Tracking and determining environmental pollutants with particle analysis is vital in mitigating potential threats to human health.
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Large amounts of dust are released from traffic emissions, industrial operations, and waste disposal activities, directly or indirectly pollute the environment, especially the soil and the atmosphere. Metal accumulation in urban soil can adversely affect human health due to exposure via many pathways, including ingestion, inhalation, and skin contact.
Since trace metals are heterogeneously distributed in urban soils, various methods for determining particle size fractions could influence the outcomes of the risk assessments. Soil dust at the air–soil interface acts as a link between the soil and the atmosphere.
Trace metals initially settle down in the soil dust before they are fully incorporated into the soil. Hence, soil dust can reveal the atmospheric contamination status better compared to bulk soils and may be a more feasible medium for determining environmental pollution levels and health risks.
Moreover, the production of nanoparticles and nanoparticle-based products is increasing exponentially; the possible presence of nanoparticles in food, biological tissue, or the environment is becoming a major cause for concern as it considerably affects ecological and human health. However, suitable methods that can analyze and characterize nanoparticles are sparse.
What is Particle Analysis?
Particle analysis involves the assessment of the chemical composition and physical properties of particles, including particle size, shape, surface and mechanical properties, and microstructure. Depending on the sample material, many of these parameters could be important and interrelated, such as particle size and surface area.
Particles in a given material must scatter light at a certain intensity to be detected and tracked. The scattering cross-section and the incident light intensity of a particle are proportional to the amount of light scattered by a particle. Thus, an increased light scattering ability translates to an increased volume where the detection of particles is possible.
Common Techniques for Particle Analysis
Particle analysis techniques such as fluorescent laser particle tracking (FLPT), wide-field fluorescence microscopy (WFFM), particle image velocimetry (PIV), and time-resolved fluorescence polarization anisotropy (TRFPA) help explore mechanical moduli of viscous fluids, transport mechanisms at sub-cellular level, and velocity profiles in fluids.
Raman spectroscopy is a technique used in particle analysis based on inelastic scattering of monochromatic light, such as laser light. In pyrolysis-gas chromatography mass spectroscopy (Py-GCMS), samples are first thermally decomposed by pyrolysis, then separated using gas chromatography and ionized and fragmented before detection by mass spectrometry. Py-GCMS is widely used to detect and quantify anthropogenic particles, including plastics and tyre and road wear particles. This technique also provides compositional and structural information about the sample particles.
The advancement of digital technologies has led to increased use of sequential imaging software for tracking particles that can scatter light when illuminated by an external light source. Techniques such as nanoparticle-tracking analysis (NTA), laser particle tracking (LPT), and enhanced dark-field scattering microscopy characterize the scattered particles as points in space created by the scattered light or as photoluminescence of the particles illuminated by a laser or a regular light source.
Some common particle-tracking techniques employ single-particle tracking (SPT) or multiple-particle tracking (MPT) software that uses different properties depending on their field of application. SPT methods utilize a single probe particle embedded in the fluid, while MPT methods utilize a set of particles tracked via several steps.
Nanosight is a commercial software that has the NTA instrument and helps track particles in a fluid and measure particle-number concentrations and particle-size distribution. This method was used for nanoparticle screening in environmental samples and also for the evaluation of nanoparticle and protein aggregation.
Several particle analysis techniques analyze the translational motion of nanoparticles or particles in colloids to determine various parameters related to micro-rheology, velocity profile determination, nanoparticle-size determination, and particle number concentration.
Particle Analysis in Environmental Monitoring
A study by Gallego-Urrea et al. compared many different particle-tracking methods with the help of video microscopy and NTA.
Video microscopy has been extensively used to investigate the movement of particles in biological samples, velocity profiles in fluids, and micro-rheology. At the same time, NTA helps determine size distributions and concentrations in liquid samples. This work discussed the advantages and drawbacks of using NTA for these applications. The authors mentioned that the applications of these techniques include sub-cellular analysis of particle paths and fluid properties, particle-size distributions, and environmental particle concentrations.
According to the study, the benefits of using NTA for assessing size distributions and nanoparticle concentration in biological, environmental, and food samples include high sensitivity, minimum sample perturbation, high resolution, and high scattering intensity. In contrast, the limitations of NTA include low sensitivity for small particles, material-dependent concentration measurement, and operator-dependant measurements.
Another study by Xiangyang et al. used an in situ sampling method to analyze the trace metal concentrations and Pb isotope compositions in different particle size fractions in bulk soil, soil dust, and corresponding road dust samples gathered from an urban environment. The aim of this work was to determine the feasibility of utilizing soil dust samples to measure trace metal contamination and possible risks in urban environments compared to road dust and bulk soil.
The Pb isotope ratios and total metal concentrations results showed that soil dust is more sensitive to anthropogenic contamination in urban areas than bulk soil. According to the authors, the novel in situ method used in this work is effective at gathering different soil dust particle size fractions from the soil surfaces, and soil dust is a key indicator of environmental contamination and possible exposure to humans in urban areas.
A recent study by Wolff et al. used Raman microscopy to detect microplastic particles in environmental samples such as seawater and wastewater. The study discussed the emission of microplastics from the municipal wastewater treatment plant effluent. The results of the study indicated a higher emission of microplastics in rainy weather and showed that 95% of all microplastic particles were in a size range of 10–100 μm.
Another recent review by Mattsson et al. discussed a select group of analytical techniques for isolating, characterizing, and quantifying anthropogenic micro- and nanoparticle contaminants in the environment. The work also explored the recent developments with these techniques and their shortcomings.
This review highlighted the challenges in particle analysis related to analytical identification, sampling harmonization, and data processing; the authors believe that addressing these limitations can considerably improve resolution, data quality, and reproducibility. In fact, even for some of the most advanced techniques, such as vibrational spectroscopy for microplastic identification, improvement is needed in the areas of data analysis, processing, and validation to obtain reliable results.
References and Further Reading
Gallego-Urrea, J.A., Tuoriniemi, J., Hassellöv, M. (2011). Applications of particle-tracking analysis to the determination of size distributions and concentrations of nanoparticles in environmental, biological and food samples. TrAC Trends in Analytical Chemistry, 30(3), pp. 473-483. ISSN 0165-9936. https://doi.org/10.1016/j.trac.2011.01.005.
Bi, X., Liang, S., Li, X. (2013). A novel in situ method for sampling urban soil dust: Particle size distribution, trace metal concentrations, and stable lead isotopes. Environmental Pollution. 177, pp. 48-57. ISSN 0269-7491. https://doi.org/10.1016/j.envpol.2013.01.045.
Mattsson, K., da Silva, V.H., Deonarine, A., Louie, S.M., Gondikas, A. (2021). Monitoring anthropogenic particles in the environment: Recent developments and remaining challenges at the forefront of analytical methods. Current Opinion in Colloid & Interface Science., 56. p. 101513. ISSN 1359-0294. https://doi.org/10.1016/j.cocis.2021.101513.
Wolff, S., Kerpen, J., Prediger, J., Barkmann, L., Müller, L. (2019). Determination of the microplastics emission in the effluent of a municipal waste water treatment plant using Raman microspectroscopy. Water Res X., 2, p. 100014. https://doi.org/10.1016/j.wroa.2018.100014 .
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