In a pre-proof article posted in the Journal of Hazardous Materials, researchers applied novel laser direct infrared imaging (LDIR) spectroscopy to achieve rapid microplastic analysis (MP analysis). The research was conducted in the range of MP>10 mm in pure matrices such as deposited air samples and bottled drinking water by applying QA/QC criteria.
Study: Assessing microplastic characteristics in bottled drinking water and air deposition samples using laser direct infrared imaging. Image Credit: David Pereiras/Shutterstock.com
With polymer abundances in the order of polyvinyl chloride < polyamide < polyethylene terephthalate or polyurethane, nine water bottles from three different brands contained an average of 96 particles/L. The laboratory and domestic rooms determined MP deposition rates between zero to 573 particles m-2 h-1. These rates amounted to 7 MP (range 0–16) being consumed in a typical meal for household deposition samples. The most prevalent polymers were polyethylene terephthalate or polyurethane, followed by polyamide and polypropylene.
The paper's results revealed a statistically significant positive correlation between the deposition of MP and the total textile surface area per unit of room volume. Power law distributions containing slopes ranging from 1.9 to 3.8 were created for the particle width, area, height, mass, and volume for both sample types. MP recovery during sample preparation appeared to be a possible improvement point for laser direct infrared imaging, i.e., LDIR, which seemed to be a relatively quick MP measuring technique for human health exposure assessment.
Laser Direct Infrared Imaging (LDIR) and Modern Microplastic Analysis
Microplastics have been found everywhere in the environment, including lakes, estuaries, rivers, oceans, and the atmosphere. A few millimeters or fewer of microplastics can be directly absorbed by cells in the intestines or lungs. As a result, particle shape and size are crucial for bioavailability. Therefore, it is essential to verify these properties of MP in air, food, and drinking water to determine their toxicologically relevant dosage metrics (TRM) for further human health exposure assessment.
How to correctly measure TRM by capturing the probability density functions (PDFs) for identifying the multidimensionality of MP characteristics was recently discussed. Human intake of airborne plastic fallouts is a significant source of MP pollution and is essential for measuring probability density functions. Airborne MPs in the indoor setting include plastic fibers and fragments from the furniture.
Laser direct infrared imaging (LDIR) has made it possible to quickly and effectively conduct microplastic analysis on particles in environmental samples to capture the probability density functions.
Focussing the IR light on the center of MP particles in a sample can be successfully achieved by laser direct infrared imaging, i.e., LDIR spectroscopy with the help of a quantum cascade laser (QCL). However, its suitability for deriving probability density functions in media relevant to human health exposure assessment has not yet been tested.
The findings of this study evaluated concentrations, particle properties, and TRMs for two sample types pertinent to human health exposure assessment to MP and the subsequent capturing of the probability density functions. Here, the sample types included air deposition (particle fallout) and bottled drinking water samples, and the study was performed using laser direct infrared imaging, i.e., LDIR spectroscopy.
The design included 24 different deposition samples taken in eight separate rooms and nine drinking water samples from three brands with cap friction applied previously to increase realism.
Characterization was performed for mass concentration and number, polymer type, length, area, width, mass, and volume using probability density functions prepared for all particles per sample type.
The latter averaged the properties and was more representative of the heterogeneity of the microparticles to which a human health exposure assessment over a more extended period could be performed.
The Investigations
As previously identified, particles rather than the entire slides were the focus of the LDIR spectroscopy for microplastic analysis in drinking water and air deposition samples. In contrast to commonly applied spectroscopy methods, the laser direct infrared imaging, i.e., LDIR, worked with a limited wavenumber range between 1800-975 cm-1.
The researchers made reasonable assumptions about the observed variations in polymer abundances because the study was not intended to identify the polymer particles' sources precisely. The relative presence discrepancies between the top two or three polymers were too slight to allow further helpful analysis.
A thorough understanding of the microplastic analysis for human health exposure assessment in terms of particle sizes and properties, as well as the various relevant absorption pathways and dietary components, was crucial. After all, the choice between human health exposure assessment via drinking water and air defined the potential toxicological characteristics of these pathways.
Using pooled microplastic analysis characteristics, density functions were created for 285 particles from bottled water samples and 1010 MP from air deposition samples. The interpretation of the level of human health exposure assessment routes rather than the level of individual samples was more relevant for estimating the average microplastic analysis for human health exposure assessment over a more extended period.
The length, area, width, mass, and volume of the microparticles found in air deposition and bottled water samples were generated as power law distributions. Compared to air deposition samples, the power law distributions slope for the area was 0.47 log units higher for bottled water samples. Therefore, the bottled water samples included comparatively more particles within a small surface area, a distinction still not fully understood as was apparent from the power law distributions.
The particle volume and mass being proportional, the power law distributions slope for volume and mass were equal with error bounds (range 1.87 to 1.94). Although the composition and form of the polymers in the samples from air deposition and bottled water varied, the variation in polymer densities was slight, and the diversity of the particle mixture was such that the variations averaged out to produce almost equal mass and volume power law distributions.
Future of MP Human Health Exposure Assessment
This study's data and probability density functions can be used for microplastic analysis for human health exposure assessment. Ultimately, exposure to MP was estimated from the deposition samples to be seven MP eaten per meal for 20 minutes.
It could not be ruled out that various techniques applied contributed to some of these variations. The estimated daily exposure produced 513 MP each day. It should be emphasized that the microplastic analysis uptake only accounted for MP fallout's contribution. It was essential to consider MP contamination in the food and the MPs released during food preparation and packaging. The water samples had substantially broader ranges of microparticles, from 7 to 364 MP/L.
The researchers in this paper could determine the MP concentrations in water bottles and household and laboratory deposition samples using laser direct infrared imaging. The most prevalent polymer in both sample types was PET or PU.
The textile surface area in the examined domestic rooms was positively linked with the deposition of MP. Power law distributions for particle height, width, volume, area, and mass were created for both sample types.
Reference
Nizamali, J., Mintenig, Svenja M., Koelmans, Albert A. (2022) Assessing microplastic characteristics in bottled drinking water and air deposition samples using laser direct infrared imaging. Journal of Hazardous Materials. https://www.sciencedirect.com/science/article/pii/S0304389422017368
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