Whether the aim is to evaluate the odour contamination of used plastic packaging, to optimise cleaning processes for recovered plastics or recycled materials or to issue recommendation for efficient and appropriate waste sorting: the correct analytical strategy and suitable analytical technology are always the basis. The Fraunhofer Institute for Process Engineering and Packaging (IVV) in Freising / Germany has both – the strategy and the technology – as well as extensive polymer analysis know-how. The results of the research projects carried out by the IVV are documented in a variety of respected scientific publications [1-4]
What approach needs to be adopted in the odour analysis of plastics, however? Dr Philipp Denk from the IVV explains that the detection of aromatic substances in food provides guidance here, because it can be applied to other application areas too. A brief look at the different operations carried out in the laboratory makes it clear what procedure is involved here:
Operation 1: sensory classification of the samples. A sensory panel consisting of several people who are trained to identify odours and describe them accurately carries out a sensory evaluation of the samples in the context of a "classification test" in accordance with DIN ISO 8587: 2010-08. Dr Philipp Denk says: "In this way, it is possible to specify an initial overall odour impression, which is important for the subsequent work". This operation is followed by a gas chromatographic analysis of the samples, combined with another sensory evaluation via olfactometry (O) using a special port on the GC to facilitate nose sniffing with the aim of characterising the individual odorants present in greater detail. For this purpose, the odorants are, however, extracted from the polymer first (see operation 2).
Operation 2: extraction of the VOC from the polymer matrix. With the help of a suitable solvent, such as dichloromethane, the components that are often only present in traces in the sample but have strong odours are extracted from the polymer matrix and are separated from the non-volatile components in a high vacuum using solvent-assisted flavour evaporation (SAFE) technology. Dr Philipp Denk reports that it is important to proceed gently here, in order to make sure that "no unstable odour-active compounds are impacted and no new ones are formed during the operation, which would distort the analysis". The extract produced is filtered and concentrated and an odour extract dilution analysis (OEDA) is carried out.
Operation 3: odour extract dilution analysis. Aroma analysis has shown that it is often the case that only very few substances determine the characteristic odour, even if many different volatile compounds may be present in a sample. In order to detect these few relevant components, the extracts from a series of consecutive dilution operations are separated via gas chromatography and are evaluated via olfactometry, i.e. they are evaluated by a sensory expert using the human nose via a sniffing port (e.g. ODP) at the end of the separation process. Dr Philipp Denk reveals: "Odour-active volatile components can be detected in this way and can be attributed by comparing them with reference substances".
Operation 4: identification of the odour sources by 2D-GC-MS/O. The compounds that are particularly responsible for the odour are identified finally by a combination of gas chromatography-mass spectrometry and olfactometry (GC-MS/O) and – in the case of interaction between different signals / substances – by two-dimensional gas chromatography-mass spectrometry in combination with olfactometry (GC-GC-MS/O).