Pyrolysis Gas Chromatography–Mass Spectrometry for Microplastics

Microplastics (MPs) are widespread environmental pollutants, originating from industrial raw materials, consumer products, and plastic fragmentation. LCGC International spoke to Damià Barceló, honorary adjunct professor in the Chemistry and Physics Department at the University of Almeria, Spain, about his work analyzing 12 common plastic polymers using pyrolysis gas chromatography–mass spectrometry (Py-GC–MS), a technique that offers precise identification and quantification. Sample collection, validation parameters, and analytical challenges are discussed, emphasizing the value of Py-GC–MS potential for microplastic research.

Q. What are microplastics and why are they ubiquitous in the environment?

Plastics are dispersed in the environment and fragment into small particles—from 1 µm to 5 mm, the so-called microplastics (MPs)—after exposure to wind, sunlight, and friction. Plastic particles smaller than 0.1 µm are referred to as nanoplastics (NPs). MPs are classified into primary and secondary types: primary MPs originate from industrial raw materials, consumer products, and cosmetics, while secondary MPs result from the fragmentation of larger plastic debris.

Why they are ubiquitous in the environment? The world produces 430 million metric tons of plastic each year. Primary MPs are commonly found in personal care products (PCPs). In Europe, estimates suggest that approximately 3215 tons of MPs are released annually from PCPs. In China, the estimated release is around 346 tons per year. In the United States, the annual release is approximately 282 tons a year.

Q. Why were the 12 specific plastic polymers chosen for analysis in this study (1), and how representative are they of environmental microplastic pollution?

They are considered the most common plastic polymers used, particularly polyethylene (PE), low density PE, and polypropylene (PP). Their low densities mean that they can easily be transported through air and water. High density MPs such as polyester (PES/PEST) and polytetrafluoroethylene (PTFE) tend to accumulate in solid matrices such as sludge and sediments and so are less mobile and less studied in the environment.

Q. How were the microplastic samples collected, and what criteria were used to select the samples for pyrolysis gas chromatography–mass spectrometry (Py-GC–MS) analysis?

In this particular case grab samples of MPs from different beaches along the Catalan Coast were collected. These samples were visible to the naked eye—consider that MPs particles are 2–5 mm in size. This was one of our first studies using the newly purchased pyrolysis GC–MS instrument and the idea was to test the performance of the equipment with real-world collected samples.

Q. How does the Py-GC–MS method enhance the identification and quantification of microplastics compared to other methods?

The Py-GC–MS method is very reliable for identification using the specific fragments of each polymer, the so-called pyrolyzate compounds. The obtained pyrograms of the real-world samples were compared to those of the in-house data base to identify the characteristic pyrozylates.

For the quantitative analysis, mass spectral search softwarewas used. A calibration curve for each plastic polymer was prepared using four concentrations of the standard polymers in 0.4–4.0 mg of MPs-CaCO₃ as solid samples. Calibration cannot be performed in water because of the different densities of the polymers that will not allow for complete solubilization in water.

Q. What validation parameters were used to ensure the accuracy and reliability of the Py-GC–MS method?

Validation was performed with authentic polymer standards of each MP sample, with comparison of the pyrolyzate compounds from the laboratory authentic standards with those of the mass specpyrolyzate library and with the ones obtained with the real-world samples. Repeatability of the spectra was evaluated during different days and at different polymer concentrations. The pyrolyzer furnace temperature of 600 °C and the electron voltage of 70 eV were maintained during all analyses.

Q. What are the main challenges associated with using Py-GC–MS? What tips would you offer a first-time user?

The use of this instrumentation requires skills in mass spectrometry and in pyrolysis. GC–MS is a well-established technique and researchers are familiar with conventional electron impact (EI) spectra. When coupled to pyrolysis, EI spectra are more complex and require further training to be confident at identification of each polymer. Characteristic pyrolyzate compounds can be familiar to several polymers, for example, styrene fragment at m/z 104, which can lead to identification mistakes of the individual polymers in complex samples. A useful tip to newcomers is to follow a course on polymer fragmentation using pyrolysis—it will be very helpful to avoid future mistakes when identifying specific polymers.

Q. Is Py-GC–MS becoming more readily adopted? Is it an under-valued technique?

It is not so commonly used as Fourier transform infrared spectroscopy (FTIR) or Raman in microplastic analysis. I believe that this is because FTIR and Raman can be combined with microscopy to determine the number and size of MPs in a given sample, in addition to the chemical composition of the polymer. Although Py-GC–MS cannot identify the size of MPs as it is a destructive technique, there is the option of the so-called double shot analysis. By using this particular option, chemical compounds present or attached to the MP surface can also be identified in the same GC–MS analysis, that is bisphenol A, polycyclic aromatic hydrocarbons, and phthalates among other chemicals. It is recommended that an expert and a well-established laboratory for MPs analysis possesses the capabilities for Py-GC–MS and either FTIR or Raman; all the possibilities for characterization and analysis of MPs as well as sorbed chemicals will then be feasible.

Q. What potential future applications could the Py-GC–MS method be used for?

Py-GC–MS can be applied to the analysis of MPs present in any sample such as water, sediments, soils, air, biota, and biological fluids. The main advantages include very small amounts of sample (0.5–1 mg), less interferences in the analysis compared to FTIR and Raman, analysis of NPs when sample filtration takes place below 1 µm before analysis, and, finally, double shot analysis with identification of other contaminants present in the MPs is straightforward.

Reference

(1) Santos, L. H. M. L. M.; Insa, S.; Arxé, M.; et al. Analysis of Microplastics in the Environment: Identification and Quantification of Trace Levels of Common Types of Plastic Polymers Using Pyrolysis-GC/MS. MethodsX 2023, 10, 102143. DOI: 10.1016/j.mex.2023.102143

Damià Barceló is an honorary adjunct professor in the Chemistry and Physics Department at the University of Almeria, Almeria, Spain. His expertise lies in the analysis, fate,risk, and removal of emerging contaminants, nanomaterials, and microplastics from water, as well as sewage epidemiology of drugs and proteins using advanced mass spectrometric techniques. He has supervised
> 67 PhD students since 1992. From 1993 to the present day, he has been editor or co-editor of 40 books on environmental chemistry.