Progress in quantitative analysis of microplastics in the environment: A review

Highlights

• Conventional analytical methods for identification of microplastics were discussed.

• Up-to-date knowledge on Pyro-GC/MS for microplastic quantification was reviewed.

• Current challenge and future research of microplastic quantification were suggested.

Abstract

The amount of plastics released into the environment has been increasing dramatically. As the physical and chemical degradation of plastics results in the production of millimeter- and micrometer-sized plastic particles, their presence has been recognized as an environmental threat. Considerable efforts have been made to identify and quantify microplastics in ecosystems. Millimeter-sized plastic particles can be monitored visually, but confirmation of the type of plastic requires spectroscopic tools such as Fourier-transform infrared spectroscopy, Raman spectroscopy, optical microscopy, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. These technologies have long been considered robust methods for the identification of diverse plastic materials, but are not suitable for the quantification of concentrations in the environment using conventional analytical tools. The accuracy of plastic identification also decreases as the size of the particles approaches the scale of a few micrometers. Here, up-to-date technical developments in the identification and quantification of microplastics using pyrolysis–gas chromatography/mass spectrometry (Pyro-GC/MS) are reviewed along with their advantages, disadvantages, and accompanying technical challenges.

Introduction

To understand the environmental significance of a newly identified group of “emerging contaminants” a great deal of effort has been directed to describing their distribution, fate, and impact on the environment [1]. However, a large information gap persists regarding a number of contaminants that cannot be identified and/or quantified due to the inadequate sensitivity of current analytical techniques [1]. A reliable analytical technique could serve as a pivotal platform for the first stage of the identification and quantification of the chemical constituents of these contaminants. After the determination of contaminants, their degradation pathways and origins can be estimated [1]. Research on emerging contaminants has become a popular subject among environmental scientists and engineers [2]. Among the various types of emerging contaminants, microplastics have attracted attention due to large quantities and wide distributions in the environment. A number of intensive efforts to study their production, fate, and environmental impacts have been made over the last 5 years (Fig. 1).

According to previous reports, surplus plastic waste materials in the landfill sites can be introduced to complex environmental matrices and become sources of microplastics. Plastic debris itself can reduce the adsorption capacity of soil and allow for the transportation of organic and inorganic pollutants into groundwater and crops [3]. These processes are potentially hazardous because humans can consume the pollutants through these sources. It has been also reported that microplastics consumed by other animals can bioaccumulate in the higher predators of a food web, ultimately leading to the harmful health issues to both wildlife and humans [4], [5].

“Microplastics” are particles less than 5 mm in diameter. There are two categories of microplastics based on their sources. Primary microplastics are manufactured with a small particle size for use as cosmetics, skin scrubbers, and microfibers in clothing. Secondary microplastics are smaller particles formed through the breakdown or/and fragmentation of larger items due to exposure to solar radiation, weathering, or gradual weight loss in the environment [6]. Plastics are widely used in various industries and countries because of their superior physical and chemical properties compared with conventional materials, such as wood and metal, and their lower costs [7]. The presence of microplastics in a variety of ecosystems must be evaluated in a transparent manner, given the potential hazards they pose.

Protocols for qualitative and quantitative analyses of microplastics have yet to be established [4], [8], [9]. Common separation-based analytic techniques such as gas chromatography (GC) cannot be employed for direct analysis due to the low volatility of plastics. In detail, GC consists of a flow control section, which is composed of a sample injector, column, and column oven; a detector; and a data processor. The samples are rapidly injected into the GC column through an injector at ≤ 300℃ to be analyzed by a detector, but plastic materials do not fully exist as gas-phase compounds in this temperature range. Spectroscopic instruments, such as Fourier-transform infrared (FTIR) and Raman spectrometers, can be used to identify types of plastics [10], [11], but are practical only for pure plastics. Pyrolysis–gas chromatography/mass spectrometry (pyro-GC/MS) thermally degrades plastics into volatile organic compounds that can be identified. By measuring the quantity of representative index chemicals of each plastic, pyro-GC/MS can quantify the plastics in environmental samples, making it a promising solution for the direct analysis of all kinds of microplastics [12], [13]. Compared with other analytical methods that require multiple pretreatment processes to obtain isolated plastic particles for identification, pyro-GC/MS offers the advantage of not requiring various pretreatment steps for the identification and quantification of microplastics in an environmental matrix. The detection of microplastics can also be affected by the selection of sampling and pretreatment methods according to impurities present in the environment, because the appearance and chemical composition of collected samples can change after pretreatment processes [10].

In this review, we survey analytical methods for the detection of the six most broadly used plastics (polyethylene: PE, polypropylene: PP, polyethylene terephthalate: PET, polystyrene: PS, polyvinyl chloride: PVC, and polyamides: PAs) along with polymeric additives for their synthesis that can be found in industrial and household materials. We also comprehensively address conventional analytical tools for the identification of pure plastics and the challenges they pose during analysis of plastics in a complex environmental matrix. The advantages of pyro-GC/MS analysis for the identification and quantification of plastics in the environmental matrix are discussed, and the practical applications of this tool are described. Current technical challenges of pyro-GC/MS and promising future studies are then suggested.