Fourier transform infrared spectroscopy (FTIR) method for microplastics identification

Fourier transform infrared spectroscopy (FTIR) is a powerful tool in the field of material characterization for the study of chemical bonds of microplastics (MPs). FTIR spectroscopy is performed using an interferometer, which allows the scanning of all the frequencies present in the IR radiation generated by the source.  By applying the Fourier transform, the interferogram is transformed into a spectrum with peaks corresponding to specific chemical bonds or molecular vibrations. The spectra are recorded in absorbance mode within the range from 4000 to 400 cm−1. 

Carbon-based polymers such as polyethylene, polypropylene and polystyrene are easily identified with FTIR because different bond compositions produce unique spectra that discriminate plastics from other organic and inorganic particles. A well-established polymer spectrum library enables not only confirmation of plastics but also identification of specific polymer types. Moreover, FTIR is a non-destructive technique which means that the same sample can be analyzed multiple times if necessary.

FTIR involves three modes: transmission, reflectance and attenuated total reflection (ATR). In transmittance mode, theinvestigated particles need to be sufficiently thin (<100 μm) to avoid total absorption in the FTIR spectrum. Reflectance μ-FTIR is most suitable for plane surfaces where refractive errors are low, while the measurements of irregularly shaped materials must account for refractive error. In contrast, ATR-FTIR spectroscopy produces stable spectra from irregular MPs.

Larger particles (> 500nm) can be analyzed by using attenuated total reflection (ATR)-FTIR, but small particles require the use of micro-FTIR (μ-FTIR) that permits simultaneous visualization, mapping, and collection of spectra. Currently, μ-ATR–FTIR offers a useful method to identify microplastics in environmental samples, with identification of plastic-like particles by microscopy and subsequent chemical confirmation by spectroscopy.

FTIR can reveal the polymer composition as well as microplastic abundance but is time-consuming method.  Although semi-automatic mapping technology significantly reduces the operating time in FTIR, it requires at least 9 hours to scan one filter paper (maximum two samples per day, including the requirement of sample replacement). 

Ref:

1. W. J. Shim, et al.,  Anal. Methods, 2016, DOI: 10.1039/C6AY02558G.
2. Primpke S. et al. , 2018, “Reference database design for the automated analysis of microplastic samples based on Fourier transform” , Analytical and Bioanalytical Chemistry (2018) 410:5131–5141 https://doi.org/10.1007/s00216-018-1156-x 
3. Chen, Yiyang et al., (2020), “Identification and quantification of microplastics using Fourier Transform Infrared Spectroscopy: current status and future prospects”, Current Opinion in Environmental Science & Health, (), S2468584420300374–.  doi:10.1016/j.coesh.2020.05.004    

By: Moe Thazin Shwe, SOLEN Research Associate – IPC panel member

Editted by: Hendra WINASTU, SOLEN Principal Associate – IPC panel coordinator

Date: 17 March 2023

Article#: SOLEN-IPC-0012