Analysis of complex particle mixtures by asymmetrical flow field-flow fractionation coupled to inductively coupled plasma time-of-flight mass spectrometry

AF4/ICP-ToF-MS for the investigation of species-specific adsorption of organometallic contaminants on natural colloidal particles

Organotin (OT) compounds, while crucial in many industrial applications, pose substantial risks to the environment and human health. The toxicity and environmental behaviour of OTs depend on their chemical form, i.e., the type and number of organic substituents. Each species thus exhibits distinct toxicity profiles and varying binding affinities to environmental colloids, which influence their mobility, bioavailability, and environmental impacts. To date, however, most studies addressed speciation and colloidal characterization separately, leaving the combined determinations of organometallics along with their carrier colloids largely elusive. Here, we develop and validate an on-line measurement system to quantify the adsorption dynamics of 10 OT species on natural colloidal particles (<500 nm). The approach integrates a versatile fractionation technique (AF4), with a state-of-the-art multi-element analyzer (ICP-ToF-MS), achieving Sn detection limits as low as 6.0 ng/L. The method separates colloid-free OT species from those bound to colloids and enables the determination of OT interactions with distinct colloidal fractions. Validated in both fractionation and detection, the method provides reliable data that could elucidate the species-specific and temporal aspects of species-colloids adsorption processes. The results feature comparative studies of 10 OT species, offering critical insights into OT mobility and distribution in environmental systems.

Electrical asymmetrical flow field-flow fractionation: Fundamentals, evolution, applications, and prospects

Among the plethora of techniques that conforms the Field-Flow Fractionation (FFF) family, electrical field-flow fractionation (ElFFF) was designed to separate different analytes based on their size and electrophoretic mobility (µ). However, major technical and operational issues made this technique to fall into oblivion. Many of those drawbacks can be circumvented if another field is employed as the main driving force for the elution in the same channel, such as the most successful and useful FFF-related technique, asymmetrical flow field-flow fractionation (AF4). The combination results in a new member of the FFF family termed as electrical asymmetrical flow field-flow fractionation (EAF4). This technique was conceptualized in 2015, and has gained attention, especially in the study of biomolecules, nanomaterials, their interactions and transformations, due to its ability to determine size-resolved electrophoretic mobility of this kind of analytes, which is not directly accessible by batch techniques based on e.g., electrophoretic light scattering. This review gives an overview of this novel analytical technique, including a summary of the theoretical elements behind this approach, followed by a discussion of the possibility to provide multiple information through its coupling with a variety of detectors. Also, the applications of EAF4 to analytes from the nano to the micro scale in diverse matrices are presented. Finally, the challenges that EAF4 faces today and trends for the near future are addressed.

A multi-technique analytical approach to support (eco)toxicological investigation of zinc oxide nanoparticles

Understanding the mechanism of toxicity of nanoparticles and their behavior in biological environments is crucial for designing materials with reduced side effects and improved performance. Among the factors influencing nanoparticle behavior in biological environments, the release and bioavailability of potentially toxic metal ions can alter equilibria and cause adverse effects. In this study, we applied two on-line Field-Flow Fractionation (FFF) strategies and compared the results with off-line benchmarking centrifugal ultrafiltration to assess a key descriptor, namely the solubility of zinc oxide (ZnO) nanoparticles. We found that, at the highest nanoparticle concentrations, the nanoparticle-ion ratio quickly reaches equilibrium, and the stability is not significantly affected by the separation technique. However, at lower concentrations, dynamic, non-equilibrium behavior occurs, and the results depend on the method used to separate the solid from the ionic fraction, where FFF yielded a more representative dissolution pattern. To support the (eco)toxicological profiling of the investigated nanoparticles, we generated experimental data on colloidal stability over typical (eco)toxicological assay durations. The Zeta Potential vs pH curves revealed two distinct scenarios typical of surfaces that have undergone significant modification, most likely due to pH-dependent dissolution and re-precipitation of surface groups. Finally, to enhance hazard assessment screening, we investigated ion-dependent toxicity and the effects of exposure to fresh water. Using an in vitro human skin model, we evaluated the cytotoxicity of fresh and aged ZnO nanoparticles (exposed for 72 h in M7), revealing time-dependent, dose-dependent, and nanoparticle-dependent cytotoxicity, with lower toxicity observed in the case of aged samples.

Basic and advanced spectrometric methods for complete nanoparticles characterization in bio/eco systems: current status and future prospects

The use of engineered nanoparticles in the environment and human life has increased in the last 20 years. The risk assessment concerning application of nanomaterials in biological systems requires their thorough characterization. Understanding the correlations between physicochemical properties of nanoparticles concerning not only the size, particle size distribution, number concentration, degree of aggregation, or agglomeration but also solubility, stability, binding affinity, surface activity, chemical composition, and nanoparticle synthesis yield allows their reliable characterization. Thus, to find the structure-function/property relationship of nanoparticles, multifaceted characterization approach based on more than one analytical technique is required. On the other hand, the increasing demand for identification and characterization of nanomaterials has contributed to the continuous development of spectrometric techniques which enables for their qualitative and quantitative analysis in complex matrices giving reproducible and reliable results. This review is aimed at providing a discussion concerning four main aspects of nanoparticle characterization: nanoparticle synthesis yield, particle size and number concentration, elemental and isotopic composition of nanoparticles, and their surface properties. The conventional and non-conventional spectrometric techniques such as spectrophotometry UV-Vis, mass spectrometric techniques working in conventional and single-particle mode, or those based on optical emission detection systems are described with special emphasis paid on their advantages and drawbacks. The application and recent advances of these methods are also comprehensively reviewed and critically discussed.

Mass-Dependent Critical Value Expressions for Particle Finding in Single-Particle ICP-TOFMS

In time-of-flight mass spectrometry (TOFMS), ion detection is often achieved via electron multiplication followed by fast analog-to-digital conversion (ADC). This detection approach is chosen over time-to-digital conversion because it extends the dynamic range of TOFMS measurements, especially for transient analyses. However, fast ADC detection also introduces measurement noise fundamental to the electron multiplication process. In previous research, we demonstrated that TOFMS signals acquired with fast ADC follow a compound Poisson distribution in which the Poisson-distributed arrival of ions at the detector is compounded with the response profile of the electron multiplier. Here, we consider the influence of mass-to-charge (m/z)-dependent detector responses and their impact on particle-finding accuracy in single-particle inductively coupled plasma TOFMS (spICP-TOFMS). In spICP-TOFMS, highly time-resolved ion signals are recorded and particle signals are distinguished from background signals based on thresholding the data at m/z-specific critical values. Through Monte Carlo modeling with measured m/z-dependent detector responses, we generate compound Poisson model distributions and critical values that accurately account for the dispersion of measured signals. We test the accuracy of critical values through the analysis of dissolved element solutions and comparison of measured versus predicted event rates above critical value thresholds. The use of m/z-dependent compound Poisson critical values reduces false-positive particle identifications by one to two orders of magnitude compared to thresholding criteria based on normal or Poisson statistics. The improved accuracy and robustness of compound Poisson critical values enables automated multi-element particle finding in spICP-TOFMS.

A two-dimensional nanoparticle characterization method combining differential mobility analyzer and single-particle inductively coupled plasma-mass spectrometry with an atomizer-enabled sample introduction (ATM-DMA-spICP-MS): Toward the analysis of heteroaggregated nanoparticles in wastewater

Characterizing engineered nanoparticles (ENPs) in complex environmental matrices remains a challenging task. This work presents a two-dimensional size analysis method by combining differential mobility analyzer (DMA) and single-particle inductively coupled plasma-mass spectrometry (spICP-MS) with a new atomizer (ATM)-enabled sample introduction that is relatively easy to operate. The tailing of electrical mobility size distributions was solved by heating the aerosol flow, where water-shelled gold nanoparticles (AuNPs) were dehydrated, effectively eliminating the tailing. The improved method has a good sizing performance and can resolve the size fractions of mixed 30 nm and 50 nm AuNPs. It can reliably analyze 7.8 × 10⁵ to 1.9 × 10⁷ # of 50 nm AuNPs (or 4.1 × 10⁵ to 10⁷ # NPs/mL, equivalent to 0.6 to 14.3 μg Au/L) with a linear response and a limit of detection of 7.8 × 10⁵ # AuNPs (equivalent to 4.1 × 10⁵ # AuNPs/mL) that is relevant to NP concentrations in surface water and wastewater samples. The potential of this method to analyze environmental samples was demonstrated by characterizing AuNPs and silver nanoparticles (AgNPs) spiked in wastewater, where both NPs were revealed to form heteroaggregates with colloids existing in wastewater. The method can even directly analyze nanosized Ag particles inherent in the wastewater before adding external AgNPs. The result indicates that ATM-DMA-spICP-MS is a relatively simple two-dimensional size analysis method that has a great potential to characterize heteroaggregated NPs in aqueous environmental samples.

Discrimination and Quantification of Soil Nanoparticles by Dual-Analyte Single Particle ICP-QMS

This study presents the new application of dual-analyte single particle inductively coupled plasma quadrupole mass spectrometry (spICP-QMS) to the discrimination and quantification of two typical soil nanoparticles (kaolinite and goethite nanoparticles, abbr. KNPs and GNPs) in three samples (SA, SB, and SC) with three detection events (Al unpaired event, Fe unpaired event, and paired event). SA was mainly composed of KNPs with a concentration of 28 443 ± 817 particle mL^-1 and a mean particle size of 140.7 ± 0.2 nm. SB was mainly composed of GNPs with a concentration of 39 283 ± 702 particle mL^-1 and a mean particle size of 141.8 ± 2.9. In SC, the concentrations of KNPs and GNPs were 22 4541 ± 1401 and 70 604 ± 1623 particle mL^-1, respectively, and the mean particle sizes of KNPs and GNPs were 140.7 ± 0.2 and 60.2 ± 0.3 nm, respectively. The accuracy of dual-analyte spICP-QMS was determined by spiking experiments, comparing these results with the measurements of other techniques, analyzing the samples in different SA and SB proportions and in different SC concentrations. Our results demonstrated that the dual-analyte spICP-QMS is a promising approach to distinguishing different kinds of natural NPs in soils.