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A Microsampling Solution for Identification of Microplastics in the Environment Using FTIR

Microplastics are one of the major constituents of marine pollution. It has been documented in watersheds from way back in the 1970s [1]. Microplastics are distinguished from other plastic particles as determined by sizes less than 5 mm. Degradation of larger pieces and direct waste runoff are seen as probable sources of a majority of microplastics.

Not only do plastic materials themselves cause injury to fish and invertebrates in the waters, but it has also been observed that microplastics are a means of concentrating toxic, hydrophobic chemicals frequently found in the Earth’s water bodies. Studies into the numerous effects of plastics on the marine ecosystem are continuing and constantly shedding light on the extensive effects of these pollutants.

Fortunately, microplastics are mainly made up of organic polymers that can be easily characterized using infrared (IR) spectroscopy. In the case of sub 5 mm microplastics, IR microscopes have a vital role to play in not only visualizing but also identifying the plastic particles. The research of small particles using a microscope that interfaces to a Fourier transform infrared (FTIR) spectrometer is usually referred to as IR microspectroscope.

The SurveyIR microspectroscopy accessory shown in Figure 1 was used to image and examine all samples. When the SurveyIR is coupled with advanced, compact, and portable FTIR spectrometers, it delivers a transportable, integrated solution for field measurements. IR spectra were recorded at 8 cm−1 resolution with a room-temperature, L-alanine-doped, deuterated triglyceride sulfate (DLATGS) detector with a potassium bromide (KBr) beamsplitter and a (KBr) window.

The SurveyIR’s all-reflecting optics enabled data collection from 4000 to 400 cm−1. It took a minute or less for data collection of all spectra. Samples run in reflection/absorption were collected with an aperture mask corresponding to 60 µm at the sample. An aperture mask corresponding to 250 µm at the sample was used to gather spectra in attenuated total reflection (ATR) mode.

Samples were taken from waters close to New York City. The first two samples were from water filtered from the Hudson River, downstream from the Newtown Creek wastewater treatment plant. The filter was then let to dry and mounted to a typical 1″ x 3″ microscope slide.

The second set of samples was taken from oysters bought at a local seafood market. The samples were gathered through a series of separation methods and the remnants filtered and mounted in the same manner as the first set.

A diamond internal reflection element (IRE) was utilized in the ATR attachment. The ATR technique was used to screen larger particles without any follow-up sample preparation, for all filtered samples. For ATR analysis, a 1″ x 3″ low-E glass microscope slide was positioned under the standard microscope slide for extra support. Smaller samples were moved to low-E glass microscope slides and flattened with a roller knife before being examined.

The white, jagged plastic particle illustrated in Figure 2 was first examined directly on the filter. As a result of fibers and particles overlapping the suspected microplastic, it was moved to a clean low-E glass microscope slide for additional analysis.

The irregular nature of the transferred particle indicates it was sheared or broken off a larger piece instead of a small microbead or similar micro-plastic. Studying the IR spectrum in Figure 3, a good library match (Figure 3, magenta) is seen for oxidized polyethylene, a very common plastic material.

Figure 3. IR spectra of the white particle from Figure 1 (top, red) and the best library match (bottom, magenta).

The appearance of the band at 1718 cm-1 (denoted by * in Figure 3) is because of oxidation of polyethylene. The observation of oxidation in the polyethylene spectrum additionally indicates this particle was broken off a larger piece of plastic as environmental elements degraded the plastic material. Due to the diverse and many applications of polyethylene and the fact that no additional components were identified within the plastic, it is difficult to establish the probable source of this specific contaminant.

Other types of plastic have more exclusive uses that can help to establish the originating source, as observed in the next example.

The red tinted particle seen in Figure 4 was initially a small, 20 µm in length, red fibrous material.

Figure 4. Flattened red fiber (approx. 60 µm in diameter) viewed in transmitted illumination using the SurveyIR microscope.

The IR spectra in Figure 5 illustrates that the main component of the red fiber (top, red) is in good agreement with the library match, polypropylene copolymer (bottom, purple). Extra bonds are noticed, consistent with a minor component.

Figure 5. IR spectra of the flattened red fiber (top, red) and the top library match (bottom, purple).

Polypropylene copolymers are typically used in carpet manufacturing and carpet could very well have been the source for the red fiber contaminant. Altogether, numerous plastic particles were found in the collected water samples with the majority size of those particles spanning from 20 to 250 µm.

After examining microplastics in the Hudson River, the team started to examine how microplastic pollution travels into the marine ecosystem. By analyzing mollusk remains, the team wanted to identify microplastics in the environment that humans could ingest. In Figure 6, a small pink particle is seen. The sample was flattened on a low-E glass substrate.

Figure 6. Small pink particle imaged with oblique illumination using the SurveyIR microscope on a low-E glass microscope slide.

Figure 7. IR spectra of the pink particle (top, red) and the best library match, Zinc stearate (bottom, magenta).

The color of the plastic material is due to a dye used in the separation process to help in visual identification of potential microplastics. IR interrogation of the small pink particle exposed a simple IR spectrum (top, red) with a good library match to zinc stearate (bottom, magenta), shown in Figure 7.

Zinc stearate is typically used as a release agent in the polymer and rubber sector, as well as in the cosmetic sector as a lubricant and thickening agent [2]. Zinc stearate was the second most widely found particulate in the oyster samples after cellulosic fibers.

Like the Hudson River filtered water samples, mollusk samples had plenty of fibers. The composition of the fibers mainly included cellulosic materials, most probably cotton, in different colors. However, the image in Figure 8 illustrates another typical manmade material found within an oyster.

Figure 8. Flattened blue fiber imaged with transmitted illumination on a low-E glass microscope slide.

The appearance of the single blue fiber was rigid and smooth, in contrast to woven natural cotton fibers. The IR spectrum of the blue fiber in Figure 9 (top, red) resulted in a good library match with a popular plastic material, polyethylene terephthalate (PET) (bottom, magenta).

Figure 9. IR spectra of the flattened blue fiber (Top, Red) and the best library match, polyethylene terephthalate (Bottom, magenta).

PET is an extremely common plastic material used in plenty of everyday items from water bottles to polyester clothing. For example, Dacron® fibers are made up of PET. The most probable source of the blue fiber is from a clothing portion. Microfibers have been observed by Verschoor, et al. and several others to shed during washing [3]. These fibers can be tiny and pass through filtration systems, eventually ending up in the marine ecosystems.

As research ascertains the extensive effects of manmade pollution on different ecosystems, microplastics have become a particularly high-priority research topic over the last couple of years. For example, beginning in July 2018, the US will start banning all microplastics in cosmetics. As revealed in this article, larger plastic debris, clothing, and perhaps cosmetic products were the sources of the microplastics found in both the mollusk and Hudson River water samples.

To help scientists examining different avenues of microplastic pollution, FTIR microspectroscopy can provide the required composition information to locate microplastics. The extra flexibility of the SurveyIR microsampling accessory improves microplastic research. As manufacturing of plastic continues to flourish, it is vital for people to understand the impact microplastics have on the environment.

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Post time: May-17-2019
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