Analytical Chemistry of Plastic Debris: Sampling, Methods, and Instrumentation

Abstract

Approaches for the collection and analysis of plastic debris in environmental matrices are rapidly evolving. Such plastics span a continuum of sizes, encompassing large (macro-), medium (micro-, typically defined as particles between 1 μm and 5 mm), and smaller (nano-) plastics. All are of environmental relevance. Particle sizes are dynamic. Large plastics may fragment over time, while smaller particles may agglomerate in the field. The diverse morphologies (fragment, fiber, sphere) and chemical compositions of microplastics further complicate their characterization. Fibers are of growing interest and present particular analytical challenges due to their narrow profiles. Compositional classes of emerging concern include tire wear, paint chips, semisynthetics (e.g., rayon), and bioplastics. Plastics commonly contain chemical additives and fillers, which may alter their toxicological potency, behavior (e.g., buoyancy), or detector response (e.g., yield fluorescence) during analysis. Field sampling methods often focus on >20 μm and even >300 μm sized particles and will thus not capture smaller microplastics (which may be most abundant and bioavailable). Analysis of a limited subgroup (selected polymer types, particle sizes, or shapes) of microplastics, while often operationally necessary, can result in an underestimation of actual sample content. These shortcomings complicate calls for toxicological studies of microplastics to be based on “environmentally relevant concentrations.” Sample matrices of interest include water (including wastewater, ice, snow), sediment (soil, dust, wastewater sludge), air, and biota. Properties of the environment, and of the particles themselves, may concentrate plastic debris in select zones (e.g., gyres, shorelines, polar ice, wastewater sludge). Sampling designs should consider such patchy distributions. Episodic releases due to weather and anthropogenic discharges should also be considered. While water grab samples and sieving are commonplace, novel techniques for microplastic isolation, such as continuous flow centrifugation, show promise. The abundance of nonplastic particulates (e.g., clay, detritus, biological material) in samples interferes with microplastic detection and characterization. Their removal is typically accomplished using a combination of gravity separation and oxidative digestion (including strong bases, peroxide, enzymes); unfortunately, aggressive treatments may damage more labile plastics. Microscope-based infrared or Raman detection is often applied to provide polymer chemistry and morphological data for individual microplastic particles. However, the sheer number of particles in many samples presents logistical hurdles. In response, instruments have been developed that employ detector arrays and rapid scanning lasers. The addition of dyes to stain particulates may facilitate spectroscopic detection of some polymer types. Most researchers provide microplastic data in the form of the abundances of polymer types within particle size, polymer, and morphology classes. Polymer mass data in samples remain rare but are essential to elucidating fate. Rather than characterizing individual particles in samples, solvent extraction (following initial sample prep, such as sediment size class sorting), combined with techniques such as thermoanalysis (e.g., pyrolysis), has been used to generate microplastic mass data. However, this may obviate the acquisition of individual particle morphology and compositional information. Alternatively, some techniques (e.g., electron and atomic force microscopy and matrix-assisted laser desorption mass spectrometry) are adept at providing highly detailed data on the size, morphology, composition, and surface chemistry of select particles. Ultimately, the analyst must select the approach best suited for their study goals. Robust quality control elements are also critical to evaluate the accuracy and precision of the sampling and analysis techniques. Further, improved efforts are required to assess and control possible sample contamination due to the ubiquitous distribution of microplastics, especially in indoor environments where samples are processed.

Identification and Evaluation of Microplastics from Tea Filter Bags Based on Raman Imaging

“…Currently, several techniques have been developed for qualitative identification and quantitative assessment of MPs [ 28 ]. Among them, the simplest approach is microscopic techniques, such as scanning electron microscopy coupled with energy dispersive X-ray spectroscopy.…”

Durable Plastic Goods: A Source of Microplastics and Chemical Additives in the Built and Natural Environments

“…While efforts to investigate microplastics in wastewaters, sludges, and effluents are increasing, available analytical methods remain inadequate to detect the full range of sizes, shapes, and types (i.e., polymer composition) present . Microplastics <100 μm in size are often unretained by sampling and preparation techniques .…”

“…Microplastics <100 μm in size are often unretained by sampling and preparation techniques . The small size of microplastics and the complexity of wastewater-related matrices complicate accurate identification and quantification of microplastics. , Polymer types beyond the “usual suspects” (i.e., polyethylene, polypropylene, PET, PS, and polyvinyl chloride) often go unquantified. Understudied polymer classes include paint/coating-associated urethanes, acrylates, and butadiene-based rubbers common in tire wear. ,,, These may be toxicologically relevant (e.g., the tire antioxidant additive degradant described previously) .…”

“…The small size of microplastics and the complexity of wastewater-related matrices complicate accurate identification and quantification of microplastics. , Polymer types beyond the “usual suspects” (i.e., polyethylene, polypropylene, PET, PS, and polyvinyl chloride) often go unquantified. Understudied polymer classes include paint/coating-associated urethanes, acrylates, and butadiene-based rubbers common in tire wear. ,,, These may be toxicologically relevant (e.g., the tire antioxidant additive degradant described previously) . Sample preparation techniques for removing analytical interferences, such as aggressive oxidation, may alter some polymers, e.g., those derived from polyamides, biopolymers, and tire wear. , Fibers from clothing and carpeting laundering are a dominant source of microplastics to municipal wastewaters. , Textiles and fabrics may also be treated with stain and water repellant (e.g., PFAS), flame retardant, colorizing compounds, plasticizers, and other additives .…”

“…Once therein, stabilization practices, land application, and later environmental processes will fragment these into ever greater numbers of smaller particles, including nanoplastics. These are likely more chemically and toxicologically active, but more difficult to detect. ,, …”