Plastics tracking strategy: Plastic-related small molecules as chemical fingerprints

Highlights

• •Plastic-related small molecules provide information about plastic's past life.
• •Small molecular chemical fingerprints could become plastic identifiers.
• •Current challenges and recommendations for chemical fingerprints are identified.

Migration and transformation of (micro)plastics are complex, and the full understanding of these processes at the macro-particle level is challenging. In this commentary, we define the characteristics of plastic-related organic small molecules as chemical fingerprints, based on which the strategies and challenges for tracking plastics were discussed. It was suggested that the chemical fingerprints, as the unique identity markers, have the potential to track any possible process of interest in the full life cycle of plastics, such as processing, use, recycling, abandonment, and environmental aging of plastics.

Plastic, as a multi-chemical cocktail, has been found to contain over 16,000 chemicals that originate from intentional or unintentional additions during production [1,2]. Additionally, plastics released into the environment contain sorbed chemicals and degradation products [3]. Plastics and plastic additives are of widespread concern due to the exposure risks to humans and the environment. However, the migration and transformation of plastics in substance flows and environments are commonly accompanied by changes in their macro-morphology and surface characteristics, which increase the difficulty of managing plastics and plastic additives in general.

Apart from the plastics themselves, we propose emphasizing plastic-related small molecules as a more diverse and sensitive component of plastics. The chemical fingerprints of plastics, which are defined as the type, concentration and proportion of small organic chemicals they contain, provide an opportunity to extend current research methods. In this commentary, the chemical fingerprint tracking strategies, the current challenges, and the future directions for plastics were identified.

1. Chemical fingerprints tracking strategy

Generally, chemicals in specific plastic applications, which can be categorized as plastic endogenous compounds (e.g., additives, processing aids, and transformation products), are proprietary information. Meanwhile, most plastics are hydrophobic and can capture other organic chemicals from specific environments [3], which are classified as exogenous compounds (e.g., organics sorbed during use or in the environment). Both the plastic endogenous and exogenous compounds, which belong to plastic-related small molecules, are considered chemical fingerprints. By analyzing the characteristics of chemical fingerprints in plastic samples (and environmental backgrounds), it is considered possible to track the processing of plastics, the mechanical recycling of plastics, the specific plastic products that generate waste, the transport pathways of plastics entering the environment, the aging of environmental plastics, etc. Those processes could be tracked by chemical fingerprints, a method we define as “chemical fingerprints tracking strategy” (Fig. 1).

Fig. 1.

Chemical fingerprints of plastics tracking opportunities and applications.

Current research has done a great deal to reveal the chemical composition of plastics and the toxicity of additives. On this basis, preliminary plastic tracking efforts have focused on distinguishing the types of plastics and specific products by chemical fingerprints, which belong to plastic endogenous compounds. For example, previous studies have elucidated the possible specific plastic products that generate waste of environmental plastics by analyzing the chemical fingerprints (endogenous compounds) of environmental and virgin plastics [4]. Analyzing those endogenous compounds is relatively simple and straightforward. Li et al. analyzed compounds obtained from non-targeted mass spectrometry data by machine learning to identify virgin and recycled polyethylene terephthalate (PET) [5]. Compared to endogenous compounds, studies on exogenous compounds (e.g., organics sorbed during use or in the environment), which are also considered to be chemical fingerprints, are limited. Currently, gaps in understanding unidentified plastic-related small molecules and their environmental behavior hinder the broad application of endogenous compounds and exogenous compounds in chemical fingerprint tracking strategies.

2. Current challenges and recommendations

The complex composition of plastics, where most chemicals and transformation products are unidentified, hinders the development of chemical fingerprint tracking strategy. Therefore, more research is urgently needed to reveal the composition of the unknown chemicals in different kinds of plastics. The identification of unknown additives and the improvement of plastic-related chemical databases will contribute to a fully comprehensive understanding of chemicals in plastics [1].

In addition to complicated composition and low transparency of chemicals in plastics, their migration and transformation along with plastics are also complex, which inevitably causes convergence and divergence of fingerprints. Therefore, sufficient fundamental research to reveal the migration and transformation mechanisms of chemicals in plastics and related factors is necessary. Further, the migration of hydrophobic chemicals in plastics can be predicted based on mass transfer kinetics [6]. Besides, the transformation of chemicals in plastic can be probed by a conceptual site model, which integrates the transformation mechanisms and related factors based on specific hydrogeochemical conditions [7].

Except for their complexity of composition, migration, and transformation, the origin of plastic-related small molecules (including the endogenous and exogenous compounds) is universal. Given the complexity and large amount of targeted or non-targeted data obtained from plastic samples, multivariate statistical analysis is a powerful tool to find specific sources or environmental behaviors of those small molecules. As a multivariate statistical analysis technique, chemometrics could be divided into unsupervised multivariate analysis (clustering and ranking) and supervised multivariate analysis (classification and regression), which aim to extract information from datasets and reduce the dimensionality of the data applied to the tracking of organic pollutants in the environment [7]. However, the drawbacks of chemometrics are that it is prone to overfitting and has difficulty in dealing with large and complex datasets. To solve such problems, supervised and unsupervised machine learning algorithms may provide candidate tools when analyzing large and complex data [5]. It should be noted that the supervised learning algorithms may need to be trained by large and comprehensive datasets to address complex environmental conditions.

The analytical and detection methodology of chemical fingerprints is challenging. Since the composition and types of plastic-related small molecules are diverse, the development of novel analytical and detection methodology is necessary: (i) comprehensive characterization of plastics-related chemicals with prioritized precision and cost-effectiveness, (ii) elimination of variations in different substrates (e.g., different types and sizes of plastics, water and sediments) and avoiding interference from higher concentrations of additives, (iii) development of semi-quantitative approaches based on high-resolution mass spectrometry, (ⅳ) improvement of high-throughput analysis and detection techniques endowing chemical fingerprinting with an even greater advantage (Fig. 1).

3. Future perspectives

The chemical fingerprints of plastics have a wide range of opportunities and applications (Fig. 1): (i) Tracking the processing of plastics. Specifically, comprehensive detection and identification of endogenous compounds could improve the transparency of plastic additives, avoid the re-addition of additives [8], and track the sources of non-intentionally added substances. (ii) Tracking the mechanical recycling of plastics. Specifically, based on relevant transformation products and exogenous compounds introduced in plastics, the virgin and recycled plastics could be distinguished, and the plastics that were contaminated during use could be tracked [5]. Based on the high concentration, types, and specificity of endogenous compounds in different plastics, the sources and apportionment of recycled plastics will be identified. [1]. (iii) Tracking the source and migration of environmental plastics. Namely, the specific plastic products that generate waste could be determined by plastic endogenous compounds (e.g., additives, processing aids and transformation products), while their transportation through sewage treatment, atmospheric deposition, river flow or direct entry from litter could be tracked by exogenous compounds (including specific environmental background compounds and potentially persistent man-made chemicals, such as PPCPs). (ⅳ) Tracking the occurrence and aging of environmental plastics. For example, plastic-related small molecules such as oligomers, antioxidants, and their transformation products may become markers for plastics to indicate the occurrence or the aging degree of plastics [9,10]. Those markers used for the quantification of plastics should be specifically present in plastics, ubiquitous in different types of plastics, difficult to leach and transform, and easy to detect. The markers of plastic aging should not be separate compounds but be the combinations (parent and transformation products) that are sensitive to aging.

At present, to realize the chemical fingerprints tracking strategy, more characterization of plastic-related small molecules by targeted and non-targeted mass spectrometry tools is required. Precise quantitative studies could reveal potential patterns of migration of plastic-related small molecules in the environment. In contrast, non-targeted analysis can provide more details on chemical fingerprints and identify the potential or unknown substances of interest. Furthermore, to improve the practicality of chemical fingerprint tracking strategy in complex environments, multivariate statistical analysis tools need to be further developed, wherein machine learning algorithms will be the future direction. Overall, with the more comprehensive understanding of plastic-related small molecules, the chemical fingerprints of plastic could be unique identity markers to track any possible process of interest in the full life cycle of plastics, such as processing, use, recycling, abandonment, and environmental fate of plastics.

CRediT authorship contribution statement

Hongtao Liu: Writing – original draft, Investigation, Conceptualization. Yuna Li: Writing – review & editing, Methodology, Conceptualization. Yongzheng Ma: Writing – review & editing, Supervision, Funding acquisition. Ying Zhang: Writing – review & editing, Methodology. Zhiguang Niu: Writing – review & editing, Supervision, Funding acquisition.

Declaration of competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.