Spectrophotometric Analysis for Microplastics Measurement

Spectrophotometric Analysis of Microplastics Measurement and Use of Thermal-Damaged Plastics Library Microscopic plastic particles ranging from a few μm to 5 mm or smaller in size are called microplastics. As a marine environmental problem that adversely affects coastal areas and marine ecosystems and thus may impact human health, microplastics have become a global concern in recent years. Since early preventive measures are necessary to protect the global environment, various analytical instruments are used to identify the sources of microplastics and examine countermeasures. Microplastics are divided into two types: primary microplastics and secondary microplastics. Primary microplastics refer to substances used in industrial abrasives, scrubbing materials and similar applications. Polyethylene (PE) and polypropylene (PP) are frequently used in these applications. Secondary microplastics are substances formed when large plastic products are reduced to 5 mm or smaller in size by external factors such as ultraviolet radiation, and contain various plastic types. Fourier transform infrared spectrophotometer (FTIR) is generally used in qualitative analysis of plastics and is currently being used in research on the actual status of discharges into rivers. However, since many microplastics in the environment are primarily degraded by ultraviolet radiation, analyses using a standard FTIR library may not yield matches. This article addresses the measurement of microplastics with sizes of a few millimeters and the use of the Shimadzu thermal-damaged plastics library to identify samples.  

Equipment: IRSpirit™ Compact FTIR

The IRSpirit Fourier transform infrared spectrophotometer has a compact, portable body measuring 390 (W) × 250 (D) × 210 (H) mm and a footprint smaller than A3-size paper. It features a unique design allowing access from both sides and can be mounted vertically in confined spaces with the 250 mm edge facing forward. In addition to providing the highest S/N ratio and resolution in its class, the IRSpirit also features the largest sample compartment in its class. This characteristic permits installation of Shimadzu accessories and other commercially available accessories, providing the user with high flexibility. For this measurement, a system was used in which the Shimadzu QATR™-S ATR measurement accessory was integrated into the IRSpirit sample compartment in the IRSpirit unit. Figure 1 shows the appearance of IRSpirit + QATR-S. [caption id="attachment_140319" align="aligncenter"] Figure 1: Appearance of IRSpirit™ + QATR™-S Units[/caption]

Measurement Sample

Figure 2 shows microplastics collected from a seashore. The microplastics exhibit various shapes, including spherical and pellet-like forms, as well as different colors. [caption id="attachment_140318" align="aligncenter"] Figure 2: Microplastics Collected from Seashore[/caption]

Measurement Method: ATR Method

In the ATR method used with FTIR, ATR is the abbreviation for Attenuated Total Reflection. The absorption spectrum of a sample surface can be obtained by placing the sample on an ATR prism and measuring the total light reflected from the sample surface. The light penetration depth in ATR is a few μm. Figure 3 shows the measurement setup. [caption id="attachment_140317" align="aligncenter"] Figure 3: ATR Measurement Setup[/caption]

Thermal-Damaged Plastics Library

The Shimadzu thermal-damaged plastics library is a library containing spectral data for 13 types of plastics in an unheated state and when heated to various temperatures between 200°C and 400°C. Degradation of plastics begins with the formation of carbon-centered radicals as a result of the dissociation of carbon-hydrogen bonds caused by heat or light energy from hydrogen. When oxygen radicals react, the formation of additional radicals occurs in a type of chain reaction and hydrogen dissociates; furthermore, inert substances form through bonding between radical pairs. Plastics degrade through this process accompanied by molecular scission and cross-linking (1). Although significant differences can be seen in degradation progression between ultraviolet degradation and thermal degradation, the factors governing degradation progression are fundamentally the same (2). Since the changes that occur in the infrared spectrum are generally similar, the thermal-damaged plastics library can in many cases also be used for qualitative analysis of ultraviolet-degraded microplastics. Figure 4 shows the infrared spectrum of acrylonitrile butadiene styrene (ABS) resin when irradiated with ultraviolet light, and Figure 5 shows the infrared spectrum of ABS resin when subjected to heating. In the case of ABS, peaks associated with OH radical and C=O radical stretching vibrations appear under both ultraviolet and thermal conditions, indicating that oxidative degradation occurs due to exposure to ultraviolet radiation and heat. [caption id="attachment_140320" align="aligncenter"] Figure 4: Infrared Spectrum of ABS Resin
(Ultraviolet Irradiation)[/caption] [caption id="attachment_140321" align="aligncenter"] Infrared Spectrum of ABS Resin (Heating)[/caption]

Measurement Results

Microplastics collected from the seashore were measured for sizes of 5 mm and smaller. Table 1 shows the measurement conditions, and Figures 6 and 7 show the sample images and measurement results for two samples. From the results in Figure 6, a match was obtained for polypropylene (PP) at 200°C for 4 hours from the thermal-damaged plastics library for the white microplastic, and from Figure 7, a match for polyethylene (PE) heated at 200°C for 2 hours was obtained for the red microplastic. It can be concluded that both microplastics degraded through oxidative degradation caused by ultraviolet radiation. [caption id="attachment_140322" align="aligncenter"] Table 1: Measurement Conditions[/caption] [caption id="attachment_140323" align="aligncenter"] Figure 6: Measurement Result for White Microplastic and
Library Search Result[/caption]

Conclusion

Microplastics collected from the seashore were measured using a compact IRSpirit FTIR. Simple and easy measurement was achieved using the ATR method. Rapid qualitative analysis of degraded microplastics was also achieved using the Shimadzu thermal-damaged plastics library. When more precise qualitative analysis is desired, it is recommended to measure plastic deliberately degraded by ultraviolet irradiation and to compare this sample with the actual sample. References (1) Hiroshi Yamanoi (2007), The Mechanisms of Polymer Degradation Discoloration and Stabilization, Journal of the Materials Life Society, Japan, 19(3), 103-108. (2) Yoshio Oki (1973), Degradation of Plastic Materials, Journal of the Metal Finishing Society of Japan, 24(4), 229-238. Original Document A613 Application News (2020), Measurement of Microplastics and Use of Thermal-Damaged Plastics Library, Shimadzu Corporation   Müjde Bilgin Senior Chemist Regional Sales Manager Ant Teknik