Monitoring microplastics in drinking water: An interlaboratory study to inform effective methods for quantifying and characterizing microplastics

Highlights

• A novel study measured the performance of common methods for microplastic analysis.

• Variation in recovery was likely due to experience and particle contamination.

• Microscopy is an accurate method for counting plastic particles down to 50 μm.

• Both FTIR and Raman spectroscopy accurately identified spiked plastic particles.

• Raman spectroscopy may be more suitable for particles smaller than 20 μm. .

• Methods to reduce time and identify smaller particles need further development. .

Abstract

California Senate Bill 1422 requires the development of State-approved standardized methods for quantifying and characterizing microplastics in drinking water. Accordingly, we led an interlaboratory microplastic method evaluation study, with 22 participating laboratories from six countries, to evaluate the performance of widely used methods: sample extraction via filtering/sieving, optical microscopy, FTIR spectroscopy, and Raman spectroscopy. Three spiked samples of simulated clean water and a laboratory blank were sent to each laboratory with a prescribed standard operating procedure for particle extraction, quantification, and characterization. The samples contained known amounts of microparticles within four size fractions (1–20 μm, 20–212 μm, 212–500 μm, >500 μm), four polymer types (PE, PS, PVC, and PET), and six colors (clear, white, green, blue, red, and orange). They also included false positives (natural hair, fibers, and shells) that may be mistaken for microplastics. Among laboratories, mean particle recovery using stereomicroscopy was 76% ± 10% (SE). For particles in the three largest size fractions, mean recovery was 92% ± 12% SD. On average, laboratory contamination from blank samples was 91 particles (± 141 SD). FTIR and Raman spectroscopy accurately identified microplastics by polymer type for 95% and 91% of particles analyzed, respectively. Per particle, FTIR spectroscopy required the longest time for analysis (12 min ± 9 SD). Participants demonstrated excellent recovery and chemical identification for particles greater than 50 μm in size, with opportunity for increased accuracy and precision through training and further method refinement. This work has informed methods and QA/QC for microplastics monitoring in drinking water in the State of California.

Introduction

Microplastics represent a potential threat to human health, with exposure through the air we breathe (Chen et al., 2020; Gasperi et al., 2015), the food we eat (Cox et al., 2019; EFSA Panel on Contaminants in the Food Chain (CONTAM), 2016), and the water we drink (Eerkes-Medrano et al., 2019). While further research is needed to understand biological fate and toxicity of microplastics in humans (Barboza et al., 2018; Carbery et al., 2018; Rist et al., 2018), there is sufficient evidence to impose a precautionary approach for monitoring and mitigation of microplastics in drinking water (Danopoulos et al., 2020; Leslie and Depledge, 2020; Senathirajah et al., 2021).

Drinking water exposure has become a focal point for the State of California, which has a legislative mandate (Senate Bill 1422) to enact routine monitoring, including adoption of standard methods for implementing that requirement (Wyer et al., 2020). Such methods do not currently exist, though there is a growing body of research from which to draw. Microscopy is a useful pre-screening tool for enumerating microplastic particles and is a comparatively quick method with low upfront and equipment costs (Primpke et al., 2020). Fourier-Transform Infrared (FTIR) and Raman spectroscopy are often used to confirm microplastic particle counts by providing chemical confirmation of material type. Several studies have previously demonstrated the capabilities of these methods (Cabernard et al., 2018; Käppler et al., 2016; Xu et al., 2019).

Establishment of standard methods for management application requires several additional steps beyond publishing methods in peer-reviewed journals. Management application requires development of a detailed standard operating procedure with sufficient specificity to ensure repeatability across laboratories. It also requires a multi-laboratory method evaluation study to determine performance characteristics, including demonstration of whether the precision and accuracy of the chosen methods are suitable for the intended purpose. These data serve as a foundation for the development of laboratory accreditation, which define expectations for well-performing laboratories.

There have been several interlaboratory studies assessing microplastic methods performance across laboratories with blind microplastic samples (Cadiou et al., 2020; Isobe et al., 2019; Michida et al., 2019; Müller et al., 2020; Van Mourik et al., 2021). However, none have been conducted using specific prescribed methodologies, which results in high of variability among labs and across methods. In addition, all these studies except for one (Müller et al., 2020) have been limited to particles larger than 50 μm, which is problematic because drinking water is typically filtered to at least this size and monitoring needs to focus on smaller particles that can potentially pass through filters (Na et al., 2021).

Here we present a multi-laboratory validation study using three microplastic measurement methods: stereomicroscopy, Raman spectroscopy, and FTIR spectroscopy. For each, we establish performance characteristics, including accuracy and precision. We also establish the time requirements to implement such methods, necessary for use in required monitoring. The study included laboratories from around the world with a range of experience, and all received samples spiked with diverse particle shapes, densities, colors and sizes, a strict protocol to follow for extraction, particle identification, counting and chemical identification. These results collectively inform recommendations for microplastic analysis in drinking water, for monitoring programs within the State of California and beyond.