Using Infrared Spectroscopy for Microplastic Analysis

Microplastics are minute pieces of plastic with diameters ranging from a few micrometers to a couple of millimeters. In recent years, microplastics have become a global marine environmental issue since they may adversely affect coastal and marine ecosystems and potentially harm the health of human beings. In light of this issue, Japan is investigating the distribution of microplastics and the amount of harmful chemical substances such as PCB that are adsorbed to such microplastics in the coastal and offshore areas of Japan and a wide area spanning from Japan to the South Pole. In addition, the government is calling on businesses to stop using microbeads in personal care products such as scrubs.¹

Microplastics are classified into primary microplastics and secondary microplastics. Primary microplastics are manufactured as materials for use in scrubs and industrial abrasives and are often made of polyethylene (PE) or polypropylene (PP). Secondary microplastics refer to plastics that have been broken down from larger debris into pieces with a diameter of 5 mm or less due to external factors such as ultraviolet radiation² and the types of such plastics are various.

A Fourier transform infrared spectrophotometer (FTIR) is suited to the analysis of microplastics since FTIRs are optimal for the qualitative analysis of organic compounds. In cases where the sample is minute and no larger than 100 μm, the use of an infrared microscope is effective. This article introduces example analyses of primary and secondary microplastics utilizing an infrared microscope.

R. Fuji

AIM-9000 Infrared Microscope

An infrared microscope system can obtain data on tiny areas with great sensitivity through adjustments to the infrared beam diameter and the use of an aperture. A visual reference of the system can be found in Figure 1.

Figure  1. IRTracerTM-100 Fourier Transform Infrared Spectrophotometer (Left) and AIM-9000 Infrared Microscope (Right)

Qualitative Analysis of a Primary Microplastic

Primary microplastic found in a scrub was arranged as a sample and measured. By dissolving the scrub in water and carrying out a number of filtrations, it was possible to separate the microplastic out of the scrub. An optical microscopic image of the microplastic on the filter can be seen in Figure 2.

Figure 2. Microplastic on the Filter (Indicated by black arrows).

One section of microplastic was removed from the filter and compacted in a diamond cell to be analyzed through infrared transmission microspectroscopy. Table 1 details the measurement environment, and Figure 3 displays the measurement sample. The measurement result, as shown in Figure 4, establishes that the microplastic is polystyrene (PS).

Figure 3. Measurement Sample.

Table 1. Measurement Conditions

Figure 4. Measurement Results.

Mapping Analysis of Secondary Microplastics

Secondary microplastics, such as those found in major bodies of freshwater and seawater, were prepared as a sample and analyzed. A polytetrafluoroethylene (PTFE) filter was employed to filter and gather microplastics dispersed in water. PTFE is useful for measuring transmission spectra with the sample left on the filter, as it only has an infrared absorption band near to 1,200 cm^-1.

Infrared transmission microspectroscopy was used to carry out mapping analysis, which allows for the categorization of the range of microplastics found on the filter. Figure 5 displays an optical microscopic image of the microplastics on the filter, while details of the measurement environment can be found in Table 2. Figures 6-(a) to (c) display the measurement results. The portion of the PTFE filter to which no sample had stuck was set as the background.

Figure 5. Microplastics on the Filter.

Table 2. Measurement Conditions

The results indicated that the microplastics in the sample were polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET). Figures 6-(a) to (c) are colored in accordance with corrected peak height values (the peak height from the baseline) of the characteristic peak of each plastic (PE: 718 cm^-1 caused by CH₂ rocking vibrations, PP: 2839 cm^-1 caused by CH₂ stretching vibrations, PET: 1724 cm^-1 caused by C=O stretching vibrations).

Figure 6-(a). PE Distribution
(Based on the corrected peak height value of 718 cm^-1)

Figure 6-(b.) PP Distribution
(Based on the corrected peak height value of 2839 cm^-1)

Figure 6-(c). PET Distribution
(Based on the corrected peak height value of 1724 cm^-1)

Points where the plastic constituent is present in great quantities are indicated in red, while points with low quantities are shown in blue. Figure 7 illustrates characteristic infrared spectra from the regions in Figures 6-(a) to (c).

Figure 7. Typical Infrared Spectra from Areas in Figs. 6-(a) to (c)

Conclusion

The AIM-9000 infrared microscope can be used to carry out qualitative and mapping analyses of microplastics extracted from scrubs or collected on a filter. It is possible to measure even tiny samples with great sensitivity, while the comprehensive library included as standard allows for rapid evaluations.

References and Further Reading

1. Website of the Ministry of the Environment (Japan) Annual Report on the Environment, the Sound Material-Cycle Society and Biodiversity in Japan 2017, Part II, Chapter 4, Section 7 "Preservation of the Marine Environment" (in Japanese)
2. Website of the Ministry of the Environment (Japan) Water / Soil / Ground Environment, Marine Litter, "2016 New Year Symposium on Marine Litter" documentation

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

For more information on this source, please visit Shimadzu Scientific Instruments.