The results of the investigations have been combined to give a method for determination of the total amount of PCA (polycyclic aromatic hydrocarbons and related sulphur and nitrogen compounds, containing three or more fused aromatics rings, which might have short or cycloalkyl groups as substituents) and not only PAH (Polynuclear Aromatic Hydrocarbons) in the extract from the tread, based on IP 391 (Institute of Petroleum , London, Method 391).
The combined method consists of elements from previously accepted methods, and is suitable for both oils and for extracts from tyres. The standard deviation is better than ±0,4 % for oils with a PCA content less than 14 %, and ±0,5 % for tyres. The results indicate that the aromatic carbons content (produced by the oil companies) can be estimated from the measurements of PCA content: conversely, PCA content can be estimated from the aromatic carbons content, as determined by, say, infrared spectroscopy. This provides a means of classifying the environmental impact of tyres in terms of their PCA content.