Analytical methods for microplastics

https://publiclab.org/notes/maxliboiron/07-19-2018/how-to-analyze-plastics-forensically

It is difficult to identify microplastics of various sizes, shapes, and polymer types fully and reliably from complex environmental matrices using a single analytical method. Here are some of the most commonly used analytical methods for identifying microplastics: 

Visual identification                                                                                                        

Microplastic (2-5mm) can be visualized and identified based on their morphology, color and size with naked eyes. Visual identification is simple, fast and easy that can be done by both experts and non-professional with brief training. However, microplastic of 1mm with transparent color is hard to be identified and can lead to misidentification.            

Microscopy                                                                                                                                 

Stereo- (or dissecting) microscopy is a widely used identification method for microplastics whose size falls in the hundreds of micron range. Scanning electron microscopy (SEM) can provide extremely clear and high-magnification images of plastic-like particles. Other advanced microscopy techniques such as polarised optical microscopy can be used to identify the microplastic’s crystallinity. These methods provide high resolution images and can differentiate microplastics from other particles. However, it can also lead to possibility of false positive and possibility of missing small and transparent plastic particles.

Fourier transform infrared (FTIR) spectroscopy                                                                                               

FTIR spectroscopy can be used to determine the chemical bonds of carbon-based polymer (microplastic) and can easily discriminate plastics from other organic and inorganic particles. It is expensive instrument and time-consuming laborious work but has more advantages such as there is no possibility of false positive data, reduction of false negative data, non-destructive analysis and automatic mapping.

Raman spectroscopy                                                                                                                                     

Raman spectroscopy has also been used to identify microplastics. The different responses and spectra between FTIR and Raman can compromise each other in complex microplastic identification. Confocal microscopy along with Raman spectroscopy can be used to identify microplastics in zooplankton samples. However, Raman spectroscopy is sensitive to the additive and pigment chemicals in microplastics that interfere the identification of polymer types. It is also expensive instrument and time-consuming laborious work. 

Thermal analysis                                                                                                                                           

The thermo-analytical technique, which measures changes in the physical and chemical properties of polymers depending on their thermal stability, has been recently tested for microplastic identification. Differential scanning calorimetry (DSC) can determine the melting temperature, glass transition temperature and crystallinity of microplastics. These measurements include destructive analysis that can identify a few polymers needed to interpret complex. Pyro-GC-MS (Pyrolysis gas chromatography mass spectrometry) is another method that analyses thermally decomposed gas from polymers. Pyro-GC-MS analysis identified the isolated potential plastic particles from sediment samples. Thermal analysis provides an alternative method to spectroscopy for chemical identification of certain polymer types.

Chemical digestion                                                                                                                      

This method involves dissolving the sample in a chemical solution to digest any organic material present and leave behind the microplastic. The microplastics can then be analyzed using microscopy or spectroscopy techniques.

Novel methods                                                                                             

Atomic force microscopy (AFM) combined with either IR or Raman spectroscopy is a potential candidate for nano-plastic analysis. IR or Raman spectroscopy combined with AFM can determine the chemical composition of the object. Another possible analytical combination that may be applicable to microplastic identification is that of automated particle tracking, image analysis, and Raman spectroscopy. 

Overall, the combination of analytical methods helps in identifying microplastics in varied and complex environmental matrices. It is important to choose the most appropriate method based on the sample type and research question. 

Ref:

1. Shim, W J and et al., 2017, “Identification methods in microplastic analysis: a review”, Anal. Methods., 2017, 9, 1384 
2. Erin Tuttle and Aron Stubbins ,2023, “An optimized acidic digestion for the isolation of microplastics from biota-rich samples and cellulose acetate matrices”, EnvironPollut., volume 322

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

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

Date: 21 February 2022

Article#: SOLEN-IPC-00010

The Former Article: https://solenvn.com/en/solid-waste-degredation/
The Latter Article: https://solenvn.com/en/stereo-or-dissecting-microscopy-identification-of-microplastics/