all cases. The method has been validated by comparison with the Ti content determined by ICP-OES after microwaved assisted acid digestion of all the samples. The methodology proposed allows the complete quantification of the (nano)particulate forms of titanium in complex matrices together with their size characterization.

The use of a commercially available short length channel (14 cm length) is proposed to improve the efficiency associated to the separation by asymmetrical flow field-flow fractionation of particles in the nanometer range respect to a standard channel (27 cm length). The effect of channel length on elution times, separation efficiency and resolution have been studied. Polystyrene particles between 50 and 500 nm in size have been used to compare the behavior of both channels. Theoretical aspects based on the different contributions on particle diffusion inside the channel during the separation process have been considered to justify the results obtained. Non-equilibrium diffusion contribution to the efficiency has shown to be the most relevant aspect to be controlled during the separation. The increment of the field strength applied through the cross-flow velocityallows the reduction of diffusion while keep elution times constant. The use of the same cross-flow in a channel with a smaller area is the key factor that justifies the better efficiencies observed along the whole size range studied (improvements that reach factors up to 4.7 in experimental efficiency respect to the standard channel were achieved). The separation of polystyrene particles of 100 and 200 nm was achieved with a resolution of 1.20, whereas a 0.66 value was obtained with the standard channel at the same elution times. Channel recoveries have been also compared under optimized conditions to ensure that no side effects are produced, including the separation of mixtures of TiO2 nanoparticles. Similar or even better values were obtained with the short length channel, with recoveries higher than 85% for all the polystyrene particles tested and 75% recovery for the TiO2 nanoparticle mixture, which justifies its use for the separation of nanoparticles, providing better resolutions without compromise elution times or recoveries.