Effective examination of minute polymer residue in marine samples
In order to obtain an insight into the nature and scope of environmental pollution by plastic debris, the scientific community is supposed to help to determine the facts. Pyrolysis GC/MC could be a useful way to characterise critical microplastic particles. Scientists from Osnabrück and Darmstadt Universities are at any rate demonstrating how the chemical composition of microplastic particles can be investigated using this tool in the context of the EU-wide introduction of the Marine Strategy Framework Directive (MSFD).
During the International Coastal Cleanup 2013 , 648,015 volunteers collected more than 5,500 tonnes of debris from about 20,783 kilometres of ocean coastline. The top ten of the debris most frequently found: cigarette butts (913 tonnes), sweet wrappers (794 tonnes), plastic bottles (426 tonnes), plastic lids (385 tonnes), (plastic) drinking straws (252 tonnes), standard supermarket plastic bags (200 tonnes), glass bottles (179 tonnes), other plastic bags and sacks (176 tonnes), paper bags (167 tonnes) and beverage cans (154 tonnes). Although no objective conclusions about how much litter reaches the sea year in, year out can be drawn from the figures, it can be presumed that the total is much worse, since the proverbial tip of the iceberg is basically all that was discovered. What can definitely be assumed, on the other hand, is that plastic residue is one of the main environmental pollutants.
It is not just environmental organisations that are convinced of this. The plastics industry and the political community think that action needs to be taken too. Steps are in the meantime being taken at different national and international levels, ranging from demands for responsible and sustainable use of polymer raw materials and products to the development of effective waste disposal and recycling concepts. Measures and projects have been initiated with which the global problem of marine debris is being tackled [2-4, 14, 15]; the focus is now on inland waterways like rivers and lakes, which also appear to be affected severely [5-8].
Wasserfloh mit Plastikpartikeln im Verdauungstrakt, deutlich sichtbar (rot) in der Fluoreszenzaufnahme. Foto: Chr. Laforsch
The problem with the smallest particles
Polymers are extremely tough structures. Although they are susceptible to high-energy sunlight as well as to chemical and mechanical influences, they never – depending on their chemical structure (i.e. there are only a few exceptions) – disintegrate completely and disappear entirely. In the course of time, a plastic breaks down into increasingly small and fine particles. By the way: microplastic includes not just artefacts but also plastic granulates that are used in the manufacturing of plastic products or in cosmetics and personal products.
In view of their size (just a few micrometres to millimetres) and the fact that they often have irregular shapes and colours, organisms that live in the water and sea birds confuse microplastic particles with food and eat them. Varying numbers of plastic fragments are frequently found in dead sea birds. It remains dubious whether microplastic caused the death of the birds; this is difficult to prove experimentally. It is reasonable to assume that the birds suffered from malnutrition when their stomachs are full of plastic. Microplastic particles may also contain additives that are harmful to health as well as pesticides, heavy metals or other toxins from the environment. This means that microplastic can also be a danger to us human beings – who are at the end of the food chain – when contaminated seafood lands on our plates.
In the course of their studies , Elke Fries et al. examined marine microplastic particles that they extracted from sand samples taken at some places on the beach at the northern end of the East Frisian island of Norderney, using all the laboratory facilities they had at their disposal.
International efforts to obtain more information
The Marine Strategy Framework Directive (MSFD)  issued by the European Commission, which focusses on effective protection of the marine environment, specifies that the nature and composition of microparticles, particularly microplastic particles, must be characterised in the marine environment. Scientists from Osnabrück and Darmstadt Universities decided to do this: in the course of their studies , Elke Fries et al. examined marine microplastic particles that they extracted from sand samples taken at some places on a section of the beach at the northern end of the East Frisian island of Norderney, using all the laboratory facilities they had at their disposal. In order to determine the type of plastic and the organic additives they contained, they made successful use in the above context of pyrolysis with subsequent gas chromatography and mass spectrometric detection (GC/MS) – claiming to be the first scientists ever to have done this  (see also [11, 12]).
Fries et al. write that the purpose of spectroscopic processes in particular is to determine the structure and composition of plastics as well as to identify them. It is reported that supercritical liquid extraction or Soxhlet extraction is frequently chosen to determine organic additives in plastic matrices, while thermoanalytical techniques are apparently required if the additives contained in the polymers are to be analysed that only dissolve or can only be extracted / hydrolysed poorly or to a limited extent.
Programm of a microplastic sample collected of the island Norderney. Image: E. Fries
A look at the technical and application details
Pyrolysis GC/MS can always prove to be the instrument of choice when the aim it to obtain structural information about macromolecules efficiently, via determination of the resulting thermal decomposition products and/or pyrolysis fragments . In serial pyrolysis GC/MS, the volatile compounds of the sample are first of all extracted thermally at low temperatures and then the same sample is pyrolysed by increasing the temperature – ideally without any modification of the system. By way of summary, Fries et al. report that it is possible in this way to extract organic additives and pyrolysis products in a single analytical procedure, while at the same time obtaining information about the material structure, i.e. the type of plastic, and the additives it contains.
With the help of existing mass spectrum databases and a comparison of retention times and mass spectrums of previously tested standard substances, Fries et al. succeeded in identifying various plastic additives in the microplastic particles found on Norderney. They included plasticisers (phthalates; DEHP, DBP, DEP, DIBP and DMP), antioxidants like 2,4-di-tert-butylphenol and such aromatic compounds as benzaldehyde, which is added – for example – to cosmetics and polymers as a fragrance. The type of plastic from which the unknown microplastic particles were made was determined by the scientists by comparing the pyrograms they recorded with those obtained during the pyrolysis of standard polymers. They identified polyethylene (PE), polypropylene (PP), polystyrene (P) and polyamide (PA) as well as chlorinated and clorosulfonated PE.
System for the fully automated pyrolysis GC/MS analysis of microplastic. Image: GERSTEL
Efficient obtainment of valuable information
Fries et al. give the following assessment of their method for the detection of organic plastic additives in marine plastic particles and of the identification of the type of polymer involved: compared with traditional solvent extraction, serial pyrolysis GC/MS has the ability to analyse the type of polymer and organic additives the particles contain in a single process operation, without the use of solvents and with no background contamination. In just one process operation using the same sample, the chromatogram of the volatile compounds is obtained first of all, followed by an interference-free pyrogram of the pyrolysis fragments, because any potentially disruptive compounds have already been removed by the preceding thermal desorption stage.
Serial pyrolysis GC/MS has the degree of sensitivity required to determine plasticisers, antioxidants and aromatic substances in microplastic particles with a mass of less than 350 µ. A possible chemical, toxic or hormonal risk that some plastic additives represent can be tested and detected with this process. It was possible to use serial pyrolysis GC/MS in the context of the introduction of the Marine Strategy Framework Directive (MSFD) to investigate the chemical composition of microplastic particles. And last but not least – to present a complete picture: Fries et al. determined the following inorganic plastic additives: titanium oxide as well as barium, sulphate and zinc compounds. Fries et al. used scanning electron microscopy (SEM) for this purpose .