Our efforts are focused on the measurement science needed to improve detection, identification, and quantification of plastic particles in environmental and human health systems. Current measurement methods for microplastics (MP) generally lack required sensitivity to detect nanoplastics (NP). Currently available microplastic and nanoplastic (MNP) materials are insufficient to develop the metrology and meet regulatory and industrial needs. Thus, MMSD’s MNP Metrology Focus Area (learn more) aims to support our diverse stakeholder groups by i) developing protocols for reproducible material production that facilitate measurement science advancement, and ii) develop needed metrology to examine MNP particle evolution in solution and airborne environments using advanced microscopy, spectroscopy, and mass spectrometry techniques. NIST efforts are focused on the needed tools to improve risk assessment and support development of sustainable commercial products.
MMSD has capabilities to quantify the structure, chemical composition, morphology and transformations of MNPs in relevant media. Our MNP test material production facilities support metrology development and include physical milling, polymer synthesis expertise, and fabrication tools for producing powders, suspensions, and plastic arrays of distinct size and morphology to assess measurement capabilities, limits, and biases. Asymmetric-flow field flow fractionation (AF4) provides high resolution separations of heterogeneous size distributions based on hydrodynamic properties that can be evaluated in-line with multiple detector modalities. Separations can be optimized for intrinsic particle properties as well as interactions with secondary components in mixtures. Additionally, populations of interest can be collected and concentrated for further ex situ analysis. Our electron and optical microscopy (EM and OM, respectively) expertise utilizes advanced sample collection, preparation, and particle manipulation methods for improved outcomes. We also have diverse, powerful mass spectrometry (MS) capabilities and instrumentation which, coupled with expertise in instrumental design, technique development, and machine learning, is advancing chemical analysis of MNP test materials and environmental samples. Investments in an optical photothermal infrared (OPTIR) microscope will provide single particle analysis with high spatial resolution. OPTIR enables both size/shape-dependent and chemical modification measurement validation of MNP signatures collected on commonly employed ensemble vibrational spectroscopy instruments.
Fundamental Metrology Development for the Chemical Characterization of Micro- and Nanoplastics in Complex Matrices
Fundamental metrology for MNPs built on well-characterized intrinsic material properties, including morphology, primary size, and particle size distribution can be implemented for both fractionated monomodal distributions and ensemble mixtures. Currently, research and regulatory partners have recognized that there are insufficient commercially available MNP test materials or documented protocols to reproducibly generate non-spherical NP materials. These restrictions have limited the ability of the field to build the necessary measurement infrastructure. Research has begun in the MNP Project to systematically evaluate size-dependent, chemical changes using spectroscopic and spectrometric fingerprints of the inherently transient NP populations resulting from bulk and microplastic evolution. Investigation of pristine and chemically altered MNPs is conducted in partnership with internal NIST collaborators to control known chemical pathways, including photochemical and thermal degradation. Measuring both changes to particle mass (size distribution) and characterizing NP of known and changing chemical composition provide the foundational measurements to accurately quantify NP in unknown mixtures and different matrices. This information supports stakeholders’ needs by providing the necessary tools to measure plastic fate, evaluate exposure in different environments, and ultimately, improve risk assessment of current and emerging technologies.
- Protocols for generating nanoplastic test materials — To address the need for NP test materials with known physicochemical properties, current efforts are focused on protocol development that reliably and reproducibly generate similar quantities and size distributions of irregular-shaped NP from different plastic sources and polymer types. Methods employ commonly available milling tools, because broad adoption is essential for data comparison and measurement validation. The protocols for material generation and characterization are meant to be used as a benchmark to assess method performance across labs and in multiple environmental and applied systems.
- Quantification of pristine and simulated aged nanoplastics in heterogeneous size distributions and sample mixtures: The project aims to develop the measurement methods necessary to improve accuracy for NP identification using spectroscopic and spectrometric methods that can differentiate contributions from physical (shape, size, molecular orientation) and chemical changes. The test material samples are fractionated and characterized using AF4 and other filtering methods to generate monomodal size populations of known plastic composition, which provide the basis for method development. Controllably aged test materials are also produced and size-separated to ensure separation methods are suitable for applied systems of interest.
Methods to investigate MNP single particle and single particle size populations are being developed in collaboration with internal NIST partners using OPTIR microscopy as a comparison to high resolution Raman imaging and sensitive wide-field IR microscopy, which improves sensitivity and ultimately accuracy for NP quantification. Using MMSD MS expertise for mass-based measurements of MNP, primary efforts for the project are to evaluate performance metrics for the current de facto method of quantitation, pyrolysis gas chromatography-MS (py-GC-MS). Because of the inherent limitations of py-GC-MS, additional efforts are focused on developing higher throughput, less destructive, higher resolution instrumentation, and machine learning-enabled analyses to improve identification and quantification accuracy. The improved data inputs for accurate elemental compositions combined with unsupervised machine learning approaches aid and enable data analysis of complex spectra that were less accessible with other methods.
Dissemination of size-dependent spectral signatures and MNP stability during sample preparation and evaluation are highest priority outcomes for regulatory and industrial partners. Ultimately, MMSD is actively engaging in collaborations with divisions across NIST and external partners to develop and chemically characterize MNP test materials, degradation pathways, and environmental samples.
Applied Metrology for Detection and Characterizatio of MNP For Environmental and Human Health
To address the limited availability of validated methods and data for sampling and identifying MNP in airborne and other relevant matrices, our program aims to provide the foundational methods and well-characterized, complex samples necessary to benchmark environmental MNP. MMSD’s efforts to develop novel portable sample capture and preparation methods are conducted in tandem with physicochemical characterization method development. Correlated measurements across multiple advanced microscopy and spectroscopy platforms support cross-laboratory validation and continued development of improved characterization tools.
- Detection and characterization of airborne plastics — The project aims to address the uncertainty associated with distinguishing different classes of aged MNP (e.g., polystyrene, polyethylene, polypropylene, etc.) in complex matrices and mixtures of material types. MMSD has been collecting airborne MNPs on membrane filters at a local municipal recycling facility using a unique air sampling methodology. The novel filter design combines nanofabrication methods and fiducial marker patterns that readily enable hyperspectral analysis of the collected complex particle mixtures. The filter design also limits sample preparation across measurement methods, enabling method development for improved detection and accurate chemical characterization directly on the filter for multiple modalities, including OM, EM, and micro-Raman spectroscopy. Further development of automated particle analysis tools will provide high throughput capabilities to support field practitioners with large sample areas and numbers. Well-characterized field samples also serve as environmentally relevant proxies to begin converging the fundamental and applied efforts in the MMSD MNP program.
- Linking MNPs and their point sources through imaging — This is an internally funded MML project that aims to investigate the correlative properties of generated plastic particles from the changing material source films under systematically changing degradation environments. Efforts are focused on exploiting OPTIR spatial resolution for particles and the high-sensitivity and linearity capabilities of existing IR and Raman imaging developed by our MML collaborators. Efforts will provide previously unreported correlative measurements of direct point of source data between progeny MNP particles and film properties, providing chemical fingerprints for processing and degradation pathways to support regulatory efforts in food safety.